hybrid plasmonic devices for optical communication and sensing1090914/fulltext01.pdf2017-04-25hybrid...

Hybrid Plasmonic Devices for Optical Communicationand Sensing

XU SUN

Doctoral Thesis in PhysicsSchool of Engineering Sciences

KTH Royal Institute of TechnologyStockholm, Sweden 2017

TRITA-FYS 2017:24ISSN 0280-316XISRN KTH/FYS/–17:24-SEISBN 978-91-7729-365-1

KTH School of Engineering SciencesSE-100 44 Stockholm

SWEDEN

Akademisk avhandling som med tillstånd av Kungl Tekniska högskolan framläggestill offentlig granskning för avläggande av teknologie doktorsexamen i fysik mån-dag den 22 maj 2017 klockan 10.00 i Sal C, Electrum, Kungl Tekniska högskolan,Kistagången 16, Kista, Stockholm.

© Xu Sun, May 2017

Tryck: Universitetsservice US AB

This thesis is dedicated to my parents, my wife and my son.

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Abstract

Silicon (Si) and Si-on-insulator (SOI) platforms based technology is well-developed and is regarded as a most promising technology for the realizationof photonics integrated circuits for optical communication, interconnect, bio-sensing, etc. However, due to silicon’s indirect bandgap, absence of detectionproperties at 1.3-1.6µm and the difficulty in shrinking device size, hybrid Sidevices are widely investigated in recent years, addressing compactness andversatility. Integration of Silicon and plasmonic materials (gold, silver andcooper are commonly used) can break the so called diffraction limit and per-mit much reduced mode size in relation to Si photonic, allowing photonicson-chip components with ultra-compact size. However, the large plasmonictemporal losses due to the damping of free electrons’ oscillations, in generalincrease with reduced mode size and hence limit the propagation length andthus applicability. Hybrid plasmonic (HP) waveguides, a multi-layer waveg-uide structure supporting a hybrid mode of surface plasmonics and Si pho-tonics, is a compromise way to integrate plasmonic materials into Si or SOIplatforms, which can guide optical waves of sub-wavelength size, and with rel-ative low propagation loss. In this thesis, several HP waveguides and devicesare developed for the purposes of optical communication and sensing.The developed HP waveguides are divided into lateral and vertical structures,where different materials are either placed side by side (lateral) or arrangedas layers in vertical direction. With lateral structures, an air gap betweenplasmonic material and the silicon core can be formed, which can be used foroptical sensing and modulation applications with very high sensitivity to therefractive index change of the materials inside the gap. The single-slot HPring resonator sensor with 2.6µm radius can give a quality factor (Q factor)of 1300 at the communication wavelength of 1.55µm with a device sensitiv-ity of 102nm/RIU (RIU = refractive index unit). For the double-slot HPwaveguide, the sensitivity is even higher. The Mach-Zehnder interferometer(MZI) with a 40µm double-slot HP waveguide has a device sensitivity around474nm/RIU. The partly opened silicon side-coupled double-slot HP ring res-onator has a device sensitivity of 687.5nm/RIU, however, at the expense oflow Q factor (∼300) due to the different modes propagated inside. Furtheroptimizations (simulation results) show that the Q factor can be improved tobe over 1000. For vertical HP waveguide structures, the fabrication is sim-pler than for the lateral ones, with easier the overlayer alignment. Further,an all-optical switching HP donut resonator with a photothermal plasmonicabsorber is developed, utilizing the thermal expansion effect of silicon to shiftthe resonant peak of the HP resonator. The active area has a radius of 10µmto match the core size of a single-mode fiber. By applying 10mW power of thedriving laser to the absorber, the resonator transmitted power can be changedby 15dB, with an average response time of 16µs. Using the same fabricationflow, and removing the oxide materials using hydrogen fluoride wet etching,a hollow HP waveguide is fabricated for liquid sensing applications. The ex-perimentally demonstrated waveguide sensitivity is about 0.68, which is morethan twice that of pure Si waveguide device. Microelectromechanical systems(MEMS) can also be integrated into vertical HP waveguides. By tuning the

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thickness of the air gap, over 20dB transmitted power change was experimen-tally demonstrated. This can be used for optical switching applications byeither changing the absorption or phase of the HP devices.All components presented here are novel HP structures showing useful per-formance in optical communications and bio-sensing applications, and can beused to enhance the performance and broaden the functionality of silicon orSOI based photonic integrated circuits.

Keywords: Silicon photonics, plasmonics, optical communication,integrated photonics, photonic sensors, hybrid plasmonic waveguide, ringresonator, Mach-Zehnder interferometer, Microelectromechanical systems(MEMS).

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Sammanfattning

Teknologin baserad på kisel (SI) och kisel- på-dielektrika (SOI) plattfor-mar är väl utvecklad och ansedd som mycket lovande för framställning av fo-toniska integrerade kretsar för optisk kommunikation, optisk förbindningstek-nik, biosensorer etc. Under senare år har emellertid hybrid-kisel komponenterblivit föremål för växande intresse på grund av svårigheten att minska kisel-komponentstorlekar, kisels indirekta bandgap och frånvaron av detekteringi våglängdsbandet 1.3-1.6µm. Hybridkiseltekologi medger minskade dimen-sioner och flexibel anvädning. Integration av kisel och plasmoniska material(guld, silver, koppar används vanligen) kan övervinna den s k diffraktionsgrän-sen och medge kratigt reducered optisk fältutbredning jämfört med kiselfoto-nik och därmed möjliggöra fotonikomponenter med väsentligt minskad storlekeller “footprint”. De betydande temporala energiförluster som är förknippademed dämpningaen av fria elektronoscillationer ökar emellerid allmänt medminskad modstorlek och begränsar därmed ljusets fortplantningsträcka ochså många tillämpningar. Hybridplasmoniska (HP) vågledare, en multiskiktsvågledarstruktur, uppvisar hybridmoder innefattande ytplasmoner och kon-ventionella dielektriska moder och är en kompromiss för att integrera plasmo-niska material i kisel eller SOI plattformar. Dessa vågledare uppvisar subvåg-längdsmoder med relativt låga förluster. I föreliggande avhandling beskrivesflera HP vågledare och komponenter för optisk kommunikation och sensorer.De tillverkade HP vågledarna kan indelas i laterala och vertikala strukturer,där olika material antingen placeras sida vi sida (lateralt) eller ordnas verti-kalt. Med laterala strukturer kan ett luftgap mellan det plasmoniska mate-rialet och en kiselkärna skapas, vilket kan användas för optiska sensorer ochmodulatorer med mycket hög känslighet för indexändringar hos materialet igapet. En enkelgaps HP ring resonator med 2.6µm radie kan ge ett godhets-tal (Q värde) på 1300 vid våglängden 1.55µm med en komponentkänslighetav 102nm/RIU (RIU=brytningsindexenhet). För en dubbelgaps HP vågelda-re är känsligheten ännu strörre. En Mach Zehnder interferometer (MZI) medett 40m dubbelgap har en komponentkänslighet runt 474nm/RIU. En delvisöppen sidkopplad HP ring resonator av kisel med dubbelgap har en känslig-het på 687.5nm/RIU, dock på bekostnad av en låg Q faktor, ungefär 300,på grund av kopplingsförluster mellan de olika moder som propagerar i ring-en resp. accessvågledare. Ytterligare optimering och simuleringar visar att Qfaktorn kan ökas till över 1000. Tillverkningen är enklare för de vertikala HPvågledarstrukturerna än för de laterala på grund av enklare överskiktsupplin-jering under litografi.Vidare utvecklades en heloptisk donut HP resonatorswitch med en fototermiskplasmonikabsorbator genom att utnyttja kislets termiska expansionseffekt föratt skifta HP resonatorns resonansvåglängd. Den aktiva ytan har en radie på10µm, anpassad till kärndiametern hos en singelmodfiber. Genom att belysaabsorbatorn med 10 mW ljuseffekt kan resonatorns transmitterade effekt änd-ras med 15 dB, med en medelresponstid på 16µs. Med samma fabrikationsflö-de och geonom att avlägsna oxidmaterialen med fluorväte våtets tillverkas enihålig HP vågledare för sensortillämpningar med vätskor. Den experiementelltdemonstrerade vågledarkänsligheten är runt 0.68, vlket är mer än dubbelt så

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mycket som i en anordning baserad på enbart kisel. Mikroelektromekaniskasystem (MEMS) kan också integreras i vertikalorienterade HP vågledare. Merän 20 dB förändring av transmitterad effekt kunde experimentellt visas ge-nom att avstämma tjockleken på luftgapet. Detta kan användas för optiskaswitchingtillämpningar genom att antingen ändra absorption eller fas i HPkomponenten.

Alla här presenterade komponenter är nya HP strukturer uppvisande an-vändbara prestanda i optisk kommunikation och biosensortillämpningar ochkan utnyttjas till att förbättra prestanda och vidga funktionalitet hos kiseleller SOI-baserade fotoniska integrerade kretsar.

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Acknowledgements

My deep gratitude goes to Assoc. Prof. Lech Wosinski, my main supervisor,for his patient guidance, encouragement, and support throughout my Ph. Dstudy. Your invaluable guidance helped me in all the time of research andscientific writing during the past years, as well as will keep encouraging mein my future career and life. Thanks to Prof. Lena Wosinska, for inviting meto your summer house and dinner.

I would like especially to thank Prof. Lars Thylén, for your patient guid-ance, valuable discussions, and constructive suggestions on the direction ofmy researches. Your immense knowledge, rigorous working attitude and en-thusiasm in research always encouraged me to work toward the best.

Many thanks to my co-supervisors, Prof. Sebastian Lourdudoss and Prof.Sergei Popov, for your kind helps and guidances in my Ph. D studies, andyour suggestions in making my study and research plan.

Special thanks to Prof. Katia Gallo, for being my advanced thesis reviewer.Thanks to Assoc. Prof. Anders Liljeborg, for your always patient guidanceand assistance in Albanova nanofabrication lab. I would also like to thanksto Prof. Daoxin Dai, for helping to solve and understand the complicatedpuzzles from simulation or experimental results. Many thanks to Prof. MinQiu and Prof. Anand Srinivasan, for your encouraging talks and positivecomments on my research work.

Thanks to all the people involved in my research work: Dr. Min Yan, foryour valuable discussions and suggestions not only on studies and researches,but also the wonderful life in Sweden; Dr. Fei Lou, for your kind and effectivehelp in the nanofabrication and optical labs in my first year of my Ph. Dstudy; Dr. Xi Chen, my good friend and kind advisor, for all your help inoptical characterization lab and other social activities.

Thank you all my friends at KTH, Dr. Yiting Chen, Dr. Yuechun Shi, Dr.Yanting Sun, Dr. Jin Dai, Miao Zhang, Shuoben Hou, Ye Tian, Dr. YichenZhao, Fan Pan, Aleksandrs Marinins, Carlos Errando Herranz, Amin Bagh-ban, Dennis Visser, Elena Vasileva, Gleb Lobov, Ruslan Ivanov, Tomas, Wu-jun Mi, Jiang Sheng, Maoxiang Guo, Dr. Jiantong Li, Dr. Fei Ye, DingyuanChen..., for sharing all the unforgettable moments at KTH.

I would like to thank China Scholarship Council (CSC) for financial sup-port.

My deepest gratitude to my parents and parents in law, who are uncondi-tionally loving, supporting and encouraging me. Thanks to my beloved wife,Dr. Ken Cheng, thank you for your support and endless love when life wastough; My son, Roy Ruicheng Sun, you are the best gift for me!

Contents

Abstract vi

Sammanfattning viii

Acknowledgements ix

List of Acronyms xiv

List of Symbols xvi

List of Publications xix

List of Figures xxi

List of Tables xxv

1 Introduction 11.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Objectives and Main Achievements . . . . . . . . . . . . . . . . . . . 21.3 Thesis Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Basic Principles and Simulation Methods 52.1 Basic principles of hybrid plasmonic waveguides . . . . . . . . . . . . 5

2.1.1 Drude model . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.1.2 Surface plasmon polaritons . . . . . . . . . . . . . . . . . . . 72.1.3 Hybrid plasmonic waveguides . . . . . . . . . . . . . . . . . . 9

2.2 Simulation methods . . . . . . . . . . . . . . . . . . . . . . . . . . . 142.2.1 Finite element method . . . . . . . . . . . . . . . . . . . . . . 142.2.2 Finite-difference time-domain method . . . . . . . . . . . . . 17

3 Fabrication and Characterization Methods 193.1 Fabrication methods . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.1.1 Technology for hybrid plasmonic waveguide fabrication . . . . 203.1.2 Fabrication flow of hybrid plasmonic structures . . . . . . . . 27

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xii CONTENTS

3.2 Characterization methods . . . . . . . . . . . . . . . . . . . . . . . . 30

4 Lateral Hybrid Plasmonic Structures 334.1 Single-slot Hybrid Plasmonic Structures . . . . . . . . . . . . . . . . 33

4.1.1 Single-slot hybrid plasmonic waveguide . . . . . . . . . . . . 344.1.2 Single-slot hybrid plasmonic bend . . . . . . . . . . . . . . . 354.1.3 Single-slot hybrid plasmonic ring sensor . . . . . . . . . . . . 37

4.2 Double-slot Hybrid Plasmonic Structures . . . . . . . . . . . . . . . 394.2.1 Double-slot hybrid plasmonic waveguide . . . . . . . . . . . . 394.2.2 Double-slot hybrid plasmonic MZI sensor . . . . . . . . . . . 424.2.3 Double-slot hybrid plasmonic ring sensor . . . . . . . . . . . 454.2.4 Further discussions on electro-optic modulator applications . 50

5 Vertical Hybrid Plasmonic Structures 515.1 All-optical switching HP donut resonator . . . . . . . . . . . . . . . 515.2 Hollow hybrid plasmonic Mach-Zehnder sensor . . . . . . . . . . . . 545.3 MEMS tunable hybrid plasmonic waveguides . . . . . . . . . . . . . 585.4 Graphene-based hybrid plasmonic optical modulator . . . . . . . . . 61

6 Conclusions and Future Work 656.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

6.1.1 Optical sensing applications . . . . . . . . . . . . . . . . . . . 656.1.2 Optical switching and modulation applications . . . . . . . . 66

6.2 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

7 Guide to Appended Papers 69

Bibliography 71

List of Acronyms

1D one-dimensional

2D two-dimensional

3D three-dimensional

a-Si:H amorphous Silicon

CMOS complementary metal oxide semiconductor

c-Si crystalline Silicon

CW continuous wave

DL detection limit

DSHP double-slot hybrid plasmonic

EBE electron beam evaporation

EBL electron beam lithography

E-field electric field

EO electro-optic

FBMS free-beam movement stage

FDTD finite-difference time-domain

FEM finite-element-method

FSR free-spectral-range

HF Hydrogen Fluoride

HP hybrid plasmonic

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xiv CONTENTS

ICP-RIE inductive coupled plasma-reactive ion etching

ICs integrated circuits

IPA 2-isopropanal

MEMS microelectromechanical system

MZI Mach-Zehnder interferometer

OSA optical-spectrum-analyzer

PECVD plasma enhanced chemical vapor deposition

Q factor quality factor

Qabs factor quality factor of absorption

Qrad factor quality factor of radiation

RC resistance-capacitance

RIU refractive index unit

SEM scanning electron microscopy

SOI Silicon-on-insulator

SPPs surface plasmon polaritons

TE transverse electric

TM transverse magnetic

List of Symbols

k0 propagation constant in free space

Lprop propagation length

me mass of electron

ne density of electrons

neff effective refractive index

ng group index

S sensitivity

Veff effective mode volume

vF Fermi velocity

β propagation constant

γ damping constant of oscillation

Γ optical confinement factor

ε0 permittivity of free space

ε∞ permittivity at infinite angular frequency

λ wavelength

ω angular frequency

ωp plasma frequency

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List of Publications

Publications included in the thesisJournal Papers

A. X. Sun, D. Dai, L. Thylén and L. Wosinski, “High-sensitivity liquid refractive-index sensor based on a Mach-Zehnder interferometer with a double-slot hy-brid plasmonic waveguid” Opt. Express 23(20), 25707-25716 (2015).

B. X. Sun, D. Dai, L. Thylén and L. Wosinski, “Double-slot hybrid plasmonicring resonator used for optical sensors and modulators”, Photonics Vol. 2,No. 4, pp. 1116-1130 (2015).

