Nanophotonics - The Emergence of a New Paradigm Richard S. Quimby Department of Physics Worcester Polytechnic Institute

Outline 1. Overview: Photonics vs. Electronics 2. Fiber Optics: transmitting information 3. Integrated Optics: processing information 4. Photonic Crystals: the new paradigm 5. Implications for Education

ElectronicsPhotonics Tubes & transistors Fiber optics discreet components 1970’s Integrated circuits VLSI Molecular electronics Planar optical waveguides Integrated optical circuits Photonic crystals 1960’s 1980’s 2000’s 1970’s 1980’s 1990’s decreasing size

ElectronicsPhotonics f ~ 10 Hz 1015 wire fiber sig in sig out v ~ 10 m/s elec phot control beam Strong elec-elec interactionWeak phot-phot interaction 5 8

Advantages of Fiber Optic Communications * Immunity to electrical interference -- aircraft, military, security * Cable is lightweight, flexible, robust -- efficient use of space in conduits * Higher data rates over longer distances -- more “bandwidth” for internet traffic

Erbium Doped Fiber Amplifiers * Compatible with transmission fibers * No polarization dependence * Little cross-talk between channels * Bit-rate and format transparent * Allows wavelength multiplexing (WDM) Advantages: Disadvantages: * Limited wavelength range for amplification

After Miniscalco, in Rare Earth Doped Fiber Lasers and Amplifiers, M. Digonnet ed.,( Marcel Dekker 1993) Erbium doped glass

fiber attenuation wavelength after Jeff Hecht, Understanding Fiber Optics, (Prentice- Hall, 1999)

Raman fiber amplifier h h hf pump scattered vibration Signal in Signal out * amplification by stimulated scattering * nonlinear process: requires high pump power

Can choose pump for desired spectral gain region typical gain bandwidth is nm (~5 THz) gain efficiency is quite low (~0.027 dB/mW) compare gain efficiency of EDFA (~5 dB/mW) need high pump power (~1 W in single-mode fiber) need long interaction lengths: distributed amplification Raman amplifier gain spectrum

Wavelength Division Multiplexing

Information capacity of fiber Spectral efficiency = (bit rate)/(channel spacing) In C-band (1530 < < 1560 nm),  f ~ 3800 GHz Compare: for all radio, TV, microwave,  f  1 GHz Max data rate in fiber = (0.1)(3800 GHz) = 380 Gbs # phone calls = (380 Gb/s) / (64 kbs/call) ~ 6 million calls Spectral efficiency can be as high as 0.8 bps/Hz = (BR)/(10 BR) = 0.1 bps/Hz [conservative] L-band and S-band increase capacity further

Fiber Bragg Gratings Periodic index of refraction modulation inside core of optical fiber: Strong reflection when  = m( /2) Applications: WDM add/drop mirrors for fiber laser wavelength stabilization/control for diode and fiber lasers

How to make fiber gratings: or:

Using fiber Bragg gratings for WDM

Other ways to separate wavelengths for WDM Or, can use a blazed diffraction grating to spatially disperse the light:

The increasing importance of integrated optics * Electronic processing speed ~ 2 (Moore’s Law) * Optical fiber bit rate capacity ~ 2 * Electronic memory access speed ~ (1.05) Soon our capacity to send information over optical fibers will outstrip our ability to switch, process, or otherwise control that information. t/(18 mo.) t/(10 mo.) t/(12 mo.)

Advantages of Integrated-Optic Circuits: Small size, low power consumption Efficiency and reliability of batch fabrication Higher speed possible (not limited by inductance, capacitance) parallel optical processing possible (WDM) Substrate platform type: Hybrid -- (near term, use existing technology) Monolithic -- (long term, ultimately cheaper, more reliable) quartz, LiNbO, Si, GaAs, other III-V semiconductors

Challenges for all-optical circuits High propagation loss (~1 dB/cm, compared with ~1 dB/km for optical fiber) coupling losses going from fiber to waveguide photons interact weakly with other photons -- need large (cm scale) interaction lengths difficult to direct light around sharp bends (using conventional waveguiding methods) electronics-based processing is a moving target

Recent progress toward monolithic platform Recently developed by Motorola (2001) strontium titanate layer relieves strain from 4.1% lattice mismatch between Si and GaAs good platform for active devices (diode lasers, amps) Silicon monolithic platform Strontium titanate layer GaAs devices

Light modulation in lithium niobate integrated optic circuit From Jeff Hecht, Understanding Fiber Optics (Prentice Hall 1999)

after Jeff Hecht, Understanding Fiber Optics (Prentice Hall 1999) Arrayed Waveguide Grating for WDM * Optical path length difference depends on wavelength * silica-on-silicon waveguide platform * good coupling between silica waveguide and silica fiber

after Jeff Hecht, Understanding Fiber Optics (Prentice Hall 1999) Echelle gratings as alternative for WDM * advances in reactive-ion etching (vertical etched facets) * use silica-on-silicon platform * smaller size than arrayed-waveguide grating * allows more functionality on chip

