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November 2008 doc.: IEEE 802.15-08-0746-00-0thz
Submission
Jifeng Liu, MIT, Microphotonics CenterSlide 1
Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)
Submission Title: Integrated Photonics for THz ApplicationsDate Submitted: 7 November, 2008Source: Jifeng Liu Company: Massachusetts Institute of TechnologyAddressVoice: FAX: E-Mail: [email protected]
Re:
Abstract: This contribution presents some promising applications of integrated photonics to THz technology in terms of modulation and THz generation.
Purpose: for discussion
Notice: This document has been prepared to assist the IEEE P802.15. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.15.
November 2008 doc.: IEEE 802.15-08-0746-00-0thz
Submission
Jifeng Liu, MIT, Microphotonics CenterSlide 2
Jifeng LiuMicrophotonics Center, Massachusetts Institute of Technology
Acknowledgement• EPIC Program, Defense Advanced Research Projects Agency (DARPA).
Integrated Photonics for THz applications
Integrated Photonics enables• Small device footprint and high level of integration• Modulation at ultra-low energy consumption• Ultra-fast, high efficiency photodetectors for sub-THz
generation by heterodyne photomixing; and• Enhanced nonlinear optical effect for THz generation to ultimately achieve hand-held THz electronics
November 2008 doc.: IEEE 802.15-08-0746-00-0thz
Submission
Jifeng Liu, MIT, Microphotonics CenterSlide 3
VISION:VISION: The goal of the Center is the creation of The goal of the Center is the creation of new materials, new materials, structures and architecturesstructures and architectures to enable the evolution of to enable the evolution of photonics from single, discrete devices to photonics from single, discrete devices to integrated photonic integrated photonic systems.systems.
Research • Roadmapping • Infrastructure
E-P synergy - Integration - Standardization - Cross-market PlatformsPower - Bandwidth - Latency - Footprint - Package - Cost
OVERVIEW:OVERVIEW: Involves 50 faculty members from 9 departments with their Involves 50 faculty members from 9 departments with their
research related to photonicsresearch related to photonics
RELATION WITH INDUSTRY:RELATION WITH INDUSTRY: Communication Technology Roadmap Consortium has Communication Technology Roadmap Consortium has
16 industrial members, including IBM, Intel, HP, NEC, 16 industrial members, including IBM, Intel, HP, NEC, Siemens……Siemens……
Microphotonics Center (MPhC) at MIT
November 2008 doc.: IEEE 802.15-08-0746-00-0thz
Submission
Jifeng Liu, MIT, Microphotonics CenterSlide 4
Integrated Photonics on Si vs. Free Space OpticsIntegrated Photonics on Si vs. Free Space Optics
Free space opticsFree space optics Integrated photonics on Si Integrated photonics on Si
Light wireLight wire Optical FiberOptical Fiber
((n~0.01, 10 µm n~0.01, 10 µm diameter)diameter)
High index contrast (HIC)High index contrast (HIC)
waveguides (waveguides (n~2, n~2,
0.5×0.2 µm0.5×0.2 µm22 Xsection) Xsection)
PhotodiodePhotodiode Vertically Vertically illuminated, BW-illuminated, BW-
efficiency trade-offefficiency trade-off
HIC Waveguide-coupled, no HIC Waveguide-coupled, no BW-efficiency trade offBW-efficiency trade off
ModulatorModulator Discrete, power-Discrete, power-hungryhungry
HIC Waveguide-coupled, HIC Waveguide-coupled, small footprint, low powersmall footprint, low power
Light Light sourcesource
Discrete lasersDiscrete lasers Integrated Lasers by chip Integrated Lasers by chip bonding or epi on Sibonding or epi on Si
November 2008 doc.: IEEE 802.15-08-0746-00-0thz
Submission
Jifeng Liu, MIT, Microphotonics CenterSlide 5
Optical Channellizer
Modulator
Filter 1
Filter n
Detector
Detector
Mul
ti-m
ode
Inte
rfer
omet
ricS
plitt
er
TIA
TIA
300 MHz to 2.2 GHz RF IN
LASER
AS-EPICBlock Diagram
Optical Channellizer
Modulator
Filter 1
Filter n
Detector
Detector
Mul
ti-m
ode
Inte
rfer
omet
ricS
plitt
er
TIA
TIA
300 MHz to 2.2 GHz RF IN
LASER
AS-EPICBlock Diagram
EPIC: Integrated optical RF channelizer on siliconEPIC: Integrated optical RF channelizer on silicon
Ge EA modulator
Tunable MZI filterMMI splitter
Ge photodetectorVertical coupler
Silicon EO modulator
W Studs
Oxide Undeclad
Deposited waveguide
Gate Contact
c-Si waveguide
Integrated photonic chip enables efficient RF channelizing with small footprint, low power consumption and low EMI.
