Nanophotonics - The Emergence of a New Paradigm Richard S. Quimby Department of Physics Worcester Polytechnic Institute
Dec 21, 2015
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/