Lech Wosinski

Silicon- and plasmonics-based nanophotonics for telecom and

interconnects

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hm

SiO2

Si

wco

hSiO2 hSi_rib

metal

SiO2 Insulator

Si substrate

HSi

Lech Wosinski

Materials- and Nano Physics, School of ICT, Royal institute of Technology, 16440 Kista, Sweden,

Lars Thylén Fei Lou

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SOI: Silicon on insulator - technology of choice • Very good optical properties at 1.5 – 1.6 µm – low losses • Low index silica in the bottom and silica or air around the core guarantee a

very high contrast of refractive index in all directions – high compactness • CMOS compatible. • Low cost

•high quality

•high precision

•low roughness

Complementary Metal Oxide Semiconductor Field Effect Transistor Gate length ~ 20 nm

A cross section of the nanowire waveguide is 220 x 500 nm and bending radius can be as small as 2 µm

•No electro-optic effect •No detection in 1.3-1.6μm region •High index contrast – coupling •Lacks efficient light emission

Needs: Drawbacks:

∆=41 (46)%

Coupling difficulty:

Big mode size mismatch ~20:1 Unacceptable coupling loss for butt coupling

Lech Wosinski

The minimum dimension of silicon photonics is restricted by the diffraction limit of light !

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Silicon Photonics at KTH

Compact Arrayed Waveguide Grating Ring resonators

Photonic crystal cavity Novel couplers

Lech Wosinski

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Si-based photonic crystal structures

Lech Wosinski

Dedicated tools - PECVD

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STS (575)

Technical description Full equipment name: Plasma Enhanced Chemical Vapor Deposition (PECVD) General purpose: Deposition of silicon oxide, silicon nitride and amorphous silicon Producer: Surface Technology Systems, UK Technical data: • Parallel plate RF excited plasma, LF generator 380 kHz, 1000W or HF generator 13.56 MHz, 300W • Gases: SiH4, N2O, N2, NH3, CF4, B2H6, GeH4, PH3, Ar, O2, He • Single wafer deposition-system with loadlock (maximum 150 mm wafer) • Configured for 100 mm wafers, glue smaller pieces on 100 mm Si wafers • Achived thickness uniformity: +/- 3 % within 100 mm Si wafer • Achived refractive index uniformity: +/- 0.0005 within 100 mm Si wafer

PECVD plasma chamber

Lech Wosinski

Dedicated tools - ICP

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ALOES (585)

Technical description Full equipment name: STS ICP Multiplex Advanced Oxide Etch (AOE) system General purpose: Deep Reactive Ion Etching of silicon oxide and silicon Technical data: • Reactive ion etching process with inductively coupled plasma • Single wafer machine with carousel (2 wafers) loadlock for 100 and 200 mm wafers • Installed gases: CF4, C3F8, C4F8, SF6, CHF3, Ar, O2 • Process pressure range: 10-95 mTorr • Plasma power max, coil: 13.56 MHz, 3 kW, platen: 13.56 MHz, 1 kW • Gases: CF4, C4F8, CHF3, SF6, H2, He, O2, N2, Ar • Achived uniformity (oxide): +/- 3 % within 100 mm Si wafer • Configured for 100 mm wafers, glue smaller pieces on 100 mm Si wafers

ICP plasma chamber

Lech Wosinski

Pilars diam 440 nm Height 2.2 µm (etching aspect ratio 5:1)

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Fabrication of Si Nanowires

Basic structure for fabrication of Si nanowires and nanowire-based components

SiO2 and Si layers can be deposited by PECVD (Plasma Enhanced Chemical Vapor Deposition) method or acquired in form of SOI (Silicon on Insulator) wafer. The deposited material is amorphous (in contrast to crystaline silicon in SOI wafer) exhibiting higher propagation losses, but can be individually engineered according to the specific needs including layers thicknesses and, to some extend, their refractive indices, as well as more complicated multi-layer structures are possible to achieve. It can be also used together with other materials on almost any substrate.

Lech Wosinski

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Fabrication of Si Nanowires Process flow for fabrication of Si nanowires and nanowire-based components

Lech Wosinski

Si Nanowires in amorphous silicon (α-Si)

5µm 250 nm

500 nm

• Patterns are generated with E-beam lithography on a negative resist. • Waveguide dimension: 500nm×250nm. • Loss of a straight waveguide: ~4dB/mm (evaluated with cut back method).

