silicon- and plasmonics-based nanophotonics for telecom
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Lech Wosinski
Silicon- and plasmonics-based nanophotonics for telecom and
interconnects
1
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
2
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
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The minimum dimension of silicon photonics is restricted by the diffraction limit of light !
3
Silicon Photonics at KTH
Compact Arrayed Waveguide Grating Ring resonators
Photonic crystal cavity Novel couplers
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Si-based photonic crystal structures
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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
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Dedicated tools - ICP
6
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
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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.
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Fabrication of Si Nanowires Process flow for fabrication of Si nanowires and nanowire-based components
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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
1520 1540 1560 1580 1600 1620-30
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-5
wavelength (nm)
resp
onse
(dB)
spectral response for TE polarization
1535 1540 1545 1550 1555 1560 1565
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Wavelength(nm)
Diff
ract
ion
effic
ienc
y(dB
)
CH1
1475 1480 1485 1490 1495 1500 1505
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-10
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Wavelength(nm)
Diff
ract
ion
effic
ienc
y(dB
)
CH2
1295 1300 1305 1310 1315 1320 1325
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Wavelength(nm)
Diff
ract
ion
effic
einc
y(dB
)
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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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
1480 1500 1520 1540 1560 15800
0.2
0.4
0.6
0.8
Coupling eff. (Meas.)Coupling eff. (Sim.)Power upPower down
Wavelength [nm] C
oupl
ing
effic
ienc
y
High efficiency nonuniform grating coupler
Y. Tang, Z. Wang, L. Wosinski, U. Westergren, and S. He, Optics Letters (2010).
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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
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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.
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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.
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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
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SEM details: plasmon slot waveguide with Au
y = -3.0733x - 6.067
-12
-10
-8
-6
-4
-2
0
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
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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.
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Novel solution: hybrid plasmonic waveguide
Analysis for the parameters of the Si-Au hybrid plasmonic waveguide
0
0.1
0.2
0.3
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0.5
0.6
0 20 40 60 80 100hslot (nm)
Pow
er c
onfin
emen
t rat
io
Pbuffer
PSiO2_cladding
Pslot
PSi
0
0.1
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0.5
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0 100 200 300 400 500
Wslot (nm)
Pow
er c
onfin
emen
t rat
io
Pbuffer
PSiO2_cladding
Pslot PSi
Power confinement in different areas depending on the thickness of the silica slot layer and its width
hm
SiO2
Si
wco
hSiO2 hSi_rib
metal
SiO2 Insulator
Si substrate
HSi
Lech Wosinski 22
250 300 350 4000
0.02
0.04
0.06
0.08
0.1
Width (nm)
Loss
(dB
/µm
)
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
wco
hSiO2 hSi_rib
metal
SiO2 Insulator
Si substrate
HSi
Loss for different core width.
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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
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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
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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.
0 2 4 6 80
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1
Interaction length (µm)
Nor
mal
ized
pow
er
g1=140 nm
Fitted (g1)
g2=185 nm
Fitted (g2)
g3=235 nm
Fitted (g3)
g4=290 nm
Fitted (g4)
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).
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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
27
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
1.35 1.4 1.45 1.5 1.55 1.6 1.65-12
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wavelength (µm)
Nor
mal
ized
Pow
er (d
B)
g=56nmg=95nm
Microdisk R = 0.525 um
1400 1500 1600-15
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0
wavelength(nm)
Nor
mal
ized
Pow
er (d
B)
g=56 nmg=95 nmg=146 nm
The experimental Q-value for a disk resonator with radius 0.525 µm is 130 (theoretical value 950).
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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)
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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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1400 1500 1600-10
-8
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0
2
wavelength(nm)
Nor
mal
ized
Pow
er (d
B)
g= 56 nmg=103 nmg=148 nm
1400 1500 1600-15
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-5
0
wavelength(nm)
Nor
mal
ized
Pow
er (d
B)
g= 60 nmg=105 nmg=145 nm
Microdisk R = 1.528 µm Microdisk R = 1.027 µm
1580 1600 1620 1640 1660-15
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0
wavelength(nm)
Nor
mal
ized
Pow
er (d
B)
70oC50oC30oC10oC
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
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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
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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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