C. X. Sun, L. Wosinski and L. Thylén, “Nanoscale surface plasmon polaritondisk resonators, a theoretical analysis”, IEEE Journal of Selected Topics inQuantum Electronics, 22(2) 4600106 (2016).

D. X. Sun, X. Chen, M. Yan, M. Qiu, L. Thylén and L. Wosinski, “All-opticalswitching using a hybrid plasmonic donut resonator with photothermal ab-sorber”, IEEE PTL, 28(15), pp. 1609-1612 (2016).

E. X. Sun, L. Thylén and L. Wosinski, “Hollow hybrid plasmonic Mach-Zehndersensor”, Opt. Lett. 42(4), pp. 807-810 (2017).

Conference Presentations

F. X. Sun, L. Thylén and L. Wosinski,“Slot hybrid plasmonic ring resonatorused for optical sensors and modulators”, presented at the Asia Communica-tions and Photonics Conference (ACP) 2015, Hong Kong, Nov. 19-23, 2015,Proceedings of the IEEE

G. X. Sun, L. Thylén and L. Wosinski, “Hollow hybrid plasmonic waveguide usedfor electro-optic phase modulation”, presented at the Asia Communicationand Photonics Conference (ACP 2016), Wuhan, China, Nov. 2-5, 2016, paper:AF3F.2. Best Conference Paper Award.

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xviii CONTENTS

H. X. Sun, L. Thylén and L. Wosinski, “MEMS tunable hybrid plasmonic-Siwaveguide”, Technical Digest of the Optical Fiber Communication Confer-ence (OFC), paper Th2A.6, Mar. 19-23, 2017, Los Angeles, USA.

Other publications

I. X. Sun, D. Dai, Y. Shi, L. Thylén and L. Wosinski. “Hybrid plasmonics:structures and applications”, review paper in preparation.

J. X. Sun, F. Lou and L. Wosinski, “Study of plasmonic analogue of EIT effectbased on hybrid plasmonic waveguide system”, presented at the Asia Commu-nications and Photonics Conference (ACP), November 12-15, 2013, Beijing,China, paper: AF4A.4.

K. X. Sun and L. Wosinski, “Double-slot hybrid plasmonic cavity used for phasemodulation and sensing”, Technical Digest of the Optical Fiber Communi-cation Conference (OFC), paper W2A.41, Mar. 22-26, 2015, Los Angeles,USA.

L. L. Wosinski, X. Sun and L. Thylén, “Hybrid plasmonic waveguides and de-vices for optical interconnects”, invited talk presented at 17th InternationalConference on Transparent Optical Networks, ICTON 2015, Budapest, Hun-gary, July 5-9, 2015, Proceedings of the IEEE.

M. L. Wosinski, X. Sun and L. Thylén, “Technological challenges in nanophotonicand plasmonic fabrication”, invited talk presented at the Asia Communica-tions and Photonics Conference (ACP) 2015, Hong Kong, Nov. 19-23, 2015,Proceedings of the IEEE.

N. L. Wosinski, X. Sun and L. Thylén, “Hybrid plasmonics for computer inter-connects and sensing”, invited talk presented at the Advanced Networks andTelecommunication Systems Conference (ANTS) 2015, Kolkata, Dec. 15-18,2015, Proceedings of the IEEE.

O. L. Wosinski, X. Sun and L. Thylén, “Silicon- and plasmonics-based nanopho-tonics for computer interconnects and sensing”, invited talk presented atthe 18th European Conference on Integrated Optics (ECIO 2016), Warsaw,18-20 May 2016.

P. L. Wosinski, X. Sun and L. Thylén, “Promise of hybrid plasmonics for opticalinterconnects”, invited talk presented at 18th International Conference onTransparent Optical Networks, ICTON 2016, Trento, Italy, July 10-14, 2016,Proceedings of the IEEE.

CONTENTS xix

Q. L. Wosinski, X. Sun and L. Thylén, “Technology of hybrid plasmonic devicesfor optical bio-sensing”, invited talk presented at 18th International Con-ference on Transparent Optical Networks, ICTON 2016, Trento, Italy, July10-14, 2016, Proceedings of the IEEE.

R. L. Wosinski, X. Sun and L. Thylén, “Nanophotonics and hybrid plasmonics:different technologies and applications”, invited talk to be presented at the“Integrated Optics: Physics and Simulations” SPIE conference, Prague Apr.24-27, 2017.

S. L. Thylén, L. Wosinski, X. Sun and D. Dai, “Integrated Nanophotonics forInformation Technologies and Sensors: Ways to solve the Present Gridlock inPerformance”, invited talk to be presented at 19th International Conferenceon Transparent Optical Networks, ICTON 2017, Girona, Spain, July 2 - 6,2017.

List of Figures

2.1 Real and imaginary parts of permittivity of gold (Au) and silver (Ag)calculated by the Drude model, fitted with the experimental data fromJohnson and Christy [24]. . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2 Planar metal-dielectric interface: (a) schematic; (b) the Hz-field distri-bution in vertical direction (y-axis). . . . . . . . . . . . . . . . . . . . . 7

2.3 Schematics of hybrid plasmonic waveguides with (a) single gap and (b)double-gap for thick gaps: t→∞ (left) and thin gaps: t→0 (right). . . 9

2.4 Analytical solutions of (a) real and (b) imaginary parts of effective re-fractive index of planar hybrid plasmonic waveguide with single gap,compared to the simulation results. . . . . . . . . . . . . . . . . . . . . . 11

2.5 Analytical solutions of planar hybrid plasmonic waveguide with double-gap: (a) the 1st, 2nd and 3rd supported optical modes; (b) the opti-mization of propagation length with different waveguide widths. h isthe width of Si core, while q = h/(2t+ h) is a parameter expressing theratio between the width of Si core and total width. . . . . . . . . . . . . 13

2.6 Finite-element-method for solving hybrid plasmonic waveguide. (a) Meshgraph of a double-slot hybrid plasmonic waveguide, and (b) numericalsolution of the supported fundamental mode. . . . . . . . . . . . . . . . 15

2.7 Axisymmetric finite-element-method for solving the eigenmode of ring/diskresonators. (a) Schematic of the surface plasmon polariton disk res-onator in cylindrical coordinates. (b) The eigenmode solved by axisym-metric finite element method. . . . . . . . . . . . . . . . . . . . . . . . . 16

2.8 Finite-difference time-domain method used for solving slot hybrid plas-monic ring resonator: (a) schematic of a slot hybrid plasmonic ring, andthe FDTD solved (b) Ex-field and (c) Hz-field distributions of the 15th

whispering-gallery mode. . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.1 Sketch of the electron beam lithography system. . . . . . . . . . . . . . 213.2 Sketch of ICP-RIE dry etching system. . . . . . . . . . . . . . . . . . . 223.3 Examples of dry and wet etching on silicon-on-insulator structure. (a)

ICP-RIE dry etching (recipe for Si etching) and (b) HF wet etching ofa silicon-on-insulator mark. . . . . . . . . . . . . . . . . . . . . . . . . . 23

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xxii List of Figures

3.4 Sketch of PECVD system. . . . . . . . . . . . . . . . . . . . . . . . . . . 243.5 Cross-section views of (a) PECVD grown amorphous Si and SiO2 and

(b) crystalline Si and thermal oxide (SiO2 from commercial used SOIwafer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.6 Sketch of EBE system and the lift-off process. . . . . . . . . . . . . . . . 263.7 Fabrication flows of SOI waveguides, double/single-slot HP waveguides,

vertical HP waveguides and hollow or MEMS tunable HP waveguides. . 293.8 Characterization setup. (a) Sketch of the coupling between single-mode

optical fiber and on-chip grating couplers. (b) Static (black arrows) anddynamic (red arrows) measurement setups. . . . . . . . . . . . . . . . . 30

4.1 Single-slot hybrid plasmonci waveguide: (a) cross-section view of thesingle-slot hybrid plasmonic waveguide, (b) E-field distribution, (c) neffand propagation length versus wslot, and (d) optical confinement factorsin Si and gap areas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

4.2 Bend properties of single-slot hybrid plasmonic waveguide. (a) and (b)Slot hybrid plasmonic bends toward left (metal layer) and right (Si core).(c) Si waveguide bend (toward left). (d) The neff and 90 bend loss ofa slot hybrid plasmonic waveguide. . . . . . . . . . . . . . . . . . . . . . 35

4.3 90 bend loss of a slot hybrid plasmonic waveguides with different radii. 364.4 Characterization results of a slot hybrid plasmonic ring sensor. (a) SEM

picture of the fabricated device. (b) Transmission responses with 100%and 60% IPA. (c) Resonant peak shifts by infiltrating with 20% to 100%IPA, and (d) their linear fitting curves in comparison to Si ring sensor. . 38

4.5 Double-slot hybrid plasmonic waveguide. (a) Schematic of the double-slot hybrid plasmonic (DSHP) waveguide connected to SOI waveguides.(b) Cross-section view of the DSHP waveguide covered by tested liq-uid. (c) Mode profile of the DSHP waveguide. The geometries are:wslot=150nm, wSi=165nm and hWG=250nm. The tested liquid is 100%2-isopropanol (IPA). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

4.6 Optical properties, including (a) effective refractive index (neff), (b) loss;optical confinement factors of (c) covering liquid (ΓIPA) and (d) Si ridge(ΓSi), of a DSHP waveguide with various wSi (100-400nm) versus wslot(20-200nm). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

4.7 Optimizations of a double-slot hybrid plasmonic waveguide. (a), (b) and(c) mode profiles of the DSHP waveguide with q parameters (q = wSi/w)of 0, 0.5 and 1, respectively. (d) Effective refractive index (neff) (blackcurves) and propagation loss (red curves) of the DSHP waveguide withvarious widths versus q parameters. (e) Optical confinement factor ofcovering liquid (ΓIPA), nano-slots (Γslot) and Si ridge (ΓSi) with differentgeometries. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4.8 Schematic of the asymmetrical Mach-Zehnder interferometer-based sen-sor employing double-slot hybrid plasmonic waveguide as the sensingarea. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

List of Figures xxiii

4.9 Scanning electron microscope (SEM) figures of the fabricated Mach-Zehnder interferometer-based sensor employing double-slot hybrid plas-monic (DSHP) waveguide as the sensing area. The widths of the Si ridgeand the slots are 165nm and 150nm, respectively. . . . . . . . . . . . . . 43

4.10 Transmission responses of the Mach-Zehnder interferometer-based sen-sor employing (a) 20µm, (b) 30µm and (c) 40µm double-slot hybridplasmonic waveguide infiltrated with 100% and 60% of IPA. (d) Fittinglines of wavelength shifts versus refractive index of tested liquids. . . . . 44

4.11 Double-slot hybrid plasmonic ring sensor. (a) Schematic of the double-slot hybrid plasmonic ring resonator. (b) Cross-section view, and (c)mode profile of the propagated wave inside the ring resonator. . . . . . . 45

4.12 Optical properties of the double-slot hybrid plasmonic ring sensor: (a)quality factors of the double-slot hybrid plasmonic ring with 150nm,250nm and 350nm wslot versus various radii. Optimization investiga-tions of (b) quality factor of absorption (Qabs), (c) effective refractiveindex (neff), and (d) sensitivity (S). . . . . . . . . . . . . . . . . . . . . 46

4.13 Scanning electron microscope (SEM) pictures of the fabricated DSHPring sensor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

4.14 Transmission responses of the fabricated DSHP ring sensor infiltratedwith 100% and 80% IPA (black curves), in comparison to the ones of aSi ring sensor (red curves). . . . . . . . . . . . . . . . . . . . . . . . . . 48

4.15 Optimized design of the double-slot hybrid plasmonic ring sensor. (a)Schematic of the ring sensor structure with loaded Q factor modification.(b) FDTD simulation results of transmission responses with differentwSi, and the loaded Q factors optimization. . . . . . . . . . . . . . . . . 49

5.1 Scanning electron microscopy (SEM) images of an HP donut resonatorwith photothermal plasmonic absorber. The upper right subfigure showsthe coupling area from Si access waveguide to hybrid plasmonic (HP)waveguide. The lower left subfigure shows the cross-section view of theHP waveguide, where the Au-Al2O3-Au layers compose the photother-mal plasmonic absorber, while the Au-SiO2-Si is the HP waveguide. . . 52

5.2 Characterization results of the hybrid plasmonic ring resonator withphotothermal absorber. (a) Transmission response of the HP donutresonator. (b) Absorption performance of the photothermal plasmonicabsorber. (c) Resonant wavelength shift of the HP donut resonatordriven by different laser powers. (d) Linear relationship between wave-length shift and optical power, which gives the switch efficiency around32.5nm/W. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

5.3 Dynamic measurement results of the all-optical switching HP donut res-onator with (a) 60µs and (b) 40µs signal periods. . . . . . . . . . . . . . 54

xxiv List of Figures

5.4 Hollow hybrid plasmonic waveguide. (a) 3D sketch of the hollow hybridplasmonic waveguide and (b) the cross-section at the Au-holes position.(c) The mode profile of the supported fundamental mode. Simulationresults of (d) the effective refractive index neff, (e) the propagation lossand (f) waveguide sensitivity S versus the width of Si ridge with differentthickness of the air gap. . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

5.5 Scanning electron microscopy (SEM) (a) top-view of the fabricated hol-low hybrid plasmonic (HP) Mach-Zehnder sensor, and (b) Cross-sectionview of the hollow HP waveguide. False colors are added to enhancecontrast. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

5.6 Characterization results of the hollow hybrid plasmonic Mach-Zehndersensor. (a) Transmission responses of the Mach-Zehnder interferometer(MZI) sensor with a 20µm hollow hybrid plasmonic (HP) waveguide inthe sensing arm, infiltrated with different concentrations of IPA. (b) Thelinear relationship between wavelength shift and refractive index changesof the MZI with 20, 30 and 40µm long hollow HP waveguides. (c) TheMZI sensitivity increases with the length of the hollow HP waveguide. . 57

5.7 Proposed hollow hybrid plasmonic waveguide for optical modulation. (a)3D sketch of the hollow hybrid plasmonic (HP) waveguide with double-gap. (b) Cross-section view at the Au-holes area, and (c) the modeprofile of the supported fundamental mode. Simulation results for: (d)the effective refractive index neff, (e) the propagation loss and (f) thewaveguide sensitivity S versus the width of Si ridge with different thick-ness of the air gap. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

5.8 MEMS tunable hybrid plasmonic waveguide. (a) Top-view and cross-section views of the MEMS tunable hybrid plasmonic waveguide with(b) “on” and (c) “off” states. Mode profiles of (d) “on” and (e) “off”states. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

5.9 Scanning electron microscopy (SEM) (a) top view and (b) cross-sectionview of the MEMS tunable hybrid plasmonic waveguide. (c) The outputpower variations as a function of wavelength for different bias voltages. 60

5.10 MEMS tunable directional coupler: (a) schematic; (b) even and oddmode of HP directional coupler when hlow=60nm; (c) coupling lengthschanged with wavelength; (d) normalized output power at one outputend of directional coupler. . . . . . . . . . . . . . . . . . . . . . . . . . . 60

5.11 Optical properties of graphene. (a) Conductivity and (b) refractive in-dex of graphene changes with chemical potential. . . . . . . . . . . . . . 61

5.12 Graphene based hybrid plasmonic modulator. (a) 3D schematic and(b) cross-section view of the proposed graphene-based hybrid plasmonicmodulator. (c) Effective refractive index (neff), and (d) propagation losschange with the driving voltage Vg. . . . . . . . . . . . . . . . . . . . . 62