Acceptable for nanophotonic devices. The best published results (commercial SOI): 0.24dB/mm.*

* W. Bogaerts, et al. J. Lightwave Technol. 23, 401-412 (2005) Lech Wosinski

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Lech Wosinski 10 10

Si-based nanophotonics for computer interconnects and telecom

Schematic configuration and optical mode profile

of a hybrid laser

AWG Demultiplexers Based on α-Si Nanowires D. Dai, L. Liu, L. Wosinski and S. He, Electronics Letters (2006).

Echelle grating triplexer N. Zhu, J. Song, L. Wosinski, S. He and L. Thylen, Optics Letters (2009).

InP Lateral Overgrowth Technology for Silicon Photonics Z. Wang, C. Junesand, W. Metaferia, C. Hu, L. Wosinski and S. Lourdudoss, J. of Materials Science and Engineering B, Vol. 177 (2012).

Hybrid Silicon (InP bonded) Electroabsorption Modulator

Design and optimization of an arbitrarily segmented traveling wave electrode for an ultrahigh speed electro-absorption modulator Y. Tang, Y. Yu, Y. Ye, U. Westergren, and S. He. Opt. Comm., (2008)

Cooperation with J. Bowers Group, UCSB, CA, USA

Future integration with microelectronic control units

Butt Coupling

Vertical Coupling

Interfacing of Silicon-on-insulator nanophotonic circuits to the real world of optical fibers

Highly efficient nonuniform grating coupler for silicon-on-insulator nanophotonic circuits Y. Tang, Z. Wang, L. Wosinski, U. Westergren, and S. He, Optics Letters (2010).

High efficiency polarization splitter based on a one-dimensional grating with a Bragg reflector, Z. Wang, Y.Tang, L. Wosinski, and S. He, IEEE Photon. Technol. Lett. (2010)

SiGe detectors/modulators

High speed optical modula-tion in Ge quantum wells Rong, Y., Ge, Y., Huo, Y., Fiorentino, M., Kamins, T. I., Ochalski, T., Thylen, L., Chacinski, M., Harris, J. S., 6th IEEE International Conference on Group IV Photonics (2009)

Cooperation with J. Harris Group, Stanford, CA, USA

50 Gb/s hybrid silicon traveling-wave electroabsorption modulator Y. Tang, H.-W. Chen, S. Jain, J. D. Peters, U. Westergren and J. E. Bowers, Opt. Exp 2011

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Guiding light and wavelength selectivity

Arrayed Waveguide Gratings for WDM communication.

1550 nm 1310 nm 1490 nm

Echelle- grating triplexer for Fiber- to-the-home communication systems

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CH3 Measured spectral responses of channel 1 for order 5, ch. 2 for order 5 and ch. 3 for order 6. The average loss per channel is 11dB and crosstalk is better than 20dB.

• Total size 320 x 270 mm • In/out tappered to 2 mm width • Waveguide loss 4dB/mm • Crosstalk -7 dB (for TE pol.) • Gaussian-shaped response. • Channel spacing: 1.5nm. • Free spectrum range: 21.7nm. • Crosstalk: -7dB. • Insersion loss: -8.5dB.

D. Dai, L. Liu, L. Wosinski and S. He, Electronics Letters (2006).

N. Zhu, J. Song, L. Wosinski, S. He and L. Thylen, Optics Letters (2009).

L. Wosinski, L. Liu, M. Dainese, and D. Dai, 13th European Conference on Integrated Optics, Copenhagen, Denmark, 25-27, 2007.

• Total size 40 x 50 µm (4 x 4 AWG) • Waveguide loss about 4dB/mm • Channel spacing 11 nm • Free spectrum range: 75nm. • Crosstalk -14 dB (for TE polarization) • Insersion loss: -6dB

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Lech Wosinski 13

Grating couplers between silicon nanophotonic circuits and fibers

Over 50% efficiency for both polarizations obtained with grating and bottom reflector.

Polarization Splitter based on a Bidirectional Grating Coupler

Z. Wang, Y.Tang, L. Wosinski, and S. He, IEEE Photon. Technol. Lett. (2010).

Index-Matching Glue BOX

TE

TM TM TE

Fiber core

Over 64% efficiency and 3dB bandwidth >70nm for TE polarization obtained with nonuniform grating.

Schematic configuration of the vertical coupling set up

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High efficiency nonuniform grating coupler

Y. Tang, Z. Wang, L. Wosinski, U. Westergren, and S. He, Optics Letters (2010).

Lech Wosinski 14

2. Diffracted light distribution

Nonuniform grating: perfect matching between the radiated light distribution and the fiber mode.

