Download Nanophotonics Lecture 1 - Groups

Introduction to nanophotonics
Alexey Belyanin
Department of Physics,
Texas A&M University
Outline
• What is nanophotonics?
– motivation
•
•
•
•
•
Principles of light guiding and confinement
Photonic crystals
Plasmonics
Optical chips and integrated photonics
Bio-nanophotonics
– Biosensors, nanoshells, imaging, therapy
• Terahertz photonics
• Exotic stuff: negative index materials, quantum
optics of semiconductor nanostructures, etc.
Nanophotonics: control of light at (sub-)wavelength scale
near-IR: 700-2000 nm
Optical communications window: 1300-1600 nm
(Why?)
Sub-wavelength scale = nanoscale for visible/near-IR light
Violates fundamental laws of diffraction??
Not applicable to near field
Not applicable to mixed photon-medium excitations: polaritons, plasmons
What kind of medium can carry
optical frequencies?
Air? Only within line of sight;
High absorption and scattering
Optical waveguides are necessary!
Copper coaxial cable? High absorption,
narrow bandwidth 300 MHz
Glass? Window glass absorbs 90% of light after 1 m.
Only 1% transmission after 2 meters.
Extra-purity silica glass?!
Loss per km, dB
Loss in silica glasses
What is dB?
Increase by 3 dB
corresponds to
doubling of power
Maximum tolerable loss
Wavelength, nm
Transmisson 95.5% of power after 1 km
P = P(0) (0.955)N after N km
P = 0.01 P(0) after 100 km: need amplifiers and repeaters
Total bandwidth ~ 100 THz!!
Optical fibers
Made by drawing molten glass from
a crucible
1965: Kao and Hockham proposed
fibers for broadband communication
1970s: commercial methods of
producing low-loss fibers by
Corning and AT&T.
1990: single-mode fiber,
capacity 622 Mbit/s
Now: capacity ~ 1Tbit/s, data rate
10 Gbit/s
Fibers opened the flood gate
Bandwidth 400 THz would allow 400
million channels with 2Mbits/sec
download speed!
Each person in the U.S. could have
his own carrier frequency, e.g.,
185,674,991,235,657 Hz.
Limitations of optical communications
In optical communications, information is transmitted over long distances
along optical fibers
However, if we want to modify, add/drop, split, or amplify signal, it needs to
be first converted to electric current, and then converted back to photons
Electronic circuits: 45 nm wires, 1 million transistors per mm2
Computing is based on controlling transport and storage of electric charges
Computing speed is limited by inertia of electrons
The interconnect bottleneck
• 109 devices per chip
• Closely spaced metal wires lead to RC delay
• Huge power dissipation due to Ohmic losses
Can electronic circuits and transmission channels be
replaced by photonic ones?!
Using photons as bits of information instead of electrons
would revolutionize data processing, optical
communications, and possibly computing
What is wrong with using electric current instead of photonic beams?
Good: electrons are small; devices are potentially scalable to a size of a
single molecule
Bad: electric current cannot be changed or modulated fast enough. Speed
is limited to nanosecond scale by circuit inductance and capacitance.
As a result, data rate is limited to a few Gb/s and transmission bandwidth
to a few GHz.
Photons travel much faster and don’t dissipate as much power
THE DREAM: could we replace electric signal processing by all-optical
signal processing?
IBM website
Futuristic silicon chip with monolithically integrated photonic and
electronic circuits
This hypothetic chip performs all-optical routing of mutliple N optical
channels each supporting 10Gbps data stream. N channels are first
demultiplexed in WDM photonic circuit, then rearranged and switched
in optical cross-connect OXC module, and multiplexed back into
another fiber with new headers in WDM multiplexer. Data packets are
buffered in optical delay line if necessary. Channels are monitored with
integrated Ge photodetector PD. CMOS logical circuits (VLSI) monitor
the performance. Electrical pads are connecting the optoelectronic chip
to other chips on a board via electrical signals.
However, dimension of optical “wires” is much larger than that of electric wires
Or optical fiber cross-section
We need to confine light to at least 1020 times smaller size than the fiber
diameter
What is the minimum confinement
scale for light at a given wavelength?
• Wave equation
• Confinement in a metal box
• Total internal reflection
EM waves in a bulk isotropic medium
E