List of Tables

2.1 Au and Ag parameters used for Drude model. . . . . . . . . . . . . . . . 62.2 Optical properties (@ 1550nm) of materials can be applied in hybrid

plasmonic waveguide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4.1 Refractive indices of aqueous solution of 2-isopropanol. . . . . . . . . . . 374.2 Comparison between different kinds of ring-type sensor. . . . . . . . . . 49

xxv

Chapter 1

Introduction

1.1 Background

Silicon (Si) or Silicon-on-insulator (SOI) platform-based photonic integrated cir-cuits (ICs) [1] show great potential for the realization of compact, low-cost andhigh performance devices for optical communication, bio-sensing, optical intercon-nect and data processing applications, having the advantages of small footprint,low absorption loss at infrared wavelength, and CMOS (complementary metal-oxide-semiconductor) compatible fabrication processes. However, due to the non-existing Pockels’ electro-optic (EO) effect in the centrosymmetric silicon, its indirectbandgap, its thermal sensitivity and diffraction limit of Si based waveguiding, othermaterials, such as semiconductor materials (Ge, GaAs, InP, etc), nonlinear mate-rials (LiNbO3, nonlinear organics, etc), and plasmonic materials (silver, gold andcopper) have been extensively studied to be integrated with Si or SOI platformto increase performance and broaden functionalities, under the common name ofhybrid Si devices.Hybrid Si-plasmonic devices are one of the hot topics in this area. Plasmonics isgenerally based on oscillations of coupled free electrons under the excitation of exter-nal electromagnetic fields, in the shape of surface plasmon polaritons (SPPs). TheSPPs can be either propagating along or localized at the interface between metaland dielectric. In comparison to the Si photonic mode, the SPPs propagating at themetal-dielectric interface have no diffraction limited modal field, and therefore deepsub-wavelength waveguiding can be realized. Generally hybrid Si-plasmonic devicescan be divided into two categories: one with separate Si and plasmonic elements[2–4], where the Si elements are used for long-range propagation and other passivecircuits, while the plasmonic elements are used as the functional devices, such asmodulators, sensors, etc. However, due to the scattering induced damping of thefree electron plasmas’ oscillations, the SPPs also suffer a huge propagation loss. Theother type of hybrid Si-plasmonic devices is based on a waveguide structure com-posed of plasmonic, low-index material and then Si as high-index material, which

1

2 CHAPTER 1. INTRODUCTION

can support a hybrid mode that is a superposition of Si photonic and plasmonicmodes, and is named as hybrid plasmonic (HP) waveguide structure [5, 6]. The HPwaveguides have the properties of both Si photonics and plasmonics and can guideoptical waves with sub-wavelength confinement, with much lower propagation lossthan the pure plasmonic ones. In this thesis work, we mainly investigate such HPwaveguides and devices in the various applications.The HP mode is formed by using surface plasmonics to enhance the evanescentfield of the (Si) photonic mode when the metal-dielectric interface(s) approachesto the Si core. Beside the properties of subwavelength confinement and relativelow propagation loss, the HP waveguides also have other properties, like polar-ization selectivity [7–10], low radiation loss of sharp bends [11–13], high Purcellfactor [14, 15], etc. In recent years, various types of HP waveguides have beendesigned and fabricated. The HP waveguide with metal cap [16] is the most inves-tigated one, with identical width of the plasmonic and Si core, and can be appliedfor subwavelength waveguiding, optical splitting [17], and coupling [18]. However,the fabrication of such waveguide requires high-precision over-layer alignment, andtherefore has a low fabrication tolerance. Other waveguide structures, like the onewith wider Si core [19], or with planar metal layer [20, 21] have been developedand can be realized with a relatively simple fabrication process, but at the expenseof large footprint. HP waveguides with lateral structures, with all componentsplaced side by side in the same plane, have also been developed [22, 23], which nor-mally contain double metal-dielectric interfaces, and can realize very large opticalconfinement in the layer of low-index material for optical sensing and modulationapplications.

1.2 Objectives and Main Achievements

Even though HP waveguides have been widely studied in recent years, the experi-mental realization of HP waveguide devices is still difficult due to the complicatedprocesses and low fabrication tolerances. Moreover, most of the experimentally re-alized HP waveguides are passive devices and for other functional applications, suchas optical sensing and switching, there are quite few experimental demonstrationsof such devices. Thus, the main objectives of this thesis are:

- To design new HP waveguide structures for optical sensing, switching andmodulation applications based on Si or SOI platforms.

- To fabricate, characterize and evaluate the performances of the designed HPdevices.

- To propose the integration between HP devices and other functional materials.

Based on a CMOS fabrication platform, we have fabricated various HP waveguidestructures for optical sensing and switching applications, which include:

1.3. THESIS OUTLINE 3

- Single-/double-slot HP waveguides and devices (including Mach-Zehnder in-terferometers and ring resonators).

- All-optical switching HP donut resonators with photothermal plasmonic ab-sorber.

- Hollow HP waveguides for sensing applications

- MEMS tunable HP waveguides.

All the fabricated structures are novel devices, never fabricated before.Using the designed HP waveguides, further integrations with nonlinear organicsand graphene have been proposed. Simulation results also show good performanceof the HP waveguides in applications as EO modulators.

1.3 Thesis Outline

In chapter 2, we will introduce basic properties of the HP mode by solving waveequations of the planar HP waveguides with single or double plasmonic-dielectric in-terfaces. Then, the simulation methods, used for numerically solving designed two-dimensional (2D) waveguide structures and devices will be briefly described. Thisincludes finite-element-method (FEM) and finite-difference time-domain (FDTD)methods.In chapter 3, operation principles of various tools for the purposes of deposition,etching, lithography, and other processing, will be introduced. Then, fabricationflows of the HP waveguides will be described.In chapter 4, the fabricated lateral HP waveguide structures, including single- anddouble-gap HP waveguides, will be presented. The optical properties of the waveg-uides, like effective refractive index (neff), propagation loss, waveguide sensitivity,bending loss, will be introduced first and then, the fabricated devices and charac-terization results will be shown and discussed.In chapter 5, the developed vertical HP waveguide structures (by overlaying mate-rial layers in the vertical direction) will be introduced, which includes an all-opticalswitching HP donut resonator, a hollow HP Mach-Zehnder sensor, a MEMS-tunableHP optical switch and a proposed HP modulator based on graphene.In chapter 6, conclusions will be drawn, which will summarize the results gainedfrom the developed HP waveguides and devices. Then, future work, based on theobtained simulation and experimental results, will be proposed, such as optimiza-tion of the performance and broadening of the functionalities of the HP devices inapplications for optical sensing, switching and modulation.

Chapter 2

Basic Principles and SimulationMethods

Surface plasmon polaritons (SPPs) originate from the electron plasma under an ex-ternal electromagnetic excitation, which can be either propagating along or localizedat the interface between materials with a high density of free electrons (negativeepsilon or plasmonic materials) and dielectric materials, e. g. metal-dielectric inter-faces. In contrast to dielectric waveguides, SPPs propagating at the metal-dielectricinterface can break the diffraction limit of light, and allow to realize waveguidingat subwavelength field size, however, at the expense of large energy losses of theoptical field due to the damping of electrons oscillations. Hybrid plasmonic (HP)[5, 6] waveguides are multi-layer structures composed of plasmonic layer(s), low-index layer(s) and high-index layer(s), which can support hybrid modes, i. e. acombination of SPPs and dielectric photonic modes. Therefore they have both,plasmonic- and photonic-like properties, like subwavelength confinement, relativelylow propagation loss, etc. In this chapter, the basic principles will be explained bysolving the wave equations of planar HP waveguides in section 2.1. Then, the sim-ulation methods used to design the HP waveguides and devices will be introducedin section 2.2.

2.1 Basic principles of hybrid plasmonic waveguides

2.1.1 Drude model

The SPPs are plasmon polariton waves propagating at the interface between any twomaterials with opposite signs of permittivity. They can be analytically describedby solving the wave equations, similarly to the conventional planar photonic waveg-uides. However, the dielectric properties of plasmonic materials are dispersive, andthe dielectric function of plasmonic materials is usually described by the Drudemodel, developed by the German physicist Paul Drude in 1900, and is given in the

5

6 CHAPTER 2. BASIC PRINCIPLES AND SIMULATION METHODS

Figure 2.1: Real and imaginary parts of permittivity of gold (Au) and silver (Ag)calculated by the Drude model, fitted with the experimental data from Johnsonand Christy [24].

following equation:εm = ε∞ − ω2

p/(ω2 + iγω), (2.1)

where ε∞ stands for the dielectric constant at infinite angular frequency; ωp =√nee2/ε0me ( ne is the density of electrons, me is the electron mass and ε0 is the

permittivity of free space) is the plasma frequency, which represents the naturalfrequency of the oscillations of the electron plasma; γ is the damping constant ofthe oscillation; ω is the angular frequency of the electromagnetic wave.

In this thesis, gold (Au) and silver (Ag) are used as the plasmonic materials, whoseparameters in Drude model are listed below:

Table 2.1: Au and Ag parameters used for Drude model.

ε0 ωp/2π(THz) γ/2π(THz)

Au 9.0685 2155.6 18.36

Ag 3.7 2196 3.772

2.1. BASIC PRINCIPLES OF HYBRID PLASMONIC WAVEGUIDES 7

Within the communication wavelength range, the Drude model calculated resultsfor permittivity are well fitted with the experimental data from Johnson and Christy[24], as shown in Fig. 2.1.

2.1.2 Surface plasmon polaritons

Figure 2.2: Planar metal-dielectric interface: (a) schematic; (b) the Hz-field distri-bution in vertical direction (y-axis).

With a one-dimensional(1D) planar metal-dielectric interface (only 1D confinementin the plane orthogonal to the propagation direction), as shown in Fig. 2.2(a), themetal half space has a dielectric function εm(ω), while the dielectric half space canbe assumed to be a lossless and homogenous material with dielectric constant, εd.The expressions of the field components (Hz, Ex and Ey) in transverse magnetic(TM) mode can be written as:

For y>0

Hz(y) = A2eiβxe−kmy; (2.2a)

Ex(y) = iA21

ωε0εmkme

iβxe−kmy; (2.2b)

Ey(y) = −A2β

ωε0εmeiβxe−kmy. (2.2c)

For y<0

Hz(y) = A1eiβxekdy; (2.3a)

Ex(y) = −iA11

ωε0εdkde

iβxe−kdy; (2.3b)

Ey(y) = −A1β

ωε0εdeiβxe−kdy. (2.3c)


A1 and A2 are the amplitudes of Hz-fields. km and kd are the decay constants inmetal and dielectric, respectively, which are defined as:

km(d) =√|β2 − k0

2εm(d)|. (2.4)

The continuity of Hz and Ex requires that:

A1 = A2; (2.5a)kdkm

= − εdεm. (2.5b)

From Eq. 2.5b, one should note that the surface waves only exist when the materials(e. g. metal and dielectric) have opposite signs of the real part of their dielectricpermittivity. Combining Eq. 2.4 and 2.5, we can get the propagation constant ofSPPs propagating at the interface between the two half spaces:

β = k0

√εdεmεd + εm

. (2.6)

Then, the effective refractive index is:

neff = β

k0=

√εdεmεd + εm

. (2.7)

By using the material parameters, the analytical results of the propagating SPPscan be obtained. Fig. 2.2(b) shows the Hz-field distribution in vertical direction(y-axis), where the metal and dielectric materials are Au and Si. The maximumamplitude of the field is located at the interface (y=0), and evanescently leaksinto the metal and dielectric materials. The highly localized field distribution isreferred to as plasmonic enhancement, which is widely used in bio-sensing [25], smallelements imaging and detecting [26, 27]. On the other hand for the TE polarizationmode, the similar deduction gives:

A1(kd + km) = 0. (2.8)

As the real part of kd and km should be positive values to achieve confinement,this condition is only fulfilled when the amplitude, A1, is zero. Thus, no surfacemodes exist for the TE polarization mode. It is necessary to clarify that for plas-monic waveguides with 2D optical confinement, the supported mode always has theelectrical-field (E-field) perpendicular to the metal surface. For lateral waveguidestructure with all the elements in a same plane, such as plasmonic slot waveguide[3] or HP waveguide with double low-index nano-slots [22], the electrical-field isin horizontal direction, which is resembling that of TE mode. To avoid confusion,such polarization mode is named as quasi-TE mode.


2.1.3 Hybrid plasmonic waveguidesThe hybrid plasmonic modes are realized in multi-layer structures, as shown in Fig.2.3(a) and (b) for single and double-gap structures respectively (Au, air and Siare used as the plasmonic, low-index and high-index materials). When the Au-airinterface(s) is close enough to the Si core, the evanescent field(s) of the photonicmode is enhanced by the surface plasmonic one, forming a hybrid mode, as shownin the right pictures of Fig. 2.3.

Figure 2.3: Schematics of hybrid plasmonic waveguides with (a) single gap and (b)double-gap for thick gaps: t→∞ (left) and thin gaps: t→0 (right).

In HP waveguide design, different materials can be used, depending on the desiredfunctions of the proposed HP devices, as listed in Table 2.2:


Table2.2:

Opticalproperties

(@1550nm

)ofm

aterialscan

beapplied

inhybrid

plasmonic

waveguide.

Ref.

Material

Refractive

indexApplied

layerGain

r33

[24]Au

0.52+10.74i

Plasmonic

--

[24]Ag

0.14+11.37i

Plasmonic

--

[24]Cu

0.72+10.66i

Plasmonic

--

[28]SiO

21.4657

Low-index

--

[29]Al2 O

31.6587

Low-index

--

[30]PM

MA

1.4809Low

-index-

-

[31,32]Er-doped

phosphateglass

1.52Low

-index1cm

-1@

1532nm-

[29,33]Er-doped

Al2 O

31.6587

Low-index

0.3cm-1

@1530nm

-

[34,35]EO

organicmaterials

∼1.7

Low-index

-100-500

pmV

[36]Si3 N

41.9963

High/low

-index-

-

[37,38]LiN

bO3

2.2119(no );2.1376(n

e )High/low

-index-

31.19pm

V

[39]LiTaO

32.1186(n

o );2.1224(ne )

High/low

-index-

27.4pm

V

[40]GaInA

sP3.1649

-3.3702High-index

∼1200cm

-1@

1550nm-

[41]c-Si

3.4757High-index

--

[42]a-Si:H

3.48High-index

--

[43]InP

3.1649High-index

--


Hybrid plasmonic waveguides with single gap

The HP waveguides with single gap are the most developed ones [5, 6, 16], withdifferent structures considering the required waveguide properties and fabricationmethods. As stated above, only the TM polarization mode can propagate as SPPs,and so is the case for the HP mode. The field components for such polarizationmode are:

Hz(y) =

Aeiβxek4[y−(t+h)] y ≥ t+ h

B+eiβxe−ik3(y−h) +B−eiβxeik3(y−h) h ≤ y ≤ t+ h

C+eiβxe−ik2y + C−eiβxeik2y 0 ≤ y ≤ h

Deiβxek1y y ≤ 0

; (2.9a)

Ex(y) = iHz(y) kiωε0εi

; (2.9b)

Ey(y) = −Hz(y) β

ωε0εi. (2.9c)

i=1, 2, 3 and 4 represent SiO2 (buffer layer), Si (high-index layer), air (low-indexlayer) and Au (plasmonic layer), respectively. ki is the decay constant in each layer,which is defined in Eq. 2.4. A, B+, B−, C+, C− and D represent the amplitudesof the Hz-field.The requirement of the continuitiy of Hz and Ex leads to:

Figure 2.4: Analytical solutions of (a) real and (b) imaginary parts of effectiverefractive index of planar hybrid plasmonic waveguide with single gap, comparedto the simulation results.


B± = 12e±ik3t(1∓ iξ4ξ3)A (y = t+ h); (2.10a)

C± = 12e±ik2h(1± ξ3/ξ2)B+ + 1

2e±ik2h(1∓ ξ3/ξ2)B− (y = h); (2.10b)

D = C+ + C− (y = 0); (2.10c)−iDξ1 = −C+ξ2 + C−ξ2 (y = 0), (2.10d)

ξi = ki

εi(i = 1, 2, 3, 4). Hence, the dispersion relation can be written as:

−C+ξ2 + C−ξ2 = −i(C+ + C−)ξ1. (2.11)

By solving Eq. 2.11 using mathematical software (Matlab is used in this thesis),the propagation constant of the supported mode can be obtained, as shown in Fig.2.4, where the real and imaginary parts of neff (the fundamental mode) are shownand compared to the results obtained from the simulation software. The geomet-rical parameters are h=250nm and t=20nm. By using this analytical method, theoptical properties of such a waveguide can be well-investigated, and optimizationscan be made for further studies of HP waveguides with 2-dimensional (2D) opticalconfinement.

