electro-optic materials and devices
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Electro-Optic Materials and Devices Basic Concepts: Electrons (electronics), Photons (photonics)
and Electro-Optics Basic Device Structures: Stripline devices; cascaded prism
and superprism devices; ring resonators and photonic bandgap devices.
More complex device structures Societal Impact (telecommunications, computing, aerospace) Why organics?
Bandwidth (frequency response)Electro-optic activity (signal-to-noise and dynamic range)Ease of processing and costCleaner and greener
Material design and production (The Science)Quantum mechanics—Molecular levelStatistical mechanics—Supramolecular (nanometer) levelMaterials Processing and Device Fabrication
Basics: Matter & Energy (EM Radiation)
The fundamental constituents of matter are electrons, protons, and neutrons. Electrons are negatively charged, light weight particles that interact with electromagnetic (EM) radiation. Photons are the fundamental particles of EM radiation (light, microwaves, radiowaves, etc.)
Information (Signals) is transmitted either by electrons or photons. Electrons have strong interactions with each other—a factor that results in signal degradation (noise) when high frequency signals are transmitted or signals are sent over significant distances. Photons have very weak interactions making them ideal for high frequency signal applications and long range transport (e.g., telephone communication).
Manipulation of electrons in semiconductor materials is the basis of modern electronics. Manipulation of photons is the basis of fiber optic and wireless communication.
An electro-optic material is required to convert electrical signals into optical (photonic) signals (to go between the electronic & photonic signal domains)
Electro-Optics: The Phenomena An electro-optic material (device) permits electrical and
optical signals to “talk” to each other through an “easily perturbed” electron distribution in the material. A low frequency (DC to 200 GHz) electric field (e.g., a television [analog] or computer [digital] signal) is used to perturb the electron distribution (e.g., -electrons of an organic chromophore) and that perturbation alters the speed of light passing through the material as the electric field component of light (photons) interacts with the perturbed charge distribution.
Because the speed of light is altered by the application of a control voltage, electro-optic materials can be described as materials with a voltage-controlled index of refraction.
Index of refraction = speed of light in vacuum/speed of light in material
Electro-Optic Devices: The on-ramps & interchanges of the information superhighway
The electro-optic effect can be used to transduce electrical information (signals) onto the internet (into optical signals). By slowing light down in one arm of the Mach Zehnder device shown below, the interference of light beams at the output can be controlled. Electrical information appears as an amplitude modulation on the optical transmission. This works equally well for analog or digital data.
Light InModulatedLight Out
DC bias electrodeground electrode
Substrate
RF electrode
V = d/(2n3r33L)
= optical wavelengthn = index of refