Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: Integrated Photonics

1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: Integrated Photonics for THz Applications Date Submitted: 7 November, 2008 Source: Jifeng Liu Company: Massachusetts Institute of Technology Address Voice: FAX: E-Mail: jfliu01@mit.edu 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.

2. Integrated Photonics for THz Applications Jifeng Liu Microphotonics Center, Massachusetts Institute of Technology • 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 Acknowledgement • EPIC Program, Defense Advanced Research Projects Agency (DARPA).

3. Microphotonics Center (MPhC) at MIT OVERVIEW: Involves 50 faculty members from 9 departments with their research related to photonics VISION: The goal of the Center is the creation of new materials, structures and architecturesto enable the evolution of photonics from single, discrete devices to integrated photonic systems. RELATION WITH INDUSTRY: Communication Technology Roadmap Consortium has 16 industrial members, including IBM, Intel, HP, NEC, Siemens…… Research • Roadmapping • Infrastructure E-P synergy - Integration - Standardization - Cross-market Platforms Power - Bandwidth - Latency - Footprint - Package - Cost

4. Integrated Photonics on Si vs. Free Space Optics

5. Ge EA modulator Tunable MZI filter Deposited waveguide Vertical coupler Silicon EO modulator MMI splitter Oxide Undeclad W Studs Gate Contact EPIC: Integrated Photonic RF Channelizer on Si • Integrated photonic chip enables efficient RF channelizing with small footprint, low power consumption and low EMI. • Can we transfer integrated photonics to THz technology? c-Si waveguide Ge photodetector

6. Ultra-low Energy, Integrated GeSi Electroabsorption Modulators(Modulation in Optical Domain)

7. Modulation: Optics Domain vs. THz Domain Mode in optical domain, λ=1.55 µm Mode in THz domain, (λ=100 µm, or 3 THz) • 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.

8. Electro-Optical vs. Electro-AbsorptionModulators Pin Pout(V) V MZI modulators -V EO modulators Pin Pout(V) Microring modulators V Pin Pout(V) EA modulators EAM • 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!

9. / Electro-Absorption (EA) Effect in Ge-on-Si Maximum /~4 at 1460 nm Quantum Confined Stark Effect (QCSE) in type I Ge quantum wells Y.-H. Kuo et al, Nature 437,1334, (2005) Franz-Keldysh (FK) effect in tensile strained Ge-on-Si Jongthammanurak et al, APL 89, 16115, (2006) • FK effect in tensile strained Ge shows maximum absorption contrast /~4-5at 1647 nm • QCSE in Ge QWs shows maximum /~4 at ~1460 nm

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)

11. Design of GeSi EAM Device Structure a-Si GeSi a-Si on a-Si GeSi a-Si off Tapered vertical coupler • 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 Liu et al, Opt. Express. 15, 623-628 (2007)

12. CMOS GeSi EAM 2.0 µm GeSi GeSi Integration of GeSi EAMs into CMOS Process ► GeSi grown between front and backend of CMOS process for electronic-photonic integration. ►Two-step UHVCVD GeSi selective growth: (1) a 30-60nm GeSi buffer at 360C; (2) rest of the growth at 600-700C Annealing at 800-850C decreases dislocation density to ~107/cm2 CMP to remove top facets GeSi M. Beals et al, Proc. SPIE. 6898, 689804 (2008)

13. Extinction 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)

14. Vpp=3 V Modulation 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 Liu et al, Nature Photonics. 2, 433-437 (2008)

15. Energy Consumption • 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)

16. Bandwidth 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. Liu et al, Nature Photonics. 2, 433-437 (2008)

17. GeSi EAM Performance Summary • Very small footprint (30 µm2) • Ultra-low energy consumption (50 fJ/bit) • >10 dB Extinction ratio • GHz bandwidth. Great potential for >100 GHz • Operation spectrum width covering half of the C-band for on-chip WDM.

18. Integrated Photonic Devices for Sub-THz and THz Generation

19. Sub-THz Generation by Heterodyne Photomixing NIR Laser f-fTHz THz photocurrent wave photomixing Ultrafast photodiode+Antenna NIR Laser f+fTHz 1/fTHz • 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)

20. Photocurrent Waveguide-Integrated: h + + + Photodetector WG - - - Dark current • 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 Benefits of Waveguide-Integrated Photodetectors Photocurrent h Free-space coupled: Photo- detector Dark current - +

21. Waveguide-coupled GeSi Photodetectors EPIC device Design for sub-THz operation: Material: pure Ge-on-Si Electron mobility: 4000 cm2 V/s Length=4 µm, Width=0.5 µm Ge thickness=0.1 µm Efficiency>80% Bandwidth (3dB)~0.4 THz for p-i-n diode; ~1 THz for uni-travelling-carrier photodiodes (UTC-PDs) with electron velocity overshoot. • >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.

22. EnhancedNonlinear Optical Effects enabled by Integrated Photonics 500 nm Optical power density in Si WG =106 W/cm2 at just 1 mW optical input! Si Raman lasers Rong et al, Nature 433, p292 (2005) Four wave mixing in Si WGs Fosteret al, Nature 441, p960 (2006) • 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)

23. Conclusions • 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

24. Room Temperature PL and Optical Gain from Band-Engineered Ge