Hybrid plasmonic waveguides with double gap

HP waveguides with double gaps have the plasmonic enhancement at both sidesof the Si core, and hence allow to realize large optical confinement factor in thelow-index material [22, 23]. The field components of TM mode can be written as:

Hz(y) =

Aeiβxek5[y−(2t+h)] y ≥ 2t+ h

B+eiβxe−ik4[y−(t+h)] +B−eiβxeik4[y−(t+h)] t+ h ≤ y ≤ 2t+ h

C+eiβxe−ik3(y−t) + C−eiβxeik3(y−t) t ≤ y ≤ t+ h

D+eiβxe−ik2y +D−eiβxeik2y 0 ≤ y ≤ t

Eeiβxek1y y ≤ 0

;

(2.12a)

Ex(y) = iHz(y) kiωε0εi

; (2.12b)

Ey(y) = −Hz(y) β

ωε0εi. (2.12c)


According to the requirement of the Hz and Ex continuity:

B± = 12e±ik4t(1∓ iξ5ξ4)A (y = 2t+ h); (2.13a)

C± = 12e±ik3h(1± ξ4/ξ3)B+ + 1

2e±ik3h(1∓ ξ4/ξ3)B− (y = t+ h); (2.13b)

D± = 12e±ik2h(1± ξ3/ξ2)C+ + 1

2e±ik2h(1∓ ξ3/ξ2)C− (y = h); (2.13c)

E = D+ +D− (y = 0); (2.13d)−iEξ1 = −D+ξ2 +D−ξ2 (y = 0), (2.13e)

The dispersion relation is then:

−D+ξ2 +D−ξ2 = −i(D+ +D−)ξ1. (2.14)

Taking the same geometry and material parameters as we used in the last section,

Figure 2.5: Analytical solutions of planar hybrid plasmonic waveguide with double-gap: (a) the 1st, 2nd and 3rd supported optical modes; (b) the optimization ofpropagation length with different waveguide widths. h is the width of Si core,while q = h/(2t + h) is a parameter expressing the ratio between the width of Sicore and total width.


the analytical solutions of the planar double-gap HP waveguide can be obtained.Fig. 2.5(a) shows the real part of the propagation constant for modes of differentorder (black curves) as a function of the thickness of the Si layer. There is no cut-offwhen the width of the Si layer tends to zero as expected. The propagation length(red curves) is defined as the distance where the amplitude of the field attenuatesto 1/e, which is

Lprop = λ0/[2πIm(neff] (2.15)

The propagation length is a frequently used parameter to examine the waveguidequality, as it is more intuitive to check the maximum distance, the wave can prop-agate in the proposed waveguide. An optimization of waveguide performance ismade in Fig. 2.5(b), where a q parameter, expressing the ratio between the widthof Si core and total waveguide width is defined. When the q parameter equals to 0or 1, the waveguide is a pure plasmonic waveguide with slot material as SiO2 or Si,respectively. The optimized propagation length for different widths can be foundby varying the q parameter from 0 to 1, which helps in future designs of double-slotHP waveguide sensors [Paper A], [Paper B].

2.2 Simulation methods

Planar HP waveguides with single/double low-index gap generally represent alltypes of HP waveguides with only 1D confinement in the plane orthogonal to thepropagation direction. In practical applications, depending on different fabricationprocesses and desired properties, HP waveguides with different structures have beendeveloped, which can offer a 2D optical confinement. However, due to the com-plexity of the waveguide structure, the analytical methods that give 1D solutionsare no longer applicable.

2.2.1 Finite element methodThe finite element method (FEM), a numerical method for solving partial differen-tial equations for problems in steady state (no time variation), is a widely appliedmethod for optical waveguide design. FEM divides the computing area into numer-ous sub-domains (mesh), and the final numerical results are obtained by gatheringthe ones of each sub-domain. Fig. 2.6(a) shows the mesh graph of a 2D double-slotHP waveguide in commercial FEM software, COMSOL Multiphysics. By settingthe material parameters of each layer, the geometrical parameters, boundary con-ditions, etc, the modes supported by the proposed waveguide can be found. Fig.2.6(b) shows the fundamental quasi-TE mode supported by such a waveguide, thecomputed effective refractive index is: 1.8663− 8.835× 10−4i. Other optical prop-erties, like confinement factors in different areas, field distributions, etc, can alsobe extracted from the obtained results.In addition, to study the waveguide properties, axisymmetric FEM [44] based on

2.2. SIMULATION METHODS 15

Figure 2.6: Finite-element-method for solving hybrid plasmonic waveguide. (a)Mesh graph of a double-slot hybrid plasmonic waveguide, and (b) numerical solutionof the supported fundamental mode.

COMSOL Multiphysics can be used to solve the partial differential equation incylindrical coordinates:

∇2r,yΦm(r, y)−m2Φm(r, y) + ε(ω

c)2Φm(r, y) = 0, (2.16)

and hence to obtain for example the eigenvalue of the mth-order whispering-gallery-like mode (Φm(r, y) = CmΦm(r, y)eimφ−iωt).This method can be applied to inves-tigate the ring/disk resonators with axisymmetric structures, as illustrated in Fig.2.7, where a surface plasmon polariton (SPP) disk resonator [Paper C] is shown.The solved eigenvalue is in the form of: ωeig = ωeig(real) + iωeig(imag), whereωeig(real) stands for the angular resonant frequency; while ωeig(imag) refers as the(temporal) damping or loss. The quality factor (Q factor) can be calculated by:

Q = ωeig(real)2ωeig(imag) . (2.17)

The Q factors of disk/ring resonators are determined by radiation and materialabsorption losses, which can be written as:

1Q

= 1Qabs

+ 1Qrad

(2.18)

where Qabs and Qrad are the quality factors of absorption and radiation, respec-tively. A combination between FEM and axisymmetric FEM can be used to inves-tigate the value of Qabs and Qrad, separately, and therefore to optimize the designof the proposed structures. The Qrad can be solved by setting lossless materials


Figure 2.7: Axisymmetric finite-element-method for solving the eigenmode ofring/disk resonators. (a) Schematic of the surface plasmon polariton disk res-onator in cylindrical coordinates. (b) The eigenmode solved by axisymmetric finiteelement method.

(only input the real part of permittivity) during the computing processes, while theQabs can be determined through the relationship between Qabs and neff(imag):

Qabs = ng2neff(imag) , (2.19)

where ng is the group index of the confined whispering-gallery mode, which isexpressed by:

ng = neff − λdneffdλ

(2.20)

For commonly used plasmonic materials (Ag, Au and Cu), the dispersion, λdneff/dλ,is much smaller than the value of neff, and therefore can be ignored in the calcula-tions. However, in the case when the plasmonic and dielectric materials are closeto their resonance [45, 46],εm + εd → 0, the dispersion will significantly influencethe group index, and so the properties of the resonator.In summary, the FEM simulations are applied in this work to design the HPwaveguides and their devices. Since the simulations are mostly processed in 2Dcross-section, the required computational power and simulation time are reason-able. Analyzing the results for the basic photonics ICs elements, e.g. waveguides

2.2. SIMULATION METHODS 17

or ring resonators, the performances of real devices can be estimated according tothe theory, such as Si coupled ring resonator, Mach-Zehner interferometer, etc.

2.2.2 Finite-difference time-domain methodFinite-difference time-domain (FDTD) method is a numerical method to solve thetime-dependent Maxwell equations. The partial differential equations are dividedinto space and time derivatives, and the space equations are solved at a given instanttime, and then at the next instant in time, until reaching the user-defined FDTDcomputing time or terminal conditions. Differently to FEM, FDTD simulationcan cover a wide frequency band within a single simulation time, and therefore it ismore suitable for simulation of real optical devices with asymmetry and complicatedstructures.

Figure 2.8: Finite-difference time-domain method used for solving slot hybrid plas-monic ring resonator: (a) schematic of a slot hybrid plasmonic ring, and the FDTDsolved (b) Ex-field and (c) Hz-field distributions of the 15th whispering-gallerymode.

Fig. 2.8 shows an example of FDTD simulation results of a single-slot ring resonator[Paper F]. By insertion of a dipole source with electrical-field perpendicular to themetal surface, the wave propagation properties at each wavelength are computed,and then the final results are obtained when the user defined FDTD time or terminalconditions are reached. Fig. 2.8(b) and (c) are the electric and magnetic fielddistributions at the resonant wavelength (∼1550nm), when the azimuthal numberm is 15. The Q factors, effective volume (Veff) and other properties can alsobe extracted from the computation results. In comparison to FEM simulations,FDTD method can deal with optical devices with arbitrary structures, like the Sibus waveguide coupled HP ring resonators [Paper B], however, at the expense oflonger simulation times.

Chapter 3

Fabrication and CharacterizationMethods

Recent progress in the area of photonic integrated circuits (ICs) is toward CMOS(complementary metal-oxide-semiconductor) compatibility that allows for mass pro-duction, low cost, high yield, and small footprint, and can replace the conventionalbulk components for the purposes of optical bio-sensing, communication, intercon-nect and many other applications. For Silicon (Si)- and Silicon-on-insulator (SOI)-based platforms, the fabrication technologies are being well-developed and offer therealization of low insertion loss-, densely integrated- and multi-functional photonicICs operating at near infrared wavelength. However, due to the non-existing Pock-els electro-optic (EO) effect in the centrosymmetric silicon and its indirect bandgap,integrations with other materials or platforms are required, which demand addi-tional processes of material deposition, epitaxy, wafer bonding and others. In thischapter, we will mainly focus on the integration between silicon and plasmonic ma-terials using Si and SOI platforms, and describe the fabrication (section 3.1) andcharacterization (section 3.2) methods applied here for realization of various hybridplasmonics (HP) waveguides and devices.

3.1 Fabrication methods

Similar to the SOI photonic ICs, the fabrication of HP devices requires lithographyand etching multiple-round processes. In addition to this, depending on differentHP structures and fabrication methods, other technologies, like thin film deposition,metal evaporation, oxidation, wet-etching, are also needed. In this section, we willbriefly introduce these technologies and tools used, as well as the process flows forHP waveguide fabrication.

19

20 CHAPTER 3. FABRICATION AND CHARACTERIZATION METHODS

3.1.1 Technology for hybrid plasmonic waveguide fabricationElectron beam lithography

Lithography is the key technique in ICs fabrication, which is applied to trans-fer the designed micro/nano patterns onto the sample with the help of photon(photolithography)- and electron (electron beam lithography)-sensitive resists. Thephotolithography is currently the only way for mass production of electronic orphotonic ICs. The resolution of photolithography is mainly dependent on the lightsource wavelength, which can be expressed by the Rayleigh equation:

W = kλ

NA. (3.1)

where k is the resolution factor ranged from 0.25 to 1 (depending on the opticalresist and fabrication platform), λ is the light source wavelength and NA is thenumerical aperture. For Mercury lamps with 436nm (G-line), 406nm (H-line) and365nm (I-line) wavelengths, the resolutions range from 500nm down to 200nm. ForArgon Fluoride Excimer laser with 193nm wavelength, the resolution can reach assmall feature size as 50nm, which is small enough for conventional photonic ICs fab-rication. However, for HP waveguide devices with subwavelength size of elements(<100nm), the photolithography is difficult to be applied. Moreover, the mask usedfor photolithography is also hard to be modified, and is more suitable for photonicICs with large number of elements for large scale production. In this respect, forexperimental applications, the electron beam lithography (EBL) is more suitablefor HP waveguide device fabrication. The principle of EBL is based on applyinga focused electron beam to draw a defined pattern on a sample coated with elec-tron sensitive resist (e-beam resist). As shown in Fig. 3.1, the EBL system (Raith150 in Albanova nanofabrication lab) consists of the scanning electron microscope(SEM) and high-precision stage movement controlling systems. Through a globalsystem control computer, the electrons, generated by electron gun (e-gun) with aSchottky field-emission filament and acceleration voltage system (200V to 30kV),are passed through the apertures with size of 30µm to 7.5µm to control the degreeof beam convergence. A smaller aperture size will result in a higher resolution,at the expense of lower beam current, and so longer exposure time. Then, withthe help of a group of electric and magnetic coils, the beam alignment, stigmationand (de)focusing are optimized. Next, the deflecting system controls the electronbeam to scan on the sample according to the user-defined patterns. The blanker isused to switch on/off the electron beam to avoid the exposure of undesired areas.The writing field (ranging from 50 to 500µm) is limited for each time exposure,which can be adjusted according to the user. Generally, the smaller writing areawill result in a more precise exposure due to lower deflecting angle of the electronbeam. After each exposure, the stage will move to the next writing field through thehigh-precision stage movement controller. However, for large scale photonic devicesexposure, each movement of writing area may cause stitching errors (the patterns

3.1. FABRICATION METHODS 21

Figure 3.1: Sketch of the electron beam lithography system.

at the boundaries of the writing filed are discontinuous), which is not allowed forlong-range optical waveguide components. To fix this problem, one can use: (1)multi-exposure of the boundaries by applying extra overlap patterns; (2) free-beammovement stage (FBMS) method: fixing the e-beam at one deflecting angle, andcontinuously moving the stage to draw the defined waveguide path. For the for-mer one, the multi-exposure will cause the deformations at the boundaries of eachwriting filed. For the latter one, due to the continuously moving stage, it is diffi-cult to write complicated patterns other than lines or circles. The most commonlyused method is by applying FBMS method to draw the patterns of long-range buswaveguides (across several writing fields), and the normal exposure method is usedto draw other tiny or complicated structures (within one writing filed).E-beam resists spin-coating and developing are also key steps for EBL processes.The e-beam resists are divided into positive and negative ones. For the positiveelectron resist (ZEP 520 and ZEP 7000 are most frequently used in this thesis),the exposed area will be removed after developing process; while for the negativeone (Ma-N 2403 is used in this thesis), the resist of the exposed areas remains, butother areas without exposure will be removed, as shown in the subfigure in Fig.


3.1. For the fabrication of HP waveguide devices, negative resist is normally usedfor patterning the waveguide structures, while the positive resists are used for otherfabrications, like grating couplings or plasmonic layers.

Inductive coupled plasma-reactive ion etching

Figure 3.2: Sketch of ICP-RIE dry etching system.

After lithography, the dry etching processes are commonly used to remove the un-protected areas, and hence to construct the on-chip components. The physical dryetching processes are based on highly anisotropic ion sputtering to bombard thesample surface, resulting in the etching process that is not selective (for all ma-terials). On the other hand the chemical etching processes use the reactive ionsto chemically remove the specific materials, and normally they are isotropic in alldirections (low directionality), but material selective. To obtain both good direc-tionality and selectivity in fabrication, a combination between physical and chemicaldry etching processes, named as the inductive coupled plasma-reactive ion etching(ICP-RIE), can be applied, which is realized by Advanced Oxide Etching (AOE)tool from Surface Technology Systems (STS) in this thesis, and the sketch is shownin Fig. 3.2. The inductively coupled coils are used to generate the required etchingplasma, and then, the plasma is accelerated by the RF (radio frequency)-biased


platen for etching purposes. Normally, the etching processes require low tempera-ture (10C is set in this tool) to avoid burning of optical or electron beam resist(due to ion bombardment), which is controlled by the helium (He) flow. Othersupplementary components are used to control the pressure, transferring processes,wafer positioning, etc. Fig. 3.3(a) shows an ICP-RIE etched Si structure (recipefor Si etching), good directionality and material selectivity result in the sharp edgesand smooth substrate.

Hydrogen fluoride wet etching

Wet etching is a generally used process to selectively remove the undesired ma-terials, like using hydrogen fluoride (HF) to etch oxide materials (SiO2, Al2O3,etc). They are now being replaced by dry etching methods due to their isotropicetching properties and unstable etching rate. However, for special applications, likemicro-electro-mechanical systems (MEMS) or other hollow type structures, wetetching has to be applied to release the structure from the substrate. Fig. 3.3(b)shows a Si mark released from SiO2 substrate by HF wet etching, and parts of theSi mark are suspended (the detailed processes can be seen in the fabrication flowsin subfigures).

Figure 3.3: Examples of dry and wet etching on silicon-on-insulator structure. (a)ICP-RIE dry etching (recipe for Si etching) and (b) HF wet etching of a silicon-on-insulator mark.


Plasma enhanced chemical vapor deposition

Due to the multi-layer structure of HP waveguide devices, material grown methodsare sometimes required to construct the high-index, low-index and plasmonic layers.Plasma enhanced chemical vapor deposition (PECVD) is the most frequently usedtool, which can deposit high quality materials for different purposes, like amor-phous Si (a-Si:H), SiO2 and Si3N4. Especially for the HP devices incompatiblewith commercially available SOI wafers, like HP waveguide with special height ofSi core or with metal layer as substrate, the material depositions are applied beforethe lithography and etching multi-round processes.