Confinement of light by index guiding higher index core lower index cladding lower index cladding need high index difference for confinement around tight bends index difference is limited in traditional waveguides limited bending radius achieved in practice -- thermal diffusion of Ti (  n~ 0.025) -- ion exchange (p for Li) (  n~ 0.15) -- ion implantation (  n~ 0.02) Examples for Lithium Niobate:

Photonic crystals: the new paradigm light confinement by photonic band-gap (PBG) no light propagation in PBG “cladding” material index of “core” can be lower than that of “cladding” light transmitted through “core” with high efficiency even around tight bends

Modified spontaneous emission First discussed by Purcell (1946) for radiating atoms in microwave cavities decay rate  #modes/(vol  f) if there are no available photon modes, spontaneous emission is “turned off” more efficient LED’s, “no-threshold” lasers modify angular distribution of emitted light

Photonic Bandgap (PBG) Concept e Electron moving through array of atoms in a solid energy bandgap Photon moving through array of dielectric objects in a solid

Early history of photonic bandgaps Proposed independently by Yablonovitch (1987) and John (1987) trial-and-error approach yielded “pseudo-PBG” in FCC lattice Iowa State Univ. group (Ho) showed theoretically that diamond structure (tetrahedral) should exhibit full PBG first PBG structure demonstrated experimentally by Yablonovitch (1991) [holes drilled in dielectric: known now as “yablonovite”] RPI group (Haus, 1992) showed that FCC lattice does give full PBG, but at higher photon energy

Intuitive picture of PBG After Yablonovitch, Scientific American Dec. 2001

First PBG material: yablonovite After Yablonivitch, require  n > 1.87

Possible PBG structures after Yablonovitch, Scientific American Dec. 2001

Prospects for 3-D PBG structures Difficult to make (theory ahead of experiment)  top down approach: controllable, not easily scaleable  bottom up approach (self-assembly): not as controllable, but easily scaleable Naturally occuring photonic crystals (but not full PBG)  butterfly wings  hairs of sea mouse  opals (also can be synthesized)

Photonic bandgap in 2-D Fan and Joannopoulos (MIT), 1997  planar waveguide geometry  can use same thin-film technology that is currently used for integrated circuits  theoretical calculations only so far Knight, Birks, and Russell (Univ. of Bath, UK), 1999  optical fiber geometry  use well-developed technology for silica-based optical fibers  experimental demonstrations

2-D Photonic Crystals After Joannopuolos, Photonic Crystals: Molding the flow of light, (Princeton Univ. Press, 1995)

Propagation along line defect light in light out after Mekis et al., Phys. Rev. Lett. 77, 3787 (1996) defect: remove dielectric material analogous to line of F-centers (atom vacancies) for electronic defect E field confined to region of defect, cannot propagate in rest of material high transmission, even around 90 degree bend light confined to plane by usual index waveguiding

Optical confinement at point defect after Joannopoulos, jdj.mit.edu/ defect: remove single dielectric unit analogous to single F-center (atom vacancy) for electronic defect very high-Q cavity resonance strongly modifies emission from atoms inside cavity potential for low-threshold lasers

Photonic Crystal Fibers after Birks, Opt. Lett. 22, 961 (1997) “holey” fiber stack rods & tubes, draw down into fiber variety of patterns, hole width/spacing ratio guiding by: -effective index -PBG

Small-core holey fiber after Knight, Optics & Photonics News, March 2002 effective index of “cladding” is close to that of air (n=1) anomalous dispersion (D>0) over wide range, including visible (enables soliton transmission) can taylor zero-dispersion for phase-matching in non- linear optical processes (ultrabroad supercontinuum)

Large-core holey fiber d  V = a  n - n 22 22 core clad after Knight, Optics & Photonics News, March 2002 effective index of “cladding” increases at shorter results in V value which becomes nearly independent of single mode requires V<2.405 (“endlessly single-mode”) single-mode for wide range of core sizes

Holey fiber with hollow core after Knight, Science 282, 1476 (1998) air core: the “holey” grail confinement by PBG first demonstrated in honeycomb structure only certain wavelengths confined by PBG propagating mode takes on symmetry of photonic crystal

Holey fiber with large hollow core after Knight, Optics & Photonics News, March 2002 high power transmission without nonlinear optical effects (light mostly in air) losses now ~1 dB/m (can be lower than index-guiding fiber, in principle) small material dispersion Special applications: guiding atoms in fiber by optical confinement nonlinear interactions in gas-filled air holes

Implications for education fundamentals are important physics is good background for adapting to new technology photonics is blurring boundaries of traditional disciplines At WPI: - new courses in photonics, lasers, nanotechnology - new IPG Photonics Laboratory (Olin Hall 205)  integrate into existing courses  developing new laboratory course

Prospects for nanophotonics after Dowling, home.earthlink.net/~jpdowling/pbgbib.html after Joannopoulos, jdj.mit.edu/