Lag Effect Etch rate depends on the etch width of openings

Relation between etch width and etch depth

Pup

SiO2

Si

Gel Pin

Pdown

High efficiency nonuniform grating coupler

Efficiency limiting factors

SiO2

Si

Gel Pin

Pup

Pdown

1. Diffraction directionality

Lech Wosinski 15

The minimum dimension of silicon photonics is restricted by the light wave diffraction limit !

To integrate nanophotonic devices into the existing CMOS technology for electronics a silicon-based plasmonic platform for photonics became a good choice.

In the 1980s researchers confirmed experimentally that light can propagate along a metal – dielectric interface interacting resonantly with mobile electrons

SPs at the interface between a metal and a dielectric material have a combined electromagnetic wave and surface charge character. They are transverse magnetic in character (H is in the y direction), and the generation of surface charge requires an electric field normal to the surface.

Lech Wosinski 16

Plasmonics - combining Si photonics with metals

Surface plasmon (SP) waveguides utilize the fact that light can be confined in

a single interface between a metal and a dielectric.

Guiding principle relies on coupled plasmon- polariton modes propagating as

electromagnetic fields coupled to surface plasma oscillations, which are

comprised of conduction electrons in the metal.

Sketch of a single interface between a metal and dielectric

Mode field pattern of the SP wave on this interface. Here, ¸0=1.55¹m,nm=0.47+j9.32 (Ag), and nd=1.0.

Lech Wosinski 17

Comparison of plasmon and dielectric waveguides

(a) Sketch of an SP waveguide formed by a gap between two metal layers. (b) Propagation constant and (c)

Hy field pattern of the fundamental mode (TM0) in structure (a) with different width w. (d) Sketch of a

conventional dielectric waveguide. (e) Propagation constant and (f) Ey field pattern of the fundamental mode

(TE0) in structure (d) with different width w. Here, λ0=1.55µm, nm=0.47+j9.32 (Ag), n1=3.63 (Si), and nd=n2=1.0.

Sub-wavelength confinement !

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Sergey I. Bozhevolnyi et al, Nature Photonics 440, p. 508. (2006)

Deep metallic V-groove waveguide

L. Liu, et. al., Opt. Express 13, 6645 (2005) P. Holmström , et. al., Appl. Phys. Lett.

97(7), 073110 (2010).

Nano-particle chain waveguide

Hybrid plasmonic waveguide R. F. Oulton, Nature Photonics 2, 496 - 500 (2008)

Surface plasmons for waveguiding - beyond the diffraction limit of light

metal

metal

h

w

tairSiO2

x

y

(a)

50nm SiO2 on Al

Al

(b)

metalh

SiO2

PMMAwxy

(c) (d)

200nm

Al

SiO2

Strip-line waveguide Slot-line waveguide

Lech Wosinski

Lech Wosinski 19

SEM details: plasmon slot waveguide with Au

y = -3.0733x - 6.067

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0 0.5 1 1.5 2Results: Propagation loss = 3.0733 dB/µm [0.8 dB/µm in L. Chen, J. Shakya, M. Lipson, Optics Letters, Vol. 31 (2006)] Loss of coupling between 500nm Si waveguide to 500nm Si wg with metal = 2.1511 dB/facet

Loss of taper/350nm = 0.3850 dB/taper

Access waveguide taper

Plasmon waveguide

Covered with gold to form slot waveguide

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Plasmonic slotline – experimental results

Lech Wosinski 20

Novel solution: hybrid plasmonic waveguide First theoretical investigations: M. Z. Alam, J. Meier, J. S. Aitchison, and M. Mojahedi, “Super mode propagation in low index medium,” CLEO/QELS 2007.

R. Salvador, A. Martinez, C. Garcia-Meca, R. Ortuno and J. Marti, “Analysis of Hybrid Dielectric Plasmonic Waveguides”, IEEE Journal of Selected Topics in Quantum Electronics 14(6) 1496 – 1501(2008).

R. F. Oulton, V. J. Sorger, D. A. Genov, D. F. P. Pile, and X. Zhang, “A hybrid plasmonic waveguide for subwavelength confinement and long range propagation,” Nat. Photonics 2(8), 496–500 (2008).

M. Fujii, J. Leuthold, and W. Freude, “Dispersion relation and loss of sub-wavelength confined mode of metal-dielectric-gap optical waveguides,” IEEE Photon. Technol. Lett. 21(6), 362–364 (2009).