Figure 3.4: Sketch of PECVD system.

The sketch of the PECVD from STS is shown in Fig. 3.4. The precursor gasmixtures are injected through a showerhead with well-controlled temperature anduniform gas flow. Then, the plasma is created by applying RF power with eitherlow frequency (380 kHz) or high frequency (13.56 MHz). The precursor gas mixtureis hence dissociated into ions and free radicals by interacting with plasma’s highlyenergetic electrons. Next, reactive species diffuse to the sample surface, whereabsorption and surface reactions take place and form the new material throughchemical reactions. The final properties of the deposited material depend on both,surface reactions and ion bombardment during the material formation, that can re-lease less mobile bi-products increasing purity and homogeneity of the new-formedlayer. Fig. 3.5(a) shows the cross-section view of a PECVD grown a-Si:H waveguideon SiO2 buffer layer, where one can see that the quality of deposited materials areslightly worse than the one from commercially available SOI wafer with crystallineSi (c-Si) and thermal oxide grown SiO2 in Fig. 3.5(b) (both of those two figures


Figure 3.5: Cross-section views of (a) PECVD grown amorphous Si and SiO2 and(b) crystalline Si and thermal oxide (SiO2 from commercial used SOI wafer.

are taken from HP waveguides, and the other material layers are not taken intoconsideration here), but enough for waveguiding purposes.

Electron beam evaporation

Electron beam evaporation (EBE) is another method used to physically grow met-als or dielectric materials. In Fig. 3.6, the sketch of the EBE system (Provac PAK600 in Electrum lab at KTH) is shown. The crucibles are filled with the sourcematerials (six crucibles containing different materials can be applied in this tool),and one of them is opened to the e-beam deflected by magnet system from thefilament. The solid phase source material is heated by the e-beam, and then trans-ferred into gaseous phase. Next the vapors of source material diffuse to the samplesurface, and then precipitate into solid phase again due to the low temperature ofthe sample. The samples are placed on a rotatory wafer holder and can be uni-formly coated with the evaporated material when rotating (10-20rpm) during theprocess. The growth rate is measured by a quartz crystal on the top of the chamberand calculated according to the user-defined parameters of material sources. Dur-ing the processes, the pressure in the chamber should be controlled to be as low aspossible (5× 10−7 mbar is usually applied), to reduce the impurities. Additionally,the growth rate should be also well-controlled (∼ 1Å/s) to improve the quality ofthe evaporated material.The growth direction of EBE is perpendicular to the surface of the sample, andtherefore the lift-off process can be applied in post processes to open the areascovered with photon or electron sensitive resist. As shown in the subfigure (Fig.3.6), the prepared samples with resist patterns are grown with materials by EBE.


Figure 3.6: Sketch of EBE system and the lift-off process.

Then, by immersing the sample into remover, the area covered with the resist willbe removed when rinse out the resist underneath. This method is widely used inthe fabrication of plasmonic devices or metal electrodes, and can also be applied inplanarization processes with Al2O3 [Paper D].

Microwave Plasma Asher

Microwave plasma asher (TePla Model 300 Plasma System in Electrum lab atKTH) is most often used for cleaning process of the wafer with resist. The oxygen(O2) plasma is generated by the microwave generator, which causes degradationof the resist or other organic materials on the sample with formation of water andCO2 as rest products. In order to avoid the damage of the sample, plasma without


physical bombardment is used.

Critical point dryer

Critical point dryer (Baltec Critical Point Dryer in Electrum lab at KTH) is usedfor drying the sample with mechanically sensitive structures, like hollow [PaperE][Paper G] or MEMS [Paper H] devices. After wet chemistry releasing pro-cesses, the samples are firstly immersed in acetone or isopropanol liquid, and thenput into the tool using a special holder. Next the liquid is washed away by liquidcarbon dioxide (CO2). After that, the liquid CO2 is heated until its temperaturegoes beyond the critical point to gaseous phase, at which the pressure is releasedslowly to allow the CO2 gas to escape from the tool.

Thermal oxidation of silicon

Thermo furnace (Electrum lab at KTH) is a tool used for thermal oxidation ofsilicon to precisely grow high quality SiO2 films. The high temperature furnaceuses O2 (dry oxidation) or water vapor (wet oxidation) to generate SiO2 films fromSi following the reactions:

Si+O2 → SiO2Si+2H2O → SiO2+2H2

The rate of wet oxidation is faster than that of dry oxidation and is normally usedfor thicker SiO2 layers, like the SiO2 buffer layer. For HP waveguides that requireoxidation process to grow very thin low-index material layers [22] (20∼100nm),usually dry oxidation method is used due to the easily controlled growth rate.

3.1.2 Fabrication flow of hybrid plasmonic structuresBy using various tools in the KTH cleanroom, many different types of HP waveguidedevices can be fabricated. Since most of them are based on the SOI platform, wewill start with the fabrication flows of SOI waveguides employing grating couplersfor the coupling between single-mode fiber and on-chip photonic devices. Commer-cially available SOI wafers (with 220-300nm top Si film) are most frequently usedfor fabrication of these structures. Sometimes, when the proposed waveguide struc-ture has some special geometry or demands other substrates than Si, the PECVDdepositions of SiO2 buffer layer and top a-Si:H film are firstly processed. Then,the first EBL is used to pattern the structures of the demanded waveguide deviceswith negative resist. After developing, dry etching is applied to remove undesiredmaterials not protected by resist. After cleaning process (either using remover ormicrowave plasma asher), the sample is re-coated with positive resist. Then, thesecond EBL is used to draw the patterns of grating couplers followed by dry etchingand cleaning processes. The SOI photonic devices are fabricated, as shown in thefabrication flow with black arrows in Fig. 3.7.


Single/Double-slot hybrid plasmonic waveguides

Based on the SOI waveguide fabrication flow, the third EBL is proceeded to openthe pattern for plasmonic layers (Ag or Au) from the positive resist, and thenvery thin Germanium (Ge) or Titanium (Ti) (5-20nm), as the adhesive layer, andthe plasmonic materials (Ag or Au) films are grown by EBE. After the lift-off andcleaning processes, the single/double-slot HP waveguides and devices are fabricated[Paper A] [Paper B] [Paper F], as shown in the fabrication flow with yellowarrows. The over-layer alignment of the third EBL is the key process during thefabrication, which requires very high accuracy (<50nm) due to the narrow slotsbetween plasmonic layers and Si core. Such alignment can be accomplished sepa-rately for each device, to reduce the alignment errors induced by stage movement(each pattern is located within one writing field). Additionally, the metal evapo-ration process should be carefully controlled regarding growth rate (should be low,0.5-1Å/s), and pressure in the chamber (should be low, <5×10−7bar), to obtainsmall grain size and sharp sidewalls of the constructed metal patterns.

Vertical hybrid plasmonic waveguides with planar metal layer

Differently to the SOI waveguide fabrication flow, additional Al2O3 material evap-oration and lift-off processes should be added after the waveguide etching (thefabrication flow with green arrows), to make sure, that the surface is flat enoughfor the post processing. After the fabrication of the grating coupler, a thin layer ofoxide material (Al2O3 or SiO2) should be grown as the low-index layer. Then, theplasmonic patterns and/or the plasmonic thermal-photon absorber [Paper D] aremade by the third EBL, and followed by the EBE material evaporation and lift-offprocesses. In comparison to the single/double-slot HP waveguides, the over-layeralignment tolerance is not so critical, which makes it possible to proceed with ex-posure of all the patterns in one step.

MEMS tunable or hollow hybrid plasmonic waveguides

For hollow [Paper E] [Paper G] or MEMS tunable [Paper H] HP waveguides,an additional Si3N4 layer should be deposited by PECVD between the plasmonicand oxide layers, to keep the soft plasmonic layer strained. After the fabrications ofthe planar metal layer with holes, the HF wet etching is applied to release the metallayer from the oxide material (Al2O3) (the fabrication flow with orange color). Theetching rate of the evaporated Al2O3 in 5% HF can be as high as 1 µm/min, sothis process can be finished within very short time. After that, the critical pointdryer is applied to dry the fabricated sample.


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3.2 Characterization methods

Figure 3.8: Characterization setup. (a) Sketch of the coupling between single-modeoptical fiber and on-chip grating couplers. (b) Static (black arrows) and dynamic(red arrows) measurement setups.

In this thesis, the transmission responses of the fabricated components are measuredby the grating coupler setup. The grating couplers, located at each end of thewaveguide component, are used to couple the optical signal from a single-modefiber to the on-chip device. Differently from the butt-coupling method, the gratingcouplers allow for the wafer-scale testing since they can couple light in and outfrom the surface of the chip. Moreover, the alignment is easy, since it does not needpolished facets and precise fiber location. A schematic of the coupling betweensingle-mode optical fibers and grating couplers is shown in Fig. 3.8(a), wherethe fibers are titled with an angle θ (to avoid the reflection from the facet) andare located above the periodic arrays of Si teeth. The phase-matching conditionbetween the output Si waveguide and the input single-mode optical fiber can be

3.2. CHARACTERIZATION METHODS 31

expressed as:koutsin(θ) = kin + q · 2π

Λ , (3.2)

where kin and kout are wave vectors of the modes propagated in Si waveguide andsingle-mode fiber, respectively. q is an integer of the diffraction order of the grating,which can be set to -1 in most practical designs, and Λ is the grating period.The characterization setups are shown in Fig. 3.8(b), where the input optical fiberis connected to a continuous wave (CW) laser source (1460-1580nm in this the-sis), together with a polarization controller to select the desired polarization mode.At the output end, the output fiber is connected to the optical-spectrum-analyzer(OSA) to show the output signals (static measurement setup). As for the dynamicmeasurement (the flow with red arrows), the output fiber is connected to a pho-todetector instead of the OSA. An oscilloscope is used to show the signal measuredby the photodetector, at the same time, it also shows the input electric signals fromthe generator used for optical switching or modulation. By measuring the responsetime, the optical switching or modulation speed can be determined. The setupcan also be applied in all-optical switching system measurement, where the opticalsignal source (CW laser with 1064nm wavelength in this figure), modulated by theelectric signals from the generator, is connected to the on-chip device, as shown inthe flow with dashed red arrows.

Chapter 4

Lateral Hybrid PlasmonicStructures

As described in the previous chapters, the polarization mode with electric field(E-field) perpendicular to the metal surface can be supported by hybrid plasmonicwaveguide with subwavelength size, which is referred to as the hybrid mode betweensurface plasmon polariton- (SPPs) and photonic ones. For photonic integrated cir-cuits (ICs), the lateral structures with all components placed side by side in thesame plane are mostly investigated due to lower fabrication complexity in compar-ison to the vertical ones and compatibility to further integration with electroniccircuits, or other planar devices, like optical transmitters and detectors. Based oncommercially available silicon-on-insulator (SOI) wafers, for photonic ICs applica-tions, the crystalline Si (c-Si) top layer is normally thinner than 300nm (220nmis frequently used), which allows to avoid higher order modes of the SOI waveg-uide in vertical directions. Due to the limitation in vertical directions, the designedwaveguide is mostly used for transvers-electric (TE) or quasi-TE polarization mode,which has lower propagation loss and better optical confinement in comparison tothe one designed for transvers-magnetic (TM) polarization mode. In this respect,lateral hybrid plasmonic (HP) structures, supporting quasi-TE polarization mode,are more suitable to be integrated with conventional SOI-based photonic ICs. Inthis chapter, we will introduce the lateral HP waveguides, including single-slot HPwaveguide and double-slot HP waveguide, and their applications in optical bio-sensing and electro-optic polymer-based modulators.

4.1 Single-slot Hybrid Plasmonic Structures

As we described in section 2.1, the HP waveguides with the capacity of subwave-length light confinement are multi-layer structures with plasmonic layer(s), low-index layer(s) and high-index layer. For lateral single-slot HP waveguides, the con-figuration is similar to the commonly investigated metal-cap HP waveguide [16],

33

34 CHAPTER 4. LATERAL HYBRID PLASMONIC STRUCTURES

but the different materials (metal, low-index and high-index ones) are placed nextto each other, as shown in Fig. 4.1(a).

4.1.1 Single-slot hybrid plasmonic waveguideA single-slot hybrid plasmonic waveguide, as illustrated in Fig. 4.1(a), is con-structed by placing the plasmonic material (Au is used here) close to a Si ridge, sothat a narrow air slot (referred as the low-index material) is formed. As it is seenfrom the E-field distribution in Fig. 4.1(b), the quasi-TE mode (Ex and Hy arethe dominant fields) can be supported, where the evanescent field is enhanced bythe plasmonic material, and therefore a high optical confinement in the narrow airslot can be realized. By adjusting the structure geometry, waveguide properties,including effective refractive index (neff), propagation length and optical confine-ment factors in the slot (Γslot) and Si (ΓSi) materials can be varied, as shown inFig. 4.1(c) and (d).

Figure 4.1: Single-slot hybrid plasmonci waveguide: (a) cross-section view of thesingle-slot hybrid plasmonic waveguide, (b) E-field distribution, (c) neff and propa-gation length versus wslot, and (d) optical confinement factors in Si and gap areas.

With wider Si ridge, the optical confinement in the high-index Si material is increas-ing, and so also the neff of the supported hybrid mode. In this case, the supportedmode tends to be a photonic-like, and the propagation length is increasing due to

4.1. SINGLE-SLOT HYBRID PLASMONIC STRUCTURES 35

lower plasmonic loss. Similarly for the influence of the width of the slot (wslot):with increasing width of the slot, the neff is smaller due to the lower confinementinduced by the plasmonic layer, which also contributes to a longer propagationlength (lower loss). Interestingly, the Γslot is first increasing with wslot to a max-imum value, and then decreasing. The optimized Γslot occurs when there is both,enough width and plasmonic enhancement between the Au and Si layers, and thendecreases due to lower plasmonic influence. It can be seen that when wSi keepsincreasing, the Γslot will tend to the value dominated by the evanescent field of apure Si waveguide with no influences of surface plasmonics.

4.1.2 Single-slot hybrid plasmonic bendThe plasmonic optical enhancement also plays a role in the reduction of radiationloss of waveguide bends, which has been investigated in HP ring/disk resonators[11–13]. The optical properties of 90 sharp bends toward left (metal layer) andright (silicon core) of a slot HP waveguide with 1µm radius are studied, and com-pared with the Si waveguide bend, as shown in Fig. 4.2. From the power distri-

Figure 4.2: Bend properties of single-slot hybrid plasmonic waveguide. (a) and(b) Slot hybrid plasmonic bends toward left (metal layer) and right (Si core). (c)Si waveguide bend (toward left). (d) The neff and 90 bend loss of a slot hybridplasmonic waveguide.


butions shown in Fig. 4.2(a) and (b), the single-slot HP waveguide bent towardsleft (in the direction of Au material) has lower optical confinement in the slot incomparison to the one bent right (in the direction of air). This can be explainedusing the conformal mapping theory [47]: the optical power tends to be radiatedout towards the opposite direction than that of the bend, and therefore the opticalpower is more like to be confined in the slot when the waveguide bends is in thedirection of air (right in this figure). As shown in Fig. 4.2(d), the single-slot HPbend towards right has a larger value of neff compared with the one towards left dueto larger optical confinement, and both of them are larger than the one of a pureSi bend, without the plasmonic enhancement effect. For the bending loss, both,the propagation and radiation losses have to be taken into consideration: the totalloss of the single-slot HP bend towards right is larger than others due to betteroptical confinement inducing larger plasmonic loss. However, in contrast to thepropagation loss of a straight waveguide described in Fig. 4.1(c), with narrow slotwidth, the 90 bend loss, especially for the one towards left, is lower than that ofthe Si waveguide bend. This indicates that high optical confinement of slot HPwaveguides can reduce radiation loss of sharp bends. The minimum value can beobtained with a slot HP bend towards left with a slot width around 15nm, wherethe 90 bend loss is lower than 0.1dB, about 2.5 times less than for the pure Sibend (∼0.25dB).

Figure 4.3: 90 bend loss of a slot hybrid plasmonic waveguides with different radii.

