doc.: ieee 802.15-08-0746-00-0thz submission jifeng liu, mit, microphotonics centerslide 0 november...

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.


Submission


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


Submission


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


Submission


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


Submission


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?


Submission


Ultra-low Energy, Integrated GeSi Ultra-low Energy, Integrated GeSi Electroabsorption ModulatorsElectroabsorption Modulators

(Modulation in Optical Domain)(Modulation in Optical Domain)


Submission


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)


Submission


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


Submission


/

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


Submission


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)


Submission


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)


Submission


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


Submission


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)


Submission


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



Submission


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!



Submission



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.


Submission


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.


Submission


Integrated Photonic Devices for Integrated Photonic Devices for Sub-THz and THz GenerationSub-THz and THz Generation


Submission


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


Submission


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


Submission


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


Submission


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)


Submission


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

doc.: ieee 802.15-08-0746-00-0thz submission jifeng liu, mit, microphotonics centerslide 0 november...

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