D. Dai, and S. He, “A silicon-based hybrid plasmonic waveguide with a metal cap for a nano-scale light confinement,” Opt. Express 17(19), 16646–16653 (2009).

First experimental confirmations: M. Wu, Z. Han and V. Van, “Conductor-gap-silicon plasmonic waveguides and passive components at subwavelength scale ,” Opt. Express 18(11), 11728–11736 (2010).

Z. Wang, D. Dai, Y. Shi, G. Somesfalean, P. Holmstrom, L. Thylen, S. He and L. Wosinski, “Experimental Realization of a Low-loss Nano-scale Si Hybrid Plasmonic Waveguide”, Technical Digest of OFC/NFOEC 2011.

Lech Wosinski 21

Novel solution: hybrid plasmonic waveguide

Analysis for the parameters of the Si-Au hybrid plasmonic waveguide

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Power confinement in different areas depending on the thickness of the silica slot layer and its width

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SiO2 Insulator

Si substrate

HSi

Lech Wosinski 22

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Substrate : Si

Layer I : SiO 2

Au

layer III : SiO 2 layer II : Au

layer IV : a - Si

wSi=variable hSi=400nm hSiO2= 56nm hAu=100nm

Experimental data Simulation

Low-loss highly-confined hybrid plasmonic waveguide

D. Dai and S. He, Optics Express, Vol. 17, No. 19 (2009).

Propagation distance for different core width and hight.

hm

SiO2

Si

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SiO2 Insulator

Si substrate

HSi

Loss for different core width.

Lech Wosinski 23

hSiO2= 30nm 150nm

wSi=200nm hSi=250nm hAu=100nm hSiO2= 30 – 150nm

Propagation loss experimentally obtained: 0.01 dB/μm (propagation length over 400 µm) for silica layer thickness 150 nm and 0.22dB/μm (propagation length 19 µm) for silica layer thickness 30 nm.

Simulated results of Ey field distribution and its vertical cross section in the fabricated hybrid waveguide structures

Design of the structure Fabrication a) silicon waveguide structure defined by e-beam lithography and Induced Coupled Plasma Reactive Ion Etching, b) grating in/out coupling defined by e-beam lithography and shallow etching by ICP-RIE, c) spin coating of spin-on-glass, d) coating wit a gold layer.

Characterization Propagation loss 0.01 dB/μm. SiO2 thickness 150nm

Propagation loss 0.22 dB/μm. SiO2 thickness 30nm

By adjusting the thickness of the silica layer and its width, the waveguide mode can be changed from plasmonic to photonic. Different solutions for strong confinement/short propagation length or weak confinement/long propagation length

Z. Wang, D. Dai, Y. Shi, G. Somesfalean, P. Holmstrom, L. Thylen, S. He and L. Wosinski, OFC 2011.

Low-loss highly-confined hybrid plasmonic waveguide

Lech Wosinski

The hybrid plasmonic waveguide structure comprises 3 µm thick SiO2 buffer layer, gold layer of thickness hAu = 100 nm, a thin silica core layer of hSiO2 = 56 nm, and amorphous silicon (a-Si:H) top ridge with a thickness hSi = 400 nm and width WSi = 170 nm. A long propagation length with loss of 0.08 dB/µm and 90o bend loss of 0.25 dB was obtained.

Hybrid plasmonic waveguide components

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Structure and properties of the waveguides

Directional couplers and splitters

0

1

Lech Wosinski 25

Experimental results for directional couplers/ splitters

Normalized power at through port as functions of g and L. The markers are measured results, and the lines are fitted curves.

In the experiment, 90 devices were tested, divided into 4 groups with g =140 nm, 185 nm, 235 nm, and 290 nm. Coupling lengths: 1.55 µm, 2.2 µm, 3.2 µm and 4.8 µm resp.

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g3=235 nm

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g4=290 nm

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Input through

cross

Excitation of hybrid plasmonic mode by butt coupling

Dark field images for devices (g=140 nm) with L = 0, 0.8 µm, 1.4 µm, whose splitting ratios are approximately 2:98, 54:46, 92:8

F. Lou, Z. Wang, D. Dai, L. Thylen, and L. Wosinski, “Experimental demonstration of ultra-compact directional couplers based on silicon hybrid plasmonic waveguides”, Applied Phys. Lett. Vol. 100, No. 24, 241105 (2012).

Lech Wosinski

Microring/disk resonators as add-drop multiplexers,WDM routers, reconfigurable devices, modulators, switches, ...