4.1. SINGLE-SLOT HYBRID PLASMONIC STRUCTURES 37

Further studies of the bend loss with various radii of the single-slot HP waveguidetowards left (to the direction of plasmonic materials) are shown in Fig. 4.3. For asharp bend (the bend radii lower than 1.5µm), the single-slot HP waveguide bendlosses are lower than those of pure Si bends. Moreover, with narrower slots wehave lower bend losses, since the radiation loss is the dominant loss rather than thepropagation loss, where the plasmonic layers play a role of “attracting” the opticalpower from radiation, and therefore result in a low loss when the plasmonic layer isclose enough to the Si ridge. However, with larger bend radii (larger than 1.5µm),the plasmonic loss dominates radiation loss.Together with the analysis of waveguide and bend properties, single-slot HP waveg-uides show the advantages of high optical confinement factor at the narrow slots(∼50%) and low bending loss (∼0.1dB of 90 bend with 1µm radius). Further-more, the single-slot HP waveguide curved towards the direction of the plasmonicmaterial is also accessible by a Si bus waveguide for light coupling. By taking intoaccount those benefits, single-slot HP ring liquid refractive index sensors with highsensitivity, compact size and large Q factors are promising.

4.1.3 Single-slot hybrid plasmonic ring sensorThe single-slot HP liquid refractive index ring sensor is experimentally investigated[Paper F], as shown in Fig. 4.4(a). The inner radii of the Si ring and Au disk are2.6µm and 2.45µm, respectively and therefore the slot width is around 150nm withdeviation of ±50nm due to overlayer alignment error. The characterization resultsare shown in Fig. 4.4(b) and (c). The Q factor is around 1300 for the resonancewavelength around 1555nm, with a free-spectral-range (FSR) of 29nm. The testedliquids are different percentages (20%-100%) of 2-isopropanol (IPA), with refractiveindices ranging from 1.334 to 1.3738 at 1550nm wavelength, as shown in Table 4.1.

Table 4.1: Refractive indices of aqueous solution of 2-isopropanol.

C 100% 80% 60% 40% 20% 0

n 1.3738 1.3694 1.3604 1.3472 1.334 1.318

By infiltrating with different test liquids, the resonant wavelength is changed ac-cording to:

∆λres = 2πr · SWG ·∆nm

, (4.1)

where λres is the resonant wavelength, SWG is the waveguide sensitivity corre-sponding to the ratio between effective refractive index change (∆neff) and refrac-tive index change of tested liquids (∆n), and m is the azimuthal number. With apositive refractive index change (toward higher percentages of IPA), the resonant


peak is shifted to longer wavelengths. The linear relationships of Eq. 4.1 are shownin Fig. 4.4(d), where the single-slot HP ring sensors with different Au disk radii(2.45µm and 2.35µm) are shown and compared to the one of a pure Si ring. Onecan see that the single-slot HP ring has better sensitivity compared to a pure Siring as expected. With larger Au disk radius, the sensitivity is higher due to thelarger optical confinement factor of the slot, as investigated in Fig. 4.1(d).

Figure 4.4: Characterization results of a slot hybrid plasmonic ring sensor. (a)SEM picture of the fabricated device. (b) Transmission responses with 100% and60% IPA. (c) Resonant peak shifts by infiltrating with 20% to 100% IPA, and (d)their linear fitting curves in comparison to Si ring sensor.

In summary, the single-slot HP ring resonator, constructed by locating a plasmonicdisk inside the conventional Si ring, has good performances in optical bio-sensingapplications. The smaller the distance between the plasmonic disk and the Siridge, the better the sensitivity which can be achieved. Furthermore, the radius ofa single-slot HP ring sensor can be decreased (<1µm is achievable) according tothe bending loss analysis, where the Q factor will not decrease as much as for thepure Si ones. In this way, single-slot HP ring sensors with small footprint and largesensitivity is achievable.

4.2. DOUBLE-SLOT HYBRID PLASMONIC STRUCTURES 39

4.2 Double-slot Hybrid Plasmonic Structures

4.2.1 Double-slot hybrid plasmonic waveguideDifferently from the single-slot one, the Double-slot hybrid plasmonic (DSHP)waveguide has plasmonic material located at both sides of the Si ridge, as shownin Fig. 4.5 (a) and (b) (Ag is used as plasmonic material). The mode profile of aDSHP waveguide infiltrated with IPA is shown in Fig, 4.5 (c), where a large opticalconfinement at both slot areas is observed. Detailed optical properties with varyinggeometrical parameters are illustrated in Fig. 4.6.

Figure 4.5: Double-slot hybrid plasmonic waveguide. (a) Schematic of the double-slot hybrid plasmonic (DSHP) waveguide connected to SOI waveguides. (b) Cross-section view of the DSHP waveguide covered by tested liquid. (c) Mode pro-file of the DSHP waveguide. The geometries are: wslot=150nm, wSi=165nm andhWG=250nm. The tested liquid is 100% 2-isopropanol (IPA).

The widths of Si ridge (wSi) are set as parameters from 100nm to 400nm, and theoptical properties are simulated as a function of slot width (wslot) from 20nm to200nm. The neff versus wslot is shown in Fig. 4.6 (a). One can see that neff de-creases with increasing of wslot, and gradually tends to the value of SOI waveguides(when wslot is larger than 500nm, not shown in this figure) due to the disappearanceof plasmonic influence. The elimination of plasmonic influence also contributes to


the decrease of propagation loss, as shown in Fig. 4.6(b).

Figure 4.6: Optical properties, including (a) effective refractive index (neff), (b)loss; optical confinement factors of (c) covering liquid (ΓIPA) and (d) Si ridge (ΓSi),of a DSHP waveguide with various wSi (100-400nm) versus wslot (20-200nm).

Moreover, one can observe that the larger wSi provides higher values of neff andlower propagation loss, due to the increasing contribution of a photonic-like modeconfined in the high-index lossless Si material. The changes of optical confinementfactors in covering material (IPA) (ΓIPA) and Si material (ΓSi) are more compli-cated than the variations of neff and propagation loss, as shown in Fig. 4.6(c) and(d). The maximum ΓIPA occurs when wSi=100nm and wslot∼30nm, and can reachas high value as 88%. The optical confinement factor is not only influenced by theplasmonic materials, but also by the high-index Si material, which plays a role indecreasing the power leaking into the substrate. In other words, the optical con-finement factor is dependent on some more complicated hybrid influences betweenphotonics and plasmonics. In order to exhaustively study the optical propertiesof the DSHP waveguide and find the optimized value for sensing applications, a qparameter, representing the ratio between the width of Si ridge and total width of


DSHP waveguide (q = wSi/w), is defined. Two special situations, q=0 or 1, repre-sent a typical plasmonic slot waveguide with slot material as IPA or Si, respectively.The mode profiles of DSHP waveguide with q parameters of 0, 0.5 and 1 are shownin Fig. 4.7(a), (b) and (c), respectively, which show the changes of optical powerconfined in the plasmonic slot waveguides and DSHP waveguide.

Figure 4.7: Optimizations of a double-slot hybrid plasmonic waveguide. (a), (b)and (c) mode profiles of the DSHP waveguide with q parameters (q = wSi/w) of0, 0.5 and 1, respectively. (d) Effective refractive index (neff) (black curves) andpropagation loss (red curves) of the DSHP waveguide with various widths versusq parameters. (e) Optical confinement factor of covering liquid (ΓIPA), nano-slots(Γslot) and Si ridge (ΓSi) with different geometries.

The values of neff increase with q parameter due to the increasing influence ofhigh-index Si material, as shown in the black curves in Fig. 4.7(d). However, thebehavior of propagation loss is more complicated, as shown in the red curves inFig. 4.7(d). There exist minimum values for each DSHP waveguide with specific qparameters. In other words, for DSHP waveguide (0<q<1), the propagation loss islower than that of pure plasmonic slot one, which indicates that the hybrid modehas lower loss as we described in former chapters. The optimized values of theoptical confinement factor in covering material (ΓIPA) and only inside the slot arealso shown in the blue and orange curves in Fig. 4.7(e), which can be as high as88% for ΓIPA and 80% for Γslot. This indicates that DSHP waveguide also has ahigher optical sensitivity to the tested materials (optical sensing applications) or


electro-optic polymers (optical modulation applications) than the pure plasmonicones, due to the better optical confinement provided by high-index Si material.Summarizing the analysis above, the DSHP waveguide has better performance inboth propagation loss and sensitivity to the materials covering or trapped into theslot than pure plasmonic slot ones; this can be applied in optical sensing and modu-lation applications. The optimized q parameters for propagation loss and sensitivityare different; one needs to carefully design the structures to satisfy the requirementsof desired applications.

4.2.2 Double-slot hybrid plasmonic MZI sensor

Based on the analysis of the DSHP waveguide, a Mach-Zehnder interferometer(MZI)-based liquid refractive index sensor has been developed [Paper A]. Theschematic is shown in Fig. 4.8, where an asymmetrical MZI with one arm as theactive DSHP sensing area is designed. Due to imbalance of propagation lossesof the reference and sensing arms, asymmetric Y-splitters, which can direct moreoptical power to the sensing arm, are applied to balance the interferometer andimprove the extinction ratio. The length of the reference arm, Lref, is 67µm witha width (wref) of 400nm. For the sensing arm, the DSHP waveguide, with lengthranging from 20 to 40µm (LHP=20-40µm), is placed between the input and out-put SOI waveguides, whose geometrical parameters are chosen as: wSi=165nm andwslot=150nm (w=465nm and q=0.355) according to the theoretical analysis in thelast section. Grating couplers [48] are situated at each end of the devices for thecoupling between optical fiber and on-chip devices.

Figure 4.8: Schematic of the asymmetrical Mach-Zehnder interferometer-based sen-sor employing double-slot hybrid plasmonic waveguide as the sensing area.


Figure 4.9: Scanning electron microscope (SEM) figures of the fabricatedMach-Zehnder interferometer-based sensor employing double-slot hybrid plasmonic(DSHP) waveguide as the sensing area. The widths of the Si ridge and the slotsare 165nm and 150nm, respectively.

The scanning electron microscope (SEM) top-view of the fabricated DSHP MZI sen-sor is shown in Fig. 4.9. The width of the lower Si reference arm is around 400nm,and the upper narrower one is the Si ridge of DSHP waveguide (wSi=165nm). TwoAg pads with 1µm width are located aside the Si ridge, building the narrow slots(wslot∼150nm) between the Si ridge and the Ag pads. MZI sensors with 20µm,30µm and 40µm DSHP waveguide lengths were fabricated, and the characteriza-tion results when infiltrated with different concentrations of IPA solutions are shownin Fig. 4.10.

Fig. 4.10(a)-(c) show the transmission responses of the MZI sensors with 20µm,30µm and 40µm-long DSHP waveguides, respectively infiltrated with 100% (blackcurves) and 60% (red curves) IPA. One can see that the resonant wavelength shiftsare larger with longer DSHP waveguides, which indicates that the DSHP waveguideas the sensing element has larger sensitivity than the reference arm (SOI waveg-uide). The wavelength shift is according to:

∆λres =∆neff,ref · (2Lin − 2πrref − Lref ) + 2

∫Ltaper

∆neff,taper(l)dlm

+2

∫Lridge

∆neff,ridge(l)dl + ∆neff,DSHP · LDSHPm

,

(4.2)

where ∆neff,ref,∆neff,taper, ∆neff,ridge and ∆neff,DSHP refer to the effective refractiveindex changes of Si reference arm, taper, ridge and DSHP waveguide, respectively.Other geometrical parameters can be found in Fig. 4.8.


Figure 4.10: Transmission responses of the Mach-Zehnder interferometer-based sen-sor employing (a) 20µm, (b) 30µm and (c) 40µm double-slot hybrid plasmonicwaveguide infiltrated with 100% and 60% of IPA. (d) Fitting lines of wavelengthshifts versus refractive index of tested liquids.

As shown in Eq. 4.2, due to the asymmetrical structure, the wavelength shift,which depends on total effective length difference between the arms, is influencedby multi-elements with either positive (the ones in sensing arm) or negative (theones in reference arm) contributions. The larger wavelength shift with longer sens-ing waveguide indicates that the DSHP waveguide indeed has much better sensi-tivity than the one of SOI waveguide in the reference arm. From the wavelengthshifts with various percentages of IPA solution (100% to 10%) illustrated in Fig.4.10(d), the corresponding sensitivities obtained from fitting lines are 89nm/RIU,222nm/RIU and 474nm/RIU (recalculated by using more accurate refractive in-dices of IPA solutions given in Table 4.1). The sensitivities do not have a strictlylinear relationship with LHP , because the prolongation of the DSHP waveguidealso shortens the less sensitive Si waveguide in the sensing arm, and therefore thesensitivity of the device increases faster than linearly.


4.2.3 Double-slot hybrid plasmonic ring sensor

In comparison to MZI, the resonant peaks of ring resonators have much narrowerlinewidths (large Q factor), which indicates that the wavelength shifts are much eas-ier to be detected due to higher extinction ratio, and therefore it can increase theperformances in sensing (low detection limit) and modulation (low power consump-tion) applications. Based on this, the DSHP ring resonator [Paper B] has beenstudied, which consists of a Si ring located between Ag circular sheets, as shownin Fig. 4.11(a) and (b). With the curved structure shown, the power distributionof the hybrid mode, investigated by an axisymmetric finite element method (FEM)as we described in chapter 3, tends to move to the outer slot of DSHP waveguide,as illustrated in Fig. 4.11(c). The Q factors versus the radii with 150nm, 250nmand 350nm wslot are shown in Fig. 4.12(a), which increase with larger radii, andthen tend to a constant value. The variation of Q factors is influenced by boththe quality factors of absorption (Qabs factor) and radiation (Qrad factor), as wediscussed in chapter 3.

Figure 4.11: Double-slot hybrid plasmonic ring sensor. (a) Schematic of the double-slot hybrid plasmonic ring resonator. (b) Cross-section view, and (c) mode profileof the propagated wave inside the ring resonator.

For the DSHP ring with small radii, the Q factor of DSHP ring is mainly influencedby the radiation loss and can be increased by increasing the radius; while for thelarge radii (larger than 5µm), due to almost total elimination of radiation loss, theabsorption loss of DSHP waveguide plays a significant role, which is a constantvalue, independent of the radius of the ring resonator, and therefore results in aconstant value of Q factor at large radii. In other words, for a DSHP ring withlarge radius, the Q factor can be considered to be approximately equal to the Qabsfactor (inversely proportional to absorption loss, as described in Eq. 2.19). Simi-larly, from the analysis of the DSHP waveguide, the behavior of Qabs as a functionof q (q = wSi/w) is shown in Fig. 4.12(b). For the DSHP rings (0<q<1) withdifferent widths, the Qabs factor is much larger than the one of pure plasmonic slotwaveguide (q=0 or 1), and optimal values of Qabs for different total slot widths


Figure 4.12: Optical properties of the double-slot hybrid plasmonic ring sensor: (a)quality factors of the double-slot hybrid plasmonic ring with 150nm, 250nm and350nm wslot versus various radii. Optimization investigations of (b) quality factorof absorption (Qabs), (c) effective refractive index (neff), and (d) sensitivity (S).

w exist for specific q parameters. Additionally, optimal values of the DSHP ringsensitivities also exist, as shown in Fig. 4.12(d), which are calculated by:

S = ∆λres∆n = λ

ng· ∆neff

∆n , (4.3)

where ∆λres is the resonant wavelength shift, and ∆n is the refractive index changeof the tested liquid. The sensitivity of the DSHP ring can reach as high valueas 700nm/RIU, when w=900nm and q=0.25, which can be further increased bydecreasing the total width (w), however, at the expense of a lower Qabs factor.This trade-off generally also exists in other types of HP waveguides, which havesimilar relationship between neff and loss.The theoretical investigations allow for design of high performance DSHP ringsensor with high Q factor and high sensitivity, however for practical applicationsan access Si waveguide directly coupled to the slot ring is necessary. To solve thisproblem, a DSHP ring with partly open area has been designed, as shown in the


SEM picture of a fabricated device in Fig. 4.13. The radius of the ring is here 6µm,and the width of the Si ring and bus waveguide is 350nm. The outer and innerslots have widths around 350nm and 250nm, respectively.

Figure 4.13: Scanning electron microscope (SEM) pictures of the fabricated DSHPring sensor.