Can we transfer integrated photonics to THz technology?
November 2008 doc.: IEEE 802.15-08-0746-00-0thz
Submission
Jifeng Liu, MIT, Microphotonics CenterSlide 6
Ultra-low Energy, Integrated GeSi Ultra-low Energy, Integrated GeSi Electroabsorption ModulatorsElectroabsorption Modulators
(Modulation in Optical Domain)(Modulation in Optical Domain)
November 2008 doc.: IEEE 802.15-08-0746-00-0thz
Submission
Jifeng Liu, MIT, Microphotonics CenterSlide 7
Modulation: Optics Domain vs. THz DomainModulation: Optics Domain vs. THz Domain
Modulators in THz domain may be possible by using controlled free carrier absorption effect, but…
Modulators in optical domain are much smaller than THz domain since the mode size scales with wavelength!
Better modulate in optical domain before conversion to THz wave.
Mode in optical domain,λ=1.55 µm
Mode in THz domain, (λ=100 µm, or 3 THz)
November 2008 doc.: IEEE 802.15-08-0746-00-0thz
Submission
Jifeng Liu, MIT, Microphotonics CenterSlide 8
Electro-Optical vs. Electro-Absorption Modulators
EO modulators are based on the index change (n) .- MZIs typically large (mm in length) and power hungry
- Mirocrorings very compact yet with limited operation spectrum range (~1 nm)EA modulators are based on field-induced absorption change ().
- Compact (<100 µm), low power consumption, ultrafast intrinsic response (<1 ps)- ~20 nm operation spectrum width
Ideal for integration!
Pin Pout(V)
EAMEA modulators
V
Pin Pout(V)V
-V
EO modulators
MZI modulators
Pin Pout(V)
Microring modulators
November 2008 doc.: IEEE 802.15-08-0746-00-0thz
Submission
Jifeng Liu, MIT, Microphotonics CenterSlide 9
/
Maximum /~4at 1460 nm
Franz-Keldysh (FK) effect in tensile strained Ge-on-SiJongthammanurak et al, APL 89, 16115, (2006)
Quantum Confined Stark Effect (QCSE)in type I Ge quantum wellsY.-H. Kuo et al, Nature 437,1334, (2005)
Electro-Absorption (EA) Effect in Ge-on-Si
)(log10
)/(log10
10
)(
10
L
LL
e
ee
lossInsertion
ratioExtinctionFOM
FK effect in tensile strained Ge shows maximum absorption contrast /~4-5 at 1647 nm
QCSE in Ge QWs shows maximum /~4 at ~1460 nm
November 2008 doc.: IEEE 802.15-08-0746-00-0thz
Submission
Jifeng Liu, MIT, Microphotonics CenterSlide 10
Design of GeSi Composition and Device Structure for Optimized Performance at 1550 nm
Adding a small amount of Si blue-shifts the bandedge and the wavelength of optimal absorption contrast.
Δα/α at 1550 nm optimized at a Si composition of 0.7-0.8%.
Liu et al, Opt. Express. 15, 623-628 (2007)
November 2008 doc.: IEEE 802.15-08-0746-00-0thz
Submission
Jifeng Liu, MIT, Microphotonics CenterSlide 11
on
off
a-Si GeSi a-Si
a-Si GeSi a-Si
Butt-coupling scheme adopted for easier process integration. Device area only 30 µm2.
