Nanophotonics -

The Emergence of a New Paradigm

Richard S. QuimbyDepartment 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

Electronics Photonics

Tubes & transistors Fiber optics

discreet components1970’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

decr

easi

ng s

ize

Electronics Photonics

f ~ 10 Hz f ~ 10 Hz10 15

wirefiber

sig in sig out

v ~ 10 m/s v ~ 10 m/selecphot

control beam

Strong elec-elec interaction Weak phot-phot interaction

58

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

fibe

r at

tenu

atio

n

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 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

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 indexcore

lower indexcladding

lower indexcladding

• 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

ener

gy 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, www.ee.ucla.edu/~pbmuri/

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 fiberafter 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 - n2

2 2core 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/