The characterization results are shown in Fig. 4.14, as well as the result of a Siring sensor without plasmonic sheets (red curves). The extinction ratio is over25dB, and the insertion loss without grating coupling loss is about 4dB. The FSRis about 14.4nm around 1550nm. By infiltrating with 100% IPA (solid curve) and80% IPA (dashed curve), the resonant wavelengths are changed according to Eq.4.1 (ignoring the influences of the coupling area), which is 3.3nm as shown, andthe corresponding sensitivity is 687.5nm/RIU, about 5 times larger than the oneof pure Si ring. The experimentally obtained sensitivity is slightly higher thanthe simulated one (∼600nm/RIU when w=950nm, q=0.35), which is caused by:(1) the coupling area is also sensitive to the liquid refractive index change; and(2) the side-wall roughness during fabrication induces larger radiation that mayincrease the confinement factor at the slots, and therefore contributes to the largersensitivity. Based on the experimental results, the detection limit (DL),

DL = fλ

SQ, (4.4)


(f is a factor expressing the fraction of the resonance linewidth that can be mea-sured, which can be estimated as 1/400 according to [49]), describing the smallestrefractive index change that can be detected by the system is 2.57×10−5 RIU.Comparing to the theoretical Qabs factor for this structure (larger than 10,000),

Figure 4.14: Transmission responses of the fabricated DSHP ring sensor infiltratedwith 100% and 80% IPA (black curves), in comparison to the ones of a Si ringsensor (red curves).

the measured loaded Q factor is much lower (∼300). The major reason for that isthe coupling loss between different modes, due to the different waveguide configura-tions of the coupling area and the DSHP ring, propagated inside the ring resonator.To compensate the coupling loss, an optimized design of the DSHP ring sensor hasbeen made, as shown in Fig. 4.15 (a): the width of Si core in partly opened areais broadened to have an identical neff with the one of DSHP ring. The simulationis done using finite difference time domain (FDTD) method. The geometry of theDSHP ring is fixed, while the wSi increases from 350nm to 500nm. Tapers are usedto connect the Si ridge from the coupling area to the DSHP ring area. The trans-mission responses are shown in Fig. 4.15(b). As one can see, the loaded Q factor ismodified to be over 1000, when the wSi is around 500nm, which means that the neffof the coupling area and that of the DSHP ring are nearly identical, and thereforethe coupling loss decreases to the minimum value. After the loaded Q factor opti-mization, the DSHP ring resonator shows a good performance in both sensitivityand detection limit (DL=5.37 × 10−6RIU after loaded Q factor modification). A


Figure 4.15: Optimized design of the double-slot hybrid plasmonic ring sensor. (a)Schematic of the ring sensor structure with loaded Q factor modification. (b) FDTDsimulation results of transmission responses with different wSi, and the loaded Qfactors optimization.

comparison between different kinds of ring-type label-free sensors is listed in Table4.2, which shows that the DSHP ring sensor has the highest sensitivity and prettylow detection limit in comparison to other solutions.

Table 4.2: Comparison between different kinds of ring-type sensor.

Waveguide r(µm) Q factor S(nm/RIU) DL(RIU) Ref.

Si ≈ 5 20000 70 3.75× 10−6 [50]

Si slot > 5 500 298 4.2× 10−5 [49]

SiN slot 70 1800 212 2.3× 10−5 [51]

Single-slot HP 3 1300 102 1.75× 10−5 [Paper F]

DSHP 6 300 687.5 2.57× 10−5 [Paper B]

(1034)? (5.37× 10−6)(? Results after loaded Q factor optimization.)


4.2.4 Further discussions on electro-optic modulatorapplications

In previous sections, we have shown high sensitivity of DSHP waveguides to theinfiltrated liquids. From Fig. 4.7(e), the optical confinement factor in the coveringliquid can reach as high value as 88%, which includes both the power confined inthe slots and that extended into the covering material. However, for electro-optic(EO) modulators, only the overlap between the optical field and the RF field in theelectrooptic medium is of importance (the narrow slots of DSHP waveguide). Theconfinement factor in the slots is about 10% lower than in the tested medium (alsocovering the structure), which can reach 80%. Another benefit of using a DSHPwaveguide structure is its low resistance-capacitance (RC) time constant due to thelow resistance (good conductivity of plasmonic material), which makes that highspeed modulation is achievable.Based on the experimental results of the DSHP ring resonator, by employing a typ-ical EO polymer with properties as n=1.5 and r33=300pm/V[34], the modulationvoltage,

V = 2∆nwslotn3r33

= 2wslotFOMn3r33

, (4.5)

for a 3-dB band shift is around 24V, which can be decreased to 5V after loaded Qfactor modification.

Chapter 5

Vertical Hybrid PlasmonicStructures

Vertical hybrid plasmonic (HP) structures are composed of different material layers(plasmonic, low-index and high-index) arranged in the vertical direction and can befabricated by easily controllable material growth methods as we described in formerchapters. In comparison to the lateral ones, the vertical HP structures are easier tointegrate with other materials, as we listed in Table 2.2, and therefore to realize var-ious functional devices utilizing the unique properties of both HP waveguides andother functional materials. Additionally, HP waveguides, with larger fabricationtolerance, can be developed using vertical structures [19, 20], which do not requirethe high precision over-layer alignment of the lateral ones. In this chapter, severalvertical HP devices for applications in optical sensing, switching and modulationwill be introduced.

5.1 All-optical switching HP donut resonator

Using the large thermal expansion property of Si material (thermo-optic coefficient∼1.8 × 10−4K−1 at 300 K), low-speed modulators and switches can be realized inSi-based photonic integrated circuits (ICs). However, the large footprint of elec-trodes and large power consumption prompted the development of new solutions,among others all-optical switching devices using phototermal plasmonic absorbers[52, 53]. Additionally, as we described in previous chapters, the radiation loss ofHP waveguide ring-type resonators is smaller than that of pure Si ones due to highoptical confinement, which contributes to realization of optical resonators with verysmall footprint and acceptable quality factors (Q factors). By combining the plas-monic photothermal absorber and a HP ring resonator, an all-optical switching HPdonut resonator [Paper D] has been developed, as shown in the scanning electronmicroscopy (SEM) images in Fig. 5.1.

51

52 CHAPTER 5. VERTICAL HYBRID PLASMONIC STRUCTURES

Figure 5.1: Scanning electron microscopy (SEM) images of an HP donut resonatorwith photothermal plasmonic absorber. The upper right subfigure shows the cou-pling area from Si access waveguide to hybrid plasmonic (HP) waveguide. Thelower left subfigure shows the cross-section view of the HP waveguide, where theAu-Al2O3-Au layers compose the photothermal plasmonic absorber, while the Au-SiO2-Si is the HP waveguide.

The HP devices are composed of two parts (can be seen from the cross-section viewin the lower left subfigure): the Au/Ag-SiO2-Si form the HP waveguide device; whilethe Au-Al2O3-Au/Ag build the photothermal plasmonic absorber. It is necessaryto mention that the lower metal layer (Au/Ag) contains both Au and Ag metals:Ag, as a low loss plasmonic material, is used to form the HP waveguide; while theAu material is evaporated above to protect the fabricated device from oxidation,and also as a part of the photothermal plasmonic absorber. The resonant peak ofa donut type resonator (donut means that the width of the ring is larger than theaccess waveguide to decrease the sidewalls scattering losses [13]) with 1.8µm radiusof the Si core is located at about 1564nm wavelength, as shown in Fig. 5.2(a), withan extinction ratio of about 20dB. The loaded Q factor is around 600, which iscomparable to other types of HP disk/ring resonators [11, 13, 54].

The Au-Al2O3-Au/Ag build the photothermal absorber, which can absorb over 75%of optical power from the driving laser with 1064nm wavelength, as shown in Fig.5.2(b). The radius of the photothermal plasmonic absorber is 10µm to match thesize of a single-mode optical fiber. Using the characterization setup, we described inFig. 3.8, the absorbed optical power from a 1064nm wavelength laser can heat the

5.1. ALL-OPTICAL SWITCHING HP DONUT RESONATOR 53

Figure 5.2: Characterization results of the hybrid plasmonic ring resonator withphotothermal absorber. (a) Transmission response of the HP donut resonator. (b)Absorption performance of the photothermal plasmonic absorber. (c) Resonantwavelength shift of the HP donut resonator driven by different laser powers. (d)Linear relationship between wavelength shift and optical power, which gives theswitch efficiency around 32.5nm/W.

Si material underneath, and therefore change the resonant peak of the HP donutresonator due to the change of refractive index of the Si material, as shown in Fig.5.2(c). By applying 10mW driving laser power, the transmitted power can increaseby 15dB for optical switching. The switch efficiency is about 32.5nm/W, which isabout two times smaller than the one based on SOI ring [52] due to lower opticalconfinement factor in the Si material. However, the size of the HP donut resonatoris much smaller than conventional SOI disk/ring resonators. In addition to this,the combination between a photothermal plasmonic absorber and HP waveguidealso provides a way to realize other types of all-optical switching devices based onHP passive waveguide structures with compact size.

For dynamic measurements, the rise and fall times are 18µs and 14µs, respectively,


Figure 5.3: Dynamic measurement results of the all-optical switching HP donutresonator with (a) 60µs and (b) 40µs signal periods.

as shown in Fig. 5.3, and, hence the corresponding energy used for 15dB opticalextinction ratio is 160nJ in average. The response time is generally at the samelevel as in other types of SOI-based thermal-optic devices [55], but the photother-mal absorber device size (300µm2) is much smaller than the electrically driven onewith electrodes (>4000µm2).

5.2 Hollow hybrid plasmonic Mach-Zehnder sensor

For optical sensing applications, lateral double-slot hybrid plasmonic (DSHP) waveg-uides have a large waveguide sensitivity (∼1.2) as we have shown in chapter 4.However, the fabrication tolerance of the DSHP structure is much lower than theone of the vertical HP waveguide with planar metal cap. The hollow hybrid plas-monic waveguide [Paper E] is a compromise way for optical sensing applicationswith vertical HP structures. Fig. 5.4 (a) and (b) show the hollow HP waveguideused for sensing applications. The Au layer is released from the oxide material toconstruct the hollow gap between Au and Si core. As we described previously, alarge optical confinement is achieved in the low index layer, here the hollow gap, asshown in the power distribution of the fundamental mode in Fig. 5.4 (c). From thesimulation results of waveguide properties, as illustrated in Fig. 5.4 (d), (e) and (f),the waveguide sensitivity of such a hollow HP waveguide can reach as high value as0.8, which is about two times larger than the one of a pure Si waveguide without

5.2. HOLLOW HYBRID PLASMONIC MACH-ZEHNDER SENSOR 55

Figure 5.4: Hollow hybrid plasmonic waveguide. (a) 3D sketch of the hollow hybridplasmonic waveguide and (b) the cross-section at the Au-holes position. (c) Themode profile of the supported fundamental mode. Simulation results of (d) theeffective refractive index neff, (e) the propagation loss and (f) waveguide sensitivityS versus the width of Si ridge with different thickness of the air gap.

the plasmonic layer. With a thinner gap, the waveguide sensitivity is larger due tohigher optical confinement, however, at the expense of larger propagation loss.

In the experiment, a Mach-Zehnder interferometer (MZI) is used to evaluate thesensing performances of the hollow HP waveguide, as shown in the SEM figuresin Fig. 5.5. The length difference, ∆l, between the sensing- and reference arms is30µm. In the sensing arm, hollow HP waveguides with 20, 30 and 40µm lengthsare used; while for the reference arm, HP waveguides (the low-index gap material,Al2O3, is not removed) with identical length are employed, in order to compensatethe loss difference between the two arms, and therefore increase the extinction ratioof the transmission response. The thickness of the Si core, air gap and Au layer are220nm, 75nm and 140nm, respectively. A thin Si3N4 layer (∼15nm) is applied forsustaining the Au layer. The width of the Si core is around 240nm.The transmission responses of the fabricated MZI sensor with a 20µm hollow HPwaveguide, infiltrated with different concentrations of isopropanol (IPA) are shownin Fig. 5.6(a). The extinction ratio is larger than 40dB, with an insertion loss ofabout 18dB. The resonant peak shift with different infiltrating liquids is expressedas:

∆λ = SHPLHP + SSi∆lm

, (5.1)


Figure 5.5: Scanning electron microscopy (SEM) (a) top-view of the fabricatedhollow hybrid plasmonic (HP) Mach-Zehnder sensor, and (b) Cross-section view ofthe hollow HP waveguide. False colors are added to enhance contrast.

where m is the azimuthal number; SHP and SSi are waveguide sensitivities of thehollow HP waveguide and Si waveguide, respectively; LHP is the length of thehollow HP waveguide, and ∆l is the length difference. Fig. 5.6(b) shows the linearrelationship between wavelength shift (∆λ) and the refractive index change (∆n)of the covering liquids fitted with the experimental data for the MZI with 20µm,30µm and 40µm hollow HP waveguides. The sensitivity of the MZI sensor, ∆l/∆n,also has a linear relationship with the length of the hollow HP waveguide, whichis shown in Fig. 5.6(c). The experimentally obtained waveguide sensitivity ofthe hollow HP waveguide can be calculated, according to Eq. 5.1, as 0.68. Theintercept is the sensitivity of the MZI sensor with only Si waveguide, which givesa value of about 70nm/RIU; hence, the sensitivity of the Si waveguide is 0.168.The experimentally obtained data is for a Si waveguide covered with thin Al2O3(∼75nm) and Si3N4 (∼15nm) layers, which is lower than the simulation results ofSOI waveguide sensitivity in Fig. 5.4(f).

5.2. HOLLOW HYBRID PLASMONIC MACH-ZEHNDER SENSOR 57

Figure 5.6: Characterization results of the hollow hybrid plasmonic Mach-Zehndersensor. (a) Transmission responses of the Mach-Zehnder interferometer (MZI) sen-sor with a 20µm hollow hybrid plasmonic (HP) waveguide in the sensing arm,infiltrated with different concentrations of IPA. (b) The linear relationship betweenwavelength shift and refractive index changes of the MZI with 20, 30 and 40µmlong hollow HP waveguides. (c) The MZI sensitivity increases with the length ofthe hollow HP waveguide.

The detection limit (DL) is estimated as:

DL = ∆λDSD

, (5.2)

where ∆λD is the lowest detectable wavelength change, and SD is the sensitivityof the device. By assuming the lowest detectable intensity to be 1dB, the cor-responding phase change of the MZI sensor with 20µm hollow HP waveguide isabout 4 × 10−5(2π), and the lowest detectable wavelength change, ∆λD, is about4 × 10−4nm. Then, the DL is around 2.8 × 10−6 RIU in this case. In our experi-mental investigation, the transmitted power differences of the MZI with 20, 30 and40µm hollow HP waveguide are within 10dB, which indicates that the propagationloss of such waveguide is less than 0.25dB/µm.Further optimizations of the hollow HP waveguide with double-gap [Paper G] aremade to enhance the performance of both sensing and modulation applications, asshown in the proposed structure in Fig. 5.7 (a) and (b). From the mode profile ofsuch waveguide, as shown in Fig. 5.7(c), the interaction between the optical modeand tested liquid is further increased in comparison to the one with single-gap. Thewaveguide sensitivity can reach as high a value as 1.14 from the simulation results,at the expense of larger propagation losses, as illustrated in the simulation resultsin Fig. 5.7(d)-(f). In addition to the larger sensitivity in sensing applications,the hollow HP waveguide with double-gap is also promising in optical modulationemploying e. g. electro-optic (EO) polymers. Similarly to the double-slot HP


Figure 5.7: Proposed hollow hybrid plasmonic waveguide for optical modulation.(a) 3D sketch of the hollow hybrid plasmonic (HP) waveguide with double-gap. (b)Cross-section view at the Au-holes area, and (c) the mode profile of the supportedfundamental mode. Simulation results for: (d) the effective refractive index neff,(e) the propagation loss and (f) the waveguide sensitivity S versus the width of Siridge with different thickness of the air gap.

waveguide we introduced in the former chapter, the plasmonic layers can be usedas the electrodes, which have very low resistance, and hence can be used for realiza-tion of devices with low resistance-capacitance (RC) time constant for high-speedmodulation.