10 dB extinction ratio at 1550 nm with ~4 dB insertion loss is predicted The same material and device structure can be used for both EA
modulator and photodetector
Tapered vertical coupler
Design of GeSi EAM Device Structure
Liu et al, Opt. Express. 15, 623-628 (2007)
November 2008 doc.: IEEE 802.15-08-0746-00-0thz
Submission
Jifeng Liu, MIT, Microphotonics CenterSlide 12
Integration of GeSi EAMs into CMOS ProcessIntegration of GeSi EAMs into CMOS Process
► GeSi grown between front and backend of CMOS process GeSi grown between front and backend of CMOS process for electronic-photonic integration.for electronic-photonic integration.
► Two-step UHVCVD GeSi selective growth:Two-step UHVCVD GeSi selective growth: (1) a 30-60nm GeSi buffer at 360C; (2) rest of the growth at 600-700C(1) a 30-60nm GeSi buffer at 360C; (2) rest of the growth at 600-700C Annealing at 800-850C decreases dislocation density to ~10Annealing at 800-850C decreases dislocation density to ~1077/cm/cm22
CMP to remove top facetsCMP to remove top facetsM. Beals et al, Proc. SPIE. 6898, 689804 (2008)
GeSiGeSiGeSi
GeSi EAM
2.0 µm
CMOS
November 2008 doc.: IEEE 802.15-08-0746-00-0thz
Submission
Jifeng Liu, MIT, Microphotonics CenterSlide 13
Extinction Ratio and Insertion LossExtinction Ratio and Insertion Loss
Maximum ER of 11 dB observed at 1536 nm. 8 dB ER achieved at 1550 nm with 3.7 dB insertion loss.
Higher ER can be achieved with a longer device.
Operation range: 1539-1553 nm, covering half of the C-band (1530-1560nm)
Liu et al, Nature Photonics. 2, 433-437 (2008)
November 2008 doc.: IEEE 802.15-08-0746-00-0thz
Submission
Jifeng Liu, MIT, Microphotonics CenterSlide 14
Modulation Depth Vs. Electric FieldModulation Depth Vs. Electric Field
Modulation depth increases pseudo-linearly at 30%/V between -4 and -6.5 V
8 dB extinction ratio can be achieved with a relatively small voltage swing of Vpp=3V
Vpp=3 V
Liu et al, Nature Photonics. 2, 433-437 (2008)
November 2008 doc.: IEEE 802.15-08-0746-00-0thz
Submission
Jifeng Liu, MIT, Microphotonics CenterSlide 15
Energy ConsumptionEnergy Consumption
2)2/1(/ ppCVbitEnergy
8 dB ER at 1550 nm can be achieved with an ultra-low energy consumption of 50 fJ/bit for the worst case scenario due to the tiny capacitance (11 fF) and relatively low Vpp (3V)
Attractive for low-power electronic-photonic integration: 100 Gb/s modulation only consumes 5 mW of power!
Liu et al, Nature Photonics. 2, 433-437 (2008)
November 2008 doc.: IEEE 802.15-08-0746-00-0thz
Submission
Jifeng Liu, MIT, Microphotonics CenterSlide 16
Liu et al, Nature Photonics. 2, 433-437 (2008)
Bandwidth MeasurementBandwidth Measurement
1.2 GHz bandwidth achieved in the prototype device
Bandwidth currently limited by a high series resistance (~15 kΩ) due to fabrication issues. Can be reduced to <100 Ω with process optimization to achieve >100 GHz bandwidth.
November 2008 doc.: IEEE 802.15-08-0746-00-0thz
Submission
Jifeng Liu, MIT, Microphotonics CenterSlide 17
GeSi EAM Performance SummaryGeSi EAM Performance Summary Very small footprint (30 µmVery small footprint (30 µm22)) Ultra-low energy consumption (50 fJ/bit)Ultra-low energy consumption (50 fJ/bit) >10 dB Extinction ratio>10 dB Extinction ratio GHz bandwidth. Great potential for >100 GHzGHz bandwidth. Great potential for >100 GHz Operation spectrum width covering half of the C-Operation spectrum width covering half of the C-
band for on-chip WDM. band for on-chip WDM.