k
k
H

c
Phase velocity
n
 - relative dielectric
permittivity;
n   refractive index

   
E, H  E0 , H 0 cos(kr  t  0 )
2c 0
k
; k
 

c

n n
n
2
Note: wavelength in a
medium is n times shorter
than in vacuum
How to confine light with
transparent material??
Total internal reflection!
Water: critical angle ~ 49o
Total internal reflection
n1 > n2
Dielectric waveguides
n > n’
What is the minimum size of the mode confined by TIR?
Basic waveguide geometries
Dielectric waveguides are used in all semiconductor lasers
For integrated photonic circuits we need to use silicon
and CMOS-compatible technology
Silicon on insulator waveguides
nc=1
nw=3.6
ns=1.5
The dream
No silicon lasers or amplifiers (why?)
No silicon detectors at wavelengths 1.3-1.6 m (why?)
Why there are no silicon lasers
k1 = k2 + kph ; kph << k1,2
k1 ~ k2
Only vertical (in k-space) transitions
are allowed
Only direct gap semiconductors are
optically active
k1
k2
Silicon
GaAs
SiO2 doped with active erbium ions and with silicon nanocrystals
From L. Pavesi talk 2005
Only simple devices have been built so far:
Modulators, beam splitters, etc.
Possible uses:
Rack-to-rack,
Board-to-board,
Chip-to-chip
connections
Beam A
Intel silicon
photonic modulator
Beam B
Modulation of light using nonlinear optics: dependence of the
refractive index from light intensity I (Kerr effect)
0
 n 
E ~ exp i
z
 c 
By changing n2, we can shift phases of the
beams A and B with respect to each other:
n  n  n2 I
 

c
nA  nB z
Coupling light into a thin film waveguide can be a problem
Coupling a 5-m diameter beam
from fiber tip into 0.4-m thin film
(Intel)
Tapered channel
grating
Guiding light in a low-index core?!
Almeida. OL 2004
Central region is 50 nm, but evanescent field
still extends to about 500 nm
Evanescent field can be used for inter-mode coupling and for sensors
Intel
Cornell group Nature 2004
Evanescent field sensors with substrate sensitized to a specific
molecule
Adsorbed molecules change the excitation angle of EM mode
Can we do better than a thin film dielectric waveguide
(mode size about 0.5 m, bending radius a few m)?
Photonic crystals!
Periodic modulation of dielectric constant blocks the
transmission of light at certain frequencies
One dimensional photonic crystal: Bragg grating
d
k 2d  2m, m  1,2,...
m
2d
k
, or  
d
m
Bragg reflection
Yablonovitch, Sci.Am. 2001
Yablonovitch, Sci.Am. 2001
Photon momentum conservation
d
2
Kg 
d
kin
When Kg = 2kin: incoming wave
is reflected
+
Kg
=
kout
Photonic band gap is formed
Light is blocked at certain
frequencies: PBG
Group velocity tends to 0 at the
edge of PBG -> enhancement of
light intensity
n1
n2
Yablonovitch, Sci.Am. 2001
“Photonic crystals – semiconductors of light”
Semiconductors
Periodic crystal lattice:
Potential for electrons
Length scale ~ 3-6 A
Photonic crystals
Periodic variation of dielectric constant
Length scale ~ 
Natural structures
Control electron states
and transport
From M. Florescu talk (JPL)
Artificial structures
Control EM wave propagation
and density of states
Natural opals
Striking colors even in the absence of pigments
From M. Florescu talk (JPL)
Yablonovitch, Sci.Am. 2001
Artificial Photonic Crystals
Requirement: overlapping of frequency gaps along different directions
 High ratio of dielectric indices
 Same average optical path in different media
 Dielectric networks should be connected
Woodpile structure
S. Lin et al., Nature (1998)
From M. Florescu talk
Inverted Opals
J. Wijnhoven & W. Vos, Science (1998)
Some 3D crystal designs
based on diamond lattice
By the way, why we don’t see photonic
band gap in all crystals?
Yablonovitch, Sci.Am. 2001
Photonic crystals can reflect light very efficiently.
How to make them confine and guide light?
Introduce a defect into the periodic structure!!
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•
Creates an allowed photon state in the photonic band gap
Can be used as a cavity in lasers
or as a microcavity for a “thresholdless” microlaser
1D structure with defect: Vertical Cavity Surface-Emitting Laser (VCSEL)
Edge-emitting laser
VCSEL
2D structure: photonic crystal fiber
Extra tight mode confinement, high mode intensity, high nonlinearity
First commercial all-optical interconnect based on PC fibers
(Luxtera)
Photonic circuits?
From Florescu talk
Intel
Note T-intersections and tight
bends, as in electric wires.
You cannot achieve it in dielectric
waveguides!