Download - Introduction to nanophotonics Alexey Belyanin Department of Physics, Texas A&M University
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Introduction to nanophotonicsAlexey Belyanin
Department of Physics,Texas A&M University
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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.
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Nanophotonics: control of light at (sub-)wavelength scale
near-IR: 700-2000 nmOptical communications window: 1300-1600 nm(Why?)
Sub-wavelength scale = nanoscale for visible/near-IR light
Violates fundamental laws of diffraction??
Not applicable to near fieldNot applicable to mixed photon-medium excitations: polaritons, plasmons
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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?!
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Loss
per
km
, dB
Wavelength, nm
Maximum tolerable loss
Transmisson 95.5% of power after 1 km P = P(0) (0.955)N after N kmP = 0.01 P(0) after 100 km: need amplifiers and repeaters
Total bandwidth ~ 100 THz!!
Loss in silica glassesWhat is dB? Increase by 3 dB corresponds to doubling of power
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Optical fibersMade 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
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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.
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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
Limitations of optical communications
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Electronic circuits: 45 nm wires, 1 million transistors per mm2
Computing is based on controlling transport and storage of electric chargesComputing speed is limited by inertia of electrons
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The interconnect bottleneck• 109 devices per chip• Closely spaced metal wires lead to RC delay• Huge power dissipation due to Ohmic losses
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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
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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.
THE DREAM: could we replace electric signal processing by all-optical signal processing?
IBM website
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Or optical fiber cross-section
However, dimension of optical “wires” is much larger than that of electric wires
We need to confine light to at least 10-20 times smaller size than the fiber diameter
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What is the minimum confinement scale for light at a given wavelength?
• Wave equation• Confinement in a metal box• Total internal reflection
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EM waves in a bulk isotropic medium
- relative dielectric permittivity;
n refractive index
nc
k
Phase velocity
nnck
cnk 022;
E
Hk
)cos(,, 000 trkHEHE
Note: wavelength in a medium is n times shorter than in vacuum
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How to confine light with transparent material??
Total internal reflection!
Water: critical angle ~ 49o
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Total internal reflection
n1 > n2
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Dielectric waveguides
n > n’
What is the minimum size of the mode confined by TIR?
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Basic waveguide geometries
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Dielectric waveguides are used in all semiconductor lasers
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Silicon on insulator waveguides
nc=1
nw=3.6
ns=1.5
For integrated photonic circuits we need to use silicon and CMOS-compatible technology
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The dream
No silicon lasers or amplifiers (why?)No silicon detectors at wavelengths 1.3-1.6 m (why?)
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k1
k2
Why there are no silicon lasers
k1 = k2 + kph ; kph << k1,2
k1 ~ k2
Only vertical (in k-space) transitions are allowed
Silicon GaAs
Only direct gap semiconductors are optically active
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SiO2 doped with active erbium ions and with silicon nanocrystals
From L. Pavesi talk 2005
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Intel silicon photonic modulator
Only simple devices have been built so far:Modulators, beam splitters, etc.
Modulation of light using nonlinear optics: dependence of the refractive index from light intensity I (Kerr effect) Innn 20
By changing n2, we can shift phases of the beams A and B with respect to each other:
zcniE exp~
Beam A
Beam B
znnc BA
Possible uses:Rack-to-rack, Board-to-board,Chip-to-chipconnections
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Coupling light into a thin film waveguide can be a problem
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Coupling a 5-m diameter beam from fiber tip into 0.4-m thin film(Intel)
Tapered channel
grating
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Almeida. OL 2004
Guiding light in a low-index core?!
Central region is 50 nm, but evanescent field still extends to about 500 nm
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Evanescent field can be used for inter-mode coupling and for sensors
Cornell group Nature 2004
Intel
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Evanescent field sensors with substrate sensitized to a specific molecule
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Adsorbed molecules change the excitation angle of EM mode
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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
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Yablonovitch, Sci.Am. 2001
One dimensional photonic crystal: Bragg grating
d
,...2,1,22 mmdk
md
dmk 2or,
Bragg reflection
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Yablonovitch, Sci.Am. 2001
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Photon momentum conservationd
kin
dK g
2
=kout
+Kg
When Kg = 2kin: incoming wave is reflected
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Photonic band gap is formed
n1 n2
Light is blocked at certain frequencies: PBG
Group velocity tends to 0 at the edge of PBG -> enhancement of light intensity
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Yablonovitch, Sci.Am. 2001
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Photonic crystals
Periodic variation of dielectric constant
Length scale ~
Artificial structures
Control EM wave propagation and density of states
Periodic crystal lattice:Potential for electrons
Length scale ~ 3-6 A
Natural structures
Control electron states and transport
Semiconductors“Photonic crystals – semiconductors of light”
From M. Florescu talk (JPL)
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Natural opals
Striking colors even in the absence of pigmentsFrom M. Florescu talk (JPL)
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Yablonovitch, Sci.Am. 2001
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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
J. Wijnhoven & W. Vos, Science (1998)J. Wijnhoven & W. Vos, Science (1998)S. Lin et al., Nature S. Lin et al., Nature (1998)(1998)
Woodpile structureWoodpile structure Inverted OpalsInverted Opals
Artificial Photonic Crystals
From M. Florescu talk
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Yablonovitch, Sci.Am. 2001
Some 3D crystal designs based on diamond lattice
By the way, why we don’t see photonic band gap in all crystals?
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Photonic crystals can reflect light very efficiently.How to make them confine and guide light?
Introduce a defect into the periodic structure!!
• 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
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1D structure with defect: Vertical Cavity Surface-Emitting Laser (VCSEL)
Edge-emitting laser
VCSEL
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2D structure: photonic crystal fiber
Extra tight mode confinement, high mode intensity, high nonlinearity
First commercial all-optical interconnect based on PC fibers(Luxtera)
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Photonic circuits?
Intel
Note T-intersections and tight bends, as in electric wires.You cannot achieve it in dielectric waveguides!
From Florescu talk