November 2008 doc.: IEEE 802.15-08-0746-00-0thz
Submission
Jifeng Liu, MIT, Microphotonics CenterSlide 18
Integrated Photonic Devices for Integrated Photonic Devices for Sub-THz and THz GenerationSub-THz and THz Generation
November 2008 doc.: IEEE 802.15-08-0746-00-0thz
Submission
Jifeng Liu, MIT, Microphotonics CenterSlide 19
Heterodyning two laser beams of slightly different wavelengths can results in an optical beating at THz frequency, which can be transformed to THz waves by an ultrafast photodetector.
Sub-THz generation by heterodyne photomixing requires photodiodes that are both high bandwidth (>hundreds of GHz) and high efficiency (responsivity)
Sub-THz Generation by Heterodyne Photomixing
NIR Laser f
NIR Laser f+fTHz
photomixing
1/fTHz
Ultrafast photodiode +Antenna
THz photocurrent wave
November 2008 doc.: IEEE 802.15-08-0746-00-0thz
Submission
Jifeng Liu, MIT, Microphotonics CenterSlide 20
Benefits of Waveguide-Integrated Photodetectors
Ph
oto
-d
ete
cto
r
h Photocurrent
Dark current+ -
Free-space coupled:
Separation of the optical path and the carrier collection path• Enhances the responsivity of the device without affecting its speed.• Enables small device area, and therefore, low absolute dark current
and low capacitance
Photodetector
Photocurrent
Dark current
Waveguide-Integrated: + + +
WG
- - -
h
November 2008 doc.: IEEE 802.15-08-0746-00-0thz
Submission
Jifeng Liu, MIT, Microphotonics CenterSlide 21
Waveguide-coupled GeSi Photodetectors
>1 A/W responsivity (90% quantum efficiency) and <0.2 nA dark current achieved.
Bandwidth > 4.5 GHz (only limited by the TIA circuitry).
Promising to achieve >0.4 THz bandwidth without sacrificing the efficiency with pure Ge uni-travelling-carrier PDs.
Design for sub-THz operation:Material: pure Ge-on-SiElectron mobility: 4000 cm2 V/sLength=4 µm, Width=0.5 µmGe thickness=0.1 µmEfficiency>80% Bandwidth (3dB)~0.4 THz for p-i-n diode; ~1 THz for uni-travelling-carrier photodiodes (UTC-PDs) with electron velocity overshoot.
EPIC device
November 2008 doc.: IEEE 802.15-08-0746-00-0thz
Submission
Jifeng Liu, MIT, Microphotonics CenterSlide 22
EnhancedEnhanced Nonlinear Optical Effects enabled by Nonlinear Optical Effects enabled by Integrated PhotonicsIntegrated Photonics
Strong optical confinement in high index contrast waveguides enables ultrahigh optical power density at low input optical power. Strong nonlinear effect demonstrated even in Si due to this reason
Can result in significantly enhanced efficiency for THz generation with low pump power using nonlinear optical materials
(difference frequency generation, photon rectification, etc)
500 nm
Optical power density in Si WG=106 W/cm2 at just 1 mW optical input!
...3)3(2)2( EEEP Si Raman lasersRong et al, Nature 433, p292(2005)
Four wave mixing inSi WGs Foster et al, Nature 441, p960 (2006)
November 2008 doc.: IEEE 802.15-08-0746-00-0thz
Submission
Jifeng Liu, MIT, Microphotonics CenterSlide 23
ConclusionsConclusions
Integrated photonics has great applications in THz technology by enabling small device footprint
and low energy consumption
• Modulation in the optics domain at ultra-low energy consumption
• Ultra-high bandwidth-efficiency photodetectors for sub-THz generation by heterodyne photomixing; and
• Enhanced nonlinear optical effect for THz generation