N × M Manhattan microring configuration with N input/output channels and add/drop ports for each of the M wavelengths 26

wco Metal

Lech Wosinski

Hybrid plasmonic waveguide ring/disk resonators

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WSi = 170 nm, hSi=400 nm, hSiO2=56 nm, hAu=100

nm, g= 56, 95, 146 nm, R= 0.525, 1.027, 1.528 µm

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The experimental Q-value for a disk resonator with radius 0.525 µm is 130 (theoretical value 950).

Lech Wosinski

Microdisk R = 0.525 um

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WSi = 170 nm, hSi=400 nm, hSiO2=56 nm,

hAu=100 nm, g= 56, 95, 146 nm, R= 0.525 µm

1500 nm 1415 nm

Presented by Björn O. Nilsson, President of IVA as "Progress in Research and Technology in Sweden 2012" at the 93rd Annual Meeting of the Royal Swedish Academy of Engineering Sciences, a Society under the Auspices of His Majesty the King of Sweden (Friday, 26th of October)

Lech Wosinski 29

F. Lou, Z. Wang, D. Dai, L. Thylen, and L. Wosinski, “A sub-wavelength microdisk based on hybrid plasmonic waveguides”, 5th International Photonics and OptoElectronics Meetings (POEM 2012), November 1-2, 2012, Wuhan, China. Best student paper award – first price.

Fei Lou (first at left) awarded by a first price

Lech Wosinski

Hybrid plasmonic waveguide disk resonators

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Microdisk R = 1.528 µm Microdisk R = 1.027 µm

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Thermal tunability of microdisk R = 0.525 µm 6 nm red shift when changing the temp. 10oC → 70oC

Lech Wosinski

Other hybrid plasmonic waveguide components

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J. Wang, X. Guan, Y. He, Y. Shi, Z. Wang, S. He, P. Holmström, L. Wosinski, L. Thylen, and D. Dai, Optics Express, Vol. 19, No. 2, pp. 838-847 (2011).

Compact power splitters

F. Lou, D. Dai and L. Wosinski, Optics Letters Vol. 37, 16, pp. 3372–3374 (2012).

Ultra-compact polarization beam splitter

Lech Wosinski

Other hybrid plasmonic waveguide components

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Hybrid plasmonic polymer-based modulators in vertical configurations.

Mach Zehnder arrangement

The electrical push-

pull circuit schematically shown.

Microring arrangement Cross-sectional view along the xy and xz planes of the Ez field distributions of a resonant mode at 1550 nm, with an azimuthal number of 6.

L. Thylén, P. Holmström, L. Wosinski, B. Jaskorzynska, M. Naruse, T. Kawazoe, M. Ohtsu, M. Yan, M. Fiorentino, U. Westergren, "Nanophotonics for Low-Power Switches," in Optical Fiber Telecommunications VIA (Chapter 6), Ed. Ivan Kaminow, Tingye Li and Alan Willner, Elsevier, 2013.

F. Lou, D. Dai, L. Thylen and L. Wosinski, “Design and analysis of ultra-compactEO polymer modulators based on hybrid plasmonic microring resonators”, Optics Express 2013

Lech Wosinski 33

𝝀0@1549 nm

MW signal

EOP modulator: comparison with Si slot structure

TM-mode supported by Au-Polymer-Si hybrid plasmonic waveguide Ey Field

Field distribution of TE-mode supported by Si-Polymer-Si slot waveguide

Ex Field

Au-P-Si

Si-P-Si

Hybrid plasmonic waveguide VS Si slot waveguide (1) Similar light confinement in low-index slot layer (EOP) (2) Reduced top contact; hence, RC –limited speed should be about twice faster in principle; (3) Decreased mode area => tight bends; at a cost of loss…

Lech Wosinski

Future? • The trend in photonic integration is towards CMOS compatible silicon

photonics, which means the reduction of material diversity as well as functional unification of photonic components.

• The size of these structures is constrained by the diffraction limit of light.

• Nevertheless the new developments in form of hybrid plasmonic waveguides allow to go below this limit keeping propagation losses on an acceptable level

• This gives good prospects for even higher integration and miniaturization of photonic circuits towards electronics – photonics integration

• Inter- and intra-core optical communication demands new architectures new technology, new devices

• Micro – nano – plasmo – meta- convergence

ACKNOWLEDGEMENT This work was supported by “the Swedish Research Council (VR) through its Linnæus Center of

Excellence ADOPT”, as well as project VR-621-2010-4379

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