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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

Nanophotonics -

Richard S. Quimby

Department of Physics

Worcester Polytechnic Institute

The Emergence of a New Paradigm

slide2

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

slide3

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

slide4

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

advantages of fiber optic communications
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

slide9

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

slide10

Erbium doped glass

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

slide11

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

fiber attenuation

wavelength

raman fiber amplifier
Raman fiber amplifier

hn

scattered

hn

pump

hf

vibration

Signal in

Signal out

* amplification by stimulated scattering

* nonlinear process: requires high pump power

slide14

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
information capacity of fiber
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

slide19

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
slide22

Other ways to separate wavelengths for WDM

Or, can use a blazed diffraction grating to spatially disperse the light:

slide23

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.

advantages of integrated optic circuits
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
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
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

slide27

Light modulation in lithium niobate integrated optic circuit

From Jeff Hecht, Understanding Fiber Optics (Prentice Hall 1999)

slide28

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)

slide29

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)

slide30

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

photonic crystals the new paradigm
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
slide32

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
slide33

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

early history of photonic bandgaps
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
Intuitive picture of PBG

After Yablonovitch, Scientific American Dec. 2001

first pbg material yablonovite
First PBG material: yablonovite

require n > 1.87

After Yablonivitch, www.ee.ucla.edu/~pbmuri/

possible pbg structures
Possible PBG structures

after Yablonovitch, Scientific American Dec. 2001

prospects for 3 d pbg structures
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
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
2-D Photonic Crystals

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

propagation along line defect

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
optical confinement at point defect
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/

photonic crystal fibers
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)

small core holey fiber
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

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
holey fiber with hollow core
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)

holey fiber with large hollow core
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
implications for education
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
Prospects for nanophotonics

after Dowling, home.earthlink.net/~jpdowling/pbgbib.html

after Joannopoulos, jdj.mit.edu/

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