Nanophotonics -

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Nanophotonics -

• The Emergence of a New Paradigm

Richard S. Quimby Department of Physics Worcester
Polytechnic Institute
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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
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Electronics
Photonics
Fiber optics discreet components
1970s
Tubes transistors
1960s
1970s
Planar optical waveguides
Integrated circuits
1980s
decreasing size
1980s
VLSI
Integrated optical circuits
2000s
1990s
Molecular electronics
Photonic crystals
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Electronics
Photonics
fiber
wire
10
15
f 10 Hz
f 10 Hz
sig in
sig out
control beam
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v 10 m/s
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v 10 m/s
elec
phot
Strong elec-elec interaction
Weak phot-phot interaction
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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
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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
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Erbium doped glass
After Miniscalco, in Rare Earth Doped Fiber
Lasers and Amplifiers, M. Digonnet ed.,( Marcel
Dekker 1993)
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after Jeff Hecht, Understanding Fiber Optics,
(Prentice-Hall, 1999)
fiber attenuation
wavelength
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Raman fiber amplifier
hn
scattered
hn
pump
hf
vibration
Signal in
Signal out
amplification by stimulated scattering
nonlinear process requires high pump power
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• 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

Raman amplifier gain spectrum
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Wavelength Division Multiplexing
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Information capacity of fiber
Spectral efficiency (bit rate)/(channel
spacing)
(BR)/(10 BR) 0.1 bps/Hz conservative
In C-band (1530 lt ? lt 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
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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

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How to make fiber gratings
or
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Using fiber Bragg gratings for WDM
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Other ways to separate wavelengths for WDM
Or, can use a blazed diffraction grating to
spatially disperse the light
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The increasing importance of integrated optics
t/(18 mo.)
Electronic processing speed 2
(Moores 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.
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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

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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

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

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

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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 LEDs, no-threshold
  lasers
• modify angular distribution of emitted light

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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
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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

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Intuitive picture of PBG
After Yablonovitch, Scientific American Dec. 2001
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First PBG material yablonovite
require ?n gt 1.87
After Yablonivitch, www.ee.ucla.edu/pbmuri/
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Possible PBG structures
after Yablonovitch, Scientific American Dec. 2001
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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)

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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

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

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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/
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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)
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Small-core holey fiber
after Knight, Optics Photonics News, March 2002

• effective index of cladding is close to that of
  air (n1)
• anomalous dispersion (Dgt0) over wide ? range,
  including visible (enables soliton transmission)
• can taylor zero-dispersion ? for phase-matching
  in non-linear optical processes (ultrabroad
  supercontinuum)

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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 Vlt2.405 (endlessly
  single-mode)
• single-mode for wide range of core sizes

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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, Science 282, 1476 (1998)
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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

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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

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