5.3 MEMS tunable hybrid plasmonic waveguides

The HP waveguides are very sensitive to the optical properties of low-index ma-terials, which are investigated in this thesis work for optical sensing applications.Moreover, the geometry of the low-index material layer also significantly influencesthe waveguide performance regarding both neff and loss. Microelectromechanicalsystems (MEMS) is a technology to realize geometrical changes in the micro-scale,and hence to mechanically change the waveguide properties for e. g. optical switch-ing applications. Fig. 5.8 shows the schematic of a MEMS tunable HP waveguide[Paper H], where the planar Au layer is applied as the mass element in MEMS,as well the plasmonic layer of the HP waveguide. Another Au layer is used as thesubstrate for electrode, as shown in the cross-section views in Fig. 5.8 (b) and(c). By applying voltage to the upper and lower electrodes, the waveguide device

5.3. MEMS TUNABLE HYBRID PLASMONIC WAVEGUIDES 59

changes to “on” state, where the upper Au layer will move toward the Si core, andreduce the thickness of low-index material (air) layer.

Figure 5.8: MEMS tunable hybrid plasmonic waveguide. (a) Top-view and cross-section views of the MEMS tunable hybrid plasmonic waveguide with (b) “on” and(c) “off” states. Mode profiles of (d) “on” and (e) “off” states.

Mode profiles of the “on” and “off” states are shown in Fig. 5.8(d) and (e). Asone can see, the mode is highly confined between Au layer and Si core with “on”state, and therefore the propagation loss of the waveguide can reach as high valueas 6.8dB/µm. Without driving voltage (“off” state), the propagation loss can bedown to 0.04dB/µm due to lower influence of the plasmonic layer.

The experimental results are shown in Fig. 5.9. An Au membrane with MEMSstructure is fabricated above an amorphous Si waveguide with 250nm height and400nm width. The detailed fabrication flow can be found in Fig. 3.7. By apply-ing voltage to the upper and lower electrodes, the transmitted power is changed,where the extinction ratio between “on” and “off” states is about 20dB. The MEMSstructure is still under optimization, and the required voltage can be reduced by in-troducing more mechanically sensitive structures, however, at the expense of lowerfabrication tolerance.

Apart from the tunable waveguide loss, the neff of such a waveguide can also bedramatically changed by tuning the thickness of the low-index material layer, whichis promising for power efficient switches, such as directional coupler (DC) and ring


Figure 5.9: Scanning electron microscopy (SEM) (a) top view and (b) cross-sectionview of the MEMS tunable hybrid plasmonic waveguide. (c) The output powervariations as a function of wavelength for different bias voltages.

Figure 5.10: MEMS tunable directional coupler: (a) schematic; (b) even and oddmode of HP directional coupler when hlow=60nm; (c) coupling lengths changedwith wavelength; (d) normalized output power at one output end of directionalcoupler.

5.4. GRAPHENE-BASED HYBRID PLASMONIC OPTICAL MODULATOR61

resonators. By taking the DC for example, as illustrated in Fig. 5.10, the couplinglength is:

Lc = λ

2(neven − nodd), (5.3)

and is highly dependent on the effective refractive index of even (neven) and odd(nodd) modes. With a lower thickness of the low-index layer, the coupling lengthis smaller due to higher optical confinement. By tuning the gap thicknesses from60nm to 120nm, the coupling length can be increased by around 2µm. Hence, thesplitting ratio of the DC can be changed. The normalized output power at oneport:

P = cos2( πL2Lc), (5.4)

can be tuned from 0 to 1 within 60nm thickness difference of the low-index layer(the interaction length, L, is set to be 20µm in simulation). In this way, a DC withtunable power splitting ratio is achieved.

5.4 Graphene-based hybrid plasmonic optical modulator

Graphene is an intensively investigated 2D material in recent years, and has uniqueproperties in both, electrical and optical applications. The conductivity of grapheneis described via the Kubo formula in a complex form consisting of the interbandand intraband contributions:

σtotal = σintra + σ′inter + iσ′′inter, (5.5)

where the intraband conductivity has the Drude-like form:

σintra = σ04µπ

1~(τ1 − iω) , (5.6)

Figure 5.11: Optical properties of graphene. (a) Conductivity and (b) refractiveindex of graphene changes with chemical potential.


The interband contribution has the forms:

σ′inter = σ0(1 + 1πarctan~ω − 2µ

~τ2− 1πarctan~ω + 2µ

~τ2); (5.7a)

σ′′inter = −σ01

2π ln(2µ+ ~ω)2 + ~2τ2

2

(2µ− ~ω)2 + ~2τ22 ; (5.7b)

where σ0 = e2/(4~) is the universal optical conductance. τ1 and τ2 are the re-laxation rates associated with the intraband and interband transitions. µ is thechemical potential of graphene. The conductivities of graphene change with differ-ent µ are shown in Fig. 5.11(a). By using the Ampere’s law,

ε(r, ω) = ε0(1 + iσ(ω)ε0ωd

), (5.8)

where d=0.33nm is the thickness of graphene, the refractive index of graphene canbe deducted, as shown in Fig. 5.11(b).

Figure 5.12: Graphene based hybrid plasmonic modulator. (a) 3D schematic and(b) cross-section view of the proposed graphene-based hybrid plasmonic modulator.(c) Effective refractive index (neff), and (d) propagation loss change with the drivingvoltage Vg.

The relationship between the driving voltage V, and chemical potential µ can beestimated by

|µ| ≈ ~vF√π|a0(V − Vo)|, (5.9)

5.4. GRAPHENE-BASED HYBRID PLASMONIC OPTICAL MODULATOR63

where V0 is the voltage offset caused by natural doping,vF = 0.9 × 106m/s is theFermi velocity of Dirac fermions in Graphene [56], and a0 ≈ 9 × 1016m−2V −1 es-timated from a parallel-plate capacitor model [57]. In this way, the mathematicalmodel of graphene is set, which can be applied in simulation software for the com-putation of waveguide modulator based on graphene.Graphene-based Si waveguide modulators have been demonstrated in [57], wherethe evanescent field of the Si photonic mode interacts with the graphene, and leadto a change of waveguide absorption. The interaction can be further enhanced byusing HP waveguide, as shown in the proposed structure in Fig. 5.12(a). Twomono-layers of graphene are transferred between the Au layer and Si core duringthe fabrication processes of the HP waveguide with planar plasmonic layer, wherea large optical confinement is achieved, as shown in Fig. 5.12(b). The optical prop-erties of neff and loss are shown in Fig. 5.12(c) and (d): with 4V driving voltage,the change of neff can reach as high value as 0.03, while the change of loss is over0.95dB/µm. In comparison to a corresponding device based on a Si waveguide(0.02-0.2dB/µm), the modulation depth of the HP waveguide with graphene is sev-eral times larger, due to the larger interaction between graphene and optical modeat the low-index material layer.Since the designed HP waveguide is compatible with conventional Si platform, onlythe active area is covered with plasmonic and graphene structures. The insertionloss, originated from the propagation loss of HP waveguide, is about 5dB with10µm length of such a HP waveguide, and the modulation extinction ratio canreach 10dB, which is large enough for optical communication applications. Thecapacitance of the 10µm long graphene HP waveguide is about 50fF, and the totalresistance, estimated from ref. [58], is about 60Ω. The associated 3-dB bandwidthis then calculated to be over 50GHz.

Chapter 6

Conclusions and Future Work

In this thesis, we have developed several hybrid plasmonic (HP) waveguides anddevices for optical sensing, switching and modulation applications, which showthe advantages with respect to small size, high performance or easy fabricationprocesses. However, further studies are still desirable for better performance andbroader functionalities. In this chapter, we will conclude the work presented in thisthesis (section 6.1), and then propose future work according to the obtained results(section 6.2).

6.1 Conclusions

6.1.1 Optical sensing applications

For optical sensing applications, both the single/double-slot HP waveguides andhollow HP waveguide show large interaction between tested liquids and opticalmode, which results in large waveguide sensitivity, and so sensitive devices. Amongthem, the double-slot HP (DSHP) waveguide shows the largest waveguide sensitiv-ity (>1.2) due to both, the high-index contrast (as the Si slot waveguide) and theplasmonic enhancement (as the plasmonic slot waveguide). Mach-Zehnder inter-ferometers (MZI) and ring resonator sensors based on such waveguides have beendeveloped. However, since the plasmonic layers beside the waveguide can block theexternal coupling from the access waveguide, the DSHP ring resonator has to bedesigned as a partly open structure, which leads to large coupling losses due tothe nonuniform structures. The single-slot HP waveguide can be applied in the Sibus waveguide coupled ring sensor, which shows a good balance between qualityfactor (Q factor) and sensitivity. Both the single-slot and double-slot HP waveg-uides are lateral structures, which are designed by placing different materials besideeach other in the same plane. The fabrication of such a waveguide requires high-precision of materials alignment as well as good quality of the plasmonic materials.

65

66 CHAPTER 6. CONCLUSIONS AND FUTURE WORK

The hollow HP waveguide is a compromise solution to increase the sensitivity ofconventional SOI waveguide, keeping low fabrication difficulty. By locating the pla-nar plasmonic layer above the Si core, an air gap as the low-index layer of the HPwaveguide can be constructed, which can be infiltrated with the tested liquids forsensing applications. In such a waveguide structure, over 50% of the optical powerinteracts with the tested liquids, and leads to a waveguide sensitivity of around 0.8.These HP devices can work compatibly with other passive SOI devices, and can beused to enhance the sensing functions of conventional Si based photonic integratedcircuits (ICs).

6.1.2 Optical switching and modulation applicationsUtilizing the Si thermal expansion effect, an all-optical switching device, comprisinga photothermal plasmonic absorber and an HP waveguide, has been experimentallyrealized. Since the HP ring/disk resonators have a good performance with smallradius, together with the small footprint of the photothermal plasmonic absorber,the all-optical switching device can be realized with compact size. In this work, theradius of the Si core is 1.8µm and that of the photothermal plasmonic absorber is10µm to match the field width of single-mode fiber cores. This size can be decreasedby applying better optical focusing method, like a lensed fiber or microscope. Thepower used for 15dB optical switching is 10mW, with an average response time of16µs.One of the benefits of the HP waveguide is the large optical confinement factor inthe low-index layer, which leads not only to high sensitivity to the optical proper-ties of the low-index materials, but also to its thickness. An HP waveguide tunableby microelectromechanical systems (MEMS) is a way to electrically change thethickness of the low-index layer, and therefore to modify the waveguide propertiesof both, effective refractive index (neff) and propagation loss. In our experiment,about 20dB extinction ratio can be achieved between “on” and “off” states, suitablefor realization of optical switching functions. Apart from this, the large change ofneff can also be used to realize tunable Si-based passive devices, like directionalcouplers (DCs). From the simulation results, a DC with tunable power splittingratio can be realized applying 60nm thickness change of the low-index layer.Similarly, using the HP waveguide structure, a large interaction between the opticalmode and graphene can be obtained, which can lead to the realization of graphene-based HP modulators. In our proposed design, the modulation depth (the change oftransmitted power per unite waveguide length when applying driving voltage) canbe about 0.95dB/µm, which is several times larger than for the corresponding Siwaveguide. Additionally, nonlinear electro-optic (EO) polymers can also be appliedas the low-index material in the HP waveguide, and can easily be spin-coated above(vertical structures) or beside (lateral structures) the Si core during the fabricationprocesses. The plasmonic layers can also be employed as electrodes, which givevery small resistance-capacitance (RC) time constants for high-speed modulation.

6.2. FUTURE WORK 67

6.2 Future work

From the simulation and experimental results of the developed HP waveguides anddevices in this thesis, we propose future continuation of this work in the followingareas:

1. Optimizations of the quality of plasmonic materials. Based on our experimen-tal investigations, the quality of plasmonic material has a significant influenceon the performances of the HP waveguides and devices. By using proper meth-ods, like annealing of the metal film in high temperature, the grain size andflatness of sidewalls can be improved. Another promising attempt is to usewafer bonding method to integrate planar crystalline metal film on the top ofthe Si core. In this way, no adhesive material (Ti and Ge are commonly usedin this work) is required, and the uniformity of the fabricated devices can bemuch improved.

2. HP photonic ICs with compact footprint. The developed vertical HP waveg-uides are also suitable for the development of passive photonic ICs with com-pact footprint. As we described in the thesis, the width of the Si core canbe designed to be below sub-wavelength size. Moreover, the interaction be-tween each photonic device is small due to high optical confinement. Bycombination of these benefits, it is promising to develop passive photonic ICs,including multimode interferometers, directional couplers, splitters, etc., withsub-wavelength size of Si cores under one planar plasmonic film.

3. Experimental demonstration of EO modulators with proper nonlinear organicmaterials or graphene. We have demonstrated good performance of single-/double-slot and hollow HP waveguides in liquid sensing applications. Withthe help of electrooptic organic materials or graphene, EO modulators withhigh-performance are promising possibilities, as described in chapters 4 and 5.In the next step a proper electrooptic material and graphene can be appliedto experimentally realize such modulators. The related fabrication methods,like poling of organic materials and transferring the graphene mono-layer,have to be investigated.

4. Surface and gas sensing applications. In this thesis, our experimental inves-tigations of HP sensors are based on liquid refractive index sensing. For thefuture continuation of this work, surface sensing with thin layer receptors canbe developed. Moreover, an experimental setup of gas sensing can also beestablished, to broaden the application scope of HP sensors.

5. Integration with gain materials. The large optical confinement factor in thelow-index layer of HP waveguides can give a large interaction between sup-ported mode and low-index gain materials, like rare earth doped glasses orpolymers, which can be used for loss compensation or even lasing functionsin future photonics ICs.

Chapter 7

Guide to Appended Papers

Paper A. X. Sun, D. Dai, L. Thylén and L. Wosinski, “High-sensitivity liquidrefractive-index sensor based on a Mach-Zehnder interferometer with a double-slothybrid plasmonic waveguid” Opt. Express 23(20), 25707-25716 (2015).

Contribution: Design, simulation, fabrication and analysis; major part of writ-ing.Paper B. X. Sun, D. Dai, L. Thylén and L. Wosinski, “Double-slot hybrid plas-monic ring resonator used for optical sensors and modulators”, Photonics Vol. 2,No. 4, pp. 1116-1130 (2015).

Contribution: Design, simulation, fabrication and analysis; major part of writ-ing.Paper C. X. Sun, L. Wosinski and L. Thylén, “Nanoscale surface plasmon polari-ton disk resonators, a theoretical Analysis”, IEEE Journal of Selected Topics inQuantum Electronics, 22(2) 4600106 (2016).

Contribution: Design, simulation, and analysis; major part of writing.Paper D. X. Sun, X. Chen, M. Yan, M. Qiu, L. Thylén and L. Wosinski, “All-optical switching using a hybrid plasmonic donut resonator with photothermal ab-sorber”, IEEE PTL, 28(15), pp. 1609-1612 (2016).

Contribution: Design, simulation, fabrication and analysis; major part of writ-ing.Paper E. X. Sun, L. Thylén and L. Wosinski, “Hollow hybrid plasmonic Mach-Zehnder sensor”, Opt. Lett. 42(4), pp. 807-810 (2017).

Contribution: Design, simulation, fabrication and analysis; major part of writ-ing.Paper F. X. Sun, L. Thylén and L. Wosinski, “Slot hybrid plasmonic ring resonatorused for optical sensors and modulators”, presented at the Asia Communicationsand Photonics Conference (ACP) 2015, Hong Kong, Nov. 19-23, 2015, Proceedingsof the IEEE.

Contribution: Design, simulation, fabrication and analysis; major part of writ-ing.

69

70 CHAPTER 7. GUIDE TO APPENDED PAPERS

Paper G. X. Sun, L. Thylén and L. Wosinski, “Hollow hybrid plasmonic waveguideused for electro-optic phase modulation”, presented at the Asia Communication andPhotonics Conference (ACP 2016), Wuhan, China, Nov. 2-5, 2016, paper: AF3F.2.

Contribution: Design, simulation, fabrication and analysis; major part of writ-ing.Paper H. X. Sun, L. Thylén and L. Wosinski, “MEMS tunable hybrid plasmonic-Si waveguide”, Technical Digest of the Optical Fiber Communication Conference(OFC), paper Th2A.6, Mar. 19-23, 2017, Los Angeles, USA.

Contribution: Design, simulation, fabrication and analysis; major part of writ-ing.

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