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Nanophotonics -

Nanophotonics -. Richard S. Quimby Department of Physics Worcester Polytechnic Institute. The Emergence of a New Paradigm. Outline. 1. Overview: Photonics vs. Electronics 2. Fiber Optics: transmitting information 3. Integrated Optics: processing information

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Nanophotonics -

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

  2. 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

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

  4. Electronics Photonics fiber wire 10 15 f ~ 10 Hz f ~ 10 Hz sig in sig out control beam 5 v ~ 10 m/s 8 v ~ 10 m/s elec phot Strong elec-elec interaction Weak phot-phot interaction

  5. 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

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

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

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

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

  10. Raman amplifier gain spectrum • Can choose pump  for desired spectral gain region • typical gain bandwidth is 30-40 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

  11. Wavelength Division Multiplexing

  12. Information capacity of fiber Spectral efficiency = (bit rate)/(channel spacing) = (BR)/(10 BR) = 0.1 bps/Hz [conservative] 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 L-band and S-band increase capacity further

  13. 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

  14. How to make fiber gratings: or:

  15. Using fiber Bragg gratings for WDM

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

  17. The increasing importance of integrated optics t/(18 mo.) * Electronic processing speed ~ 2 (Moore’s Law) t/(10 mo.) * Optical fiber bit rate capacity ~ 2 t/(12 mo.) * 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.

  18. 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

  19. 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

  20. Recent progress toward monolithic platform GaAs devices • 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) Strontium titanate layer Silicon monolithic platform

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

  22. 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)

  23. 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 after Jeff Hecht, Understanding Fiber Optics (Prentice Hall 1999)

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

  25. 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

  26. 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

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

  28. 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

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

  30. First PBG material: yablonovite require n > 1.87 After Yablonivitch, www.ee.ucla.edu/~pbmuri/

  31. Possible PBG structures after Yablonovitch, Scientific American Dec. 2001

  32. 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)

  33. 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

  34. 2-D Photonic Crystals After Joannopuolos, Photonic Crystals: Molding the flow of light, (Princeton Univ. Press, 1995)

  35. after Mekis et al., Phys. Rev. Lett. 77, 3787 (1996) light out light in Propagation along line defect • 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

  36. Optical confinement at point defect • 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 after Joannopoulos, jdj.mit.edu/

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

  38. 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)

  39. 2 2 2 V = a  n - n  clad core Large-core holey fiber after Knight, Optics & Photonics News, March 2002 d  • 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

  40. Holey fiber with hollow core • 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 after Knight, Science282, 1476 (1998)

  41. Holey fiber with large hollow core • 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 after Knight, Optics & Photonics News, March 2002 • Special applications: • guiding atoms in fiber by optical confinement • nonlinear interactions in gas-filled air holes

  42. 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

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

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