Silicon Nanophotonics: a Current Status

Silicon Nanophotonics: A current status

A. J. Metcalf Nathaniel Kinsey

2013 Fall ECE 695 Nanophotonics

2 Outline

Silicon photonics studies the generation, transmission, modulation, processing, and detection of light using silicon as a medium

• Introduction ▫ The main motivation behind Si nanophotonics ▫ Goals for success

• Optical Integration for computing and interconnects ▫ Challenges ▫ Devices ▫ Progress and examples

• Applications beyond computing ▫ Nonlinear optics ▫ Biology/Medicine ▫ Security ▫ Quantum Optics

• Conclusions

www.washington.edu

3 Lots of Data Estimated 15 Billion connected devices by 2015 -Intel

Medical -EEtimes Imaging

Vehicle Network infotainment Applications

Digital Security Media Surveillance

www.ppinfosolutions.com Test and Digital Measurement signage

apps.isimpleness.com 4 Motivation Old bottleneck =Transistor Size New bottleneck = Speed of information transfer

Purdue Supercomputer # of cores Required Databus BW

Transistor Count Vs. Clock Speed RC delay Smaller device Noise dimensions Power Loss

Tilera GX100 (100 core processor) 5 Photonic Solutions http://www.cs4fn.org Advantage of Light Signals have large bandwidth Data travels at speed of light Less power dissipation Less cross-talk

• Electro-Optic integration Long Haul Communication ▫ Electronics Optical fiber  Fully integrated in Si ▫ Photonics  Large footprint  Mostly discrete components ▫ Plasmonics  Less mature then Photonics Envisioned  Small footprint Fully  High speed integrated  Additional functionality to devices “superchip”

Gerhard Abstreiter, Physics World (1992) 6

Challenges for Photonic integration

CMOS Compatible • Monolithic integration Low cost material Easy Difficult Infrastructure in place Electronics Modulators Waveguides Photodetectors Multiplexers Light sources Properties of Si •High refractive index •No EO effect •Small Kerr effect •High thermo-optic coef.

• Hybrid integration[near term] Si Discrete components added to common substrate InP Example: InP laser bonded to Si substrate 7 Roadmap • Develop ▫ Emitter ▫ Modulator ▫ Passive components ▫ Low loss waveguides

• Integrate ▫ All Active and Passive components on-chip ▫ Small Footprint

http://nanophotonics.labs.masdar.ac.ae

Leuthold, Optics & Photonics News, May 2013 8 Where are we now?

Proposed Si Silicon on Active Devices Integrated Insulator (SOI) Optical “Superchip” becomes standard switching in CMOS EO integration MZI 1970 1980 1990 2000 2010

Silicon suggested as photonic Passive devices waveguide

Lipson IBM Soref 9 Guiding Light • Diffraction Limited • High core index • Low cladding index Lecture 8, ECE 695, V. Shalaev • Extreme confinement

10 Passive devices

• Optical Diode • Filters • Bragg gratings • Mach-Zehnder interferometers Li Fan, Jian Wang, Leo Varghese et al. Science, • Directional Couplers (2012) Purdue Groups (Weiner & Qi)

TyMon Barwicz, et al. Nature Phot. (2006) IMEC (Ghent University) (MIT Groups (E Ippen & Henry Smith) 11 Light sources

Di Liang and John E. Bowers, Nature Focus Article (2010) • Indirect Bandgap • Radiative recombination of carriers • Two photon absorption (TPA) • Solutions ▫ Off-chip laser ▫ Heterogeneous on-chip integration 12 Raman Lasers

• Scattering of photon by optical phonon. • Incident light is pumped in 1) Rayleigh scattering 2) Stokes

• Small waveguide cross section (INTEL) Mario. Paniccia group, Nature (2005) ▫ lower power threshold ▫ Low power negates TPA problem

Mario Paniccia (INTEL), Nature (2007) 13 Hybrid Si Lasers All optical source

• Near term solution • Single frequency of light • Could couple with micro-ring Hyundai Park, et al. Opt. Express, (2005) J. Bowers Group UCSB

Images: J. E. Bowers, Nature (2010) 14

Modulators Fabry-Perot or ring resonator

• No Pockels effect • Kerr effect small • Thermo-optic effect (slow) • Plasma dispersion effect ▫ Free charges effect refractive index Narrow resonance Small footprint Temperature dependent Mach-Zehnder Interferometer Stringent fabrication

http://www.simof.fotonik.dtu.dk/ Large footprint Broader resonance 15 Modulator Advances

• MZI injection (IBM) • Microdisk Depletion (sandia) • MRR injection (Lipson) • MRR depletion (kotura, Sun) • MZI depletion (intel) • PhC injection (Natomi Group)

Qianfan Xu, et al. Nature (2005) (Lipson group) 16 Photodetectors Solomon Assefa, et al. Nature • Successful examples (2010) (Vlasov group) ▫ Germanium on silicon ▫ Avalanche (APD)

Monolithic germanium/silicon avalanche photodiodes with 340 GHz gain– bandwidth product

Yimin Kang, et al. Nature Photon. (2009) Intel & UCSD 17 Industry

4x (Laser + Modulator + Detector) 12.5Gb/s 50Gbs photonic link

Hybrid Lasers •DFB •InP deposited on Si wafer •1310nm •No cooling required •APD Photodiode

Industry 25Gbps per channel

Blue optical waveguides yellow copper wires

integrated optical and electrical circuit

IBM press release (10 Dec 2012) www.IBM.com 19

Other areas of interest Si

• Si is ideal material for passive integrated optical circuitry due to high refractive index • Unique properties of Si make it interesting in other areas ▫ Strong optical confinement  Enhancement of Kerr non-linear process ▫ Low cost CMOS-compatible  Easy to realize device designs  Well known fabrication techniques

20 Photonics beyond computing

• Integrated photonics has other uses beyond computing

▫ Nonlinear Optics

▫ Photonic Crystals

▫ Medicine/Biology

▫ Surfaces

▫ Photovoltaics

▫ Quantum Optics

Silicon Photonics: Nonlinear

• Stimulated Raman scattering ▫ Collision of photon with particle causes shift in λ ▫ Introducing pump & signal (shifted λ ) scattering rate is greatly increased ▫ χ(3) effect ▫ Gain → lasers, amplifiers • 1.511 – 1.591 µm tunable range • 28 nm gain bandwidth ▫ Nonlinear mixing → OPA • +2.9 dB gain

Paniccia (Intel), Nature, vol. 433, no. 20, 2005. Lipson, Nature, vol. 144, no. 22, 2006. 22

Silicon Photonics: Nonlinear • Super continuum generation

• Plasmon assisted generation ▫ Purcell enhancement

• The high fields result in emission of “hot electrons” before relaxation

Agarwal, Nat. Photon., vol. 7, 2013. 23

Silicon Photonics: Photonic Crystals

• Silicon light sources ▫ Erbium doped Si ▫ Photonic crystal cavity: large Q, small V ▫ Tune light wavelength

• Particle Trapping ▫ Spectroscopy ▫ Single molecule/atom constructor

D. Erickson, Nano Lett., vol. 10, 2010. L. Dal Negro, Appl. Phys. Lett., vol. 92, 2008. 24

Silicon Photonics: Medical/Biological

Waveguide • Biological spectroscopy Low index layer ▫ Specimen bind to sites on waveguide resonator Substrate ▫ Shifts the refractive index/resonant frequency ▫ Detected using interference/coupling Sun, Analytica Chimica Acta, vol. 620, 2008.

• Lab-on-a-chip ▫ Glucose meter ▫ Virus ▫ Cancer

Pugin, Appl. Surf. Sci., vol. 256S, 2009. Baily, Royal Society of Chem. Analyst, vol. 136, 2011. Baily, Royal Society of Chem. Analyst, vol. 136, 2011. 25

Silicon Photonics: Photovoltaics

• Silicon is the primary material for solar cell applications

• Using structured surfaces to aid the in-coupling of light • Increase the critical angle

• Increase the efficiency of solar cells

H. Keppner, Science, vol. 285, 1999. K. Crozier, ACS Nano, vol. 7, no. 6, 2013. F. Ferrazza, Appl. Phys. Lett., vol. 73, no. 14, 1998. 26

Silicon Photonics: Photovoltaics

• Primary problem with thin-film solar cells ▫ Small interaction length

• Use structured surfaces to increase the reflected angle (anomalous reflection)

• Microstructuring: Bragg grating, PCs

• Nanostructuring: meta-materials/surfaces ▫ Generalized Snell’s Law

J. Joannopoulos, Opt. Express, vol. 15, no. 25, 2007.

Silicon Photonics: Quantum Optics

• Interference of single photons can be used for calculations (analog-esque)

• Network shown is used to perform unitary matrix transformations ▫ Solution exponential on CPUs - limited to 20 parameters Walther, Nature Photonics, vol. 7, 2013. • Each coupler (beam splitter) and arm (phase shift) is chosen at random (25 independent parameters)

• Probability of exit of photons and their interference represents a matrix transformation

Walmsley, Science, vol. 339, 2013.

Silicon Photonics: Quantum Optics

• Light exhibits both wave-like and particle- like behavior.

J. Wheeler, “Mathematical Foundations of Quantum Mechanics”, Academic Press, 1978. • Traditionally though, experiment determines the state (i.e. you cannot observe both states at the same time).

• Experiment decides the state which is observed

Image from: J. L . O’Brien, Science, vol. 338, 2012. • Delayed-Choice Experiment ▫ 2nd beam splitter inserted: interference effects (waves) ▫ 2nd beam splitter removed: path-choice (particles)

Silicon Photonics: Quantum Optics • Quantum beam splitter (Hadamard logic gate) ▫ Allows a superposition of states ▫ A superimposed photon (ancilla photon) is used

• Changing the state of the incident photon α[0,π/2] the system is may be changed between photon(α=0) and wave (α=π/2)

• Unlike traditional delayed-choice experiments this cannot be predicted by classical means Fox, “Quantum Optics: An Introduction” • Illustrates simultaneous existence of both particle and wave states

J. L . O’Brien, Science, vol. 338, 2012. 30

Conclusions

Silicon has long been established as an ideal material for passive integrated optical circuitry due

• high refractive index ▫ Ideal for passive devices ▫ strong optical confinement ▫ Enhancement of Kerr non-linearity • CMOS-compatible manufacturability.

Challenges • inversion symmetry of the silicon crystal lattice ▫ No Pockels effect, makes modulation difficult • Indirect bandgap ▫ Lasing in Si is difficult • Diffraction limited ▫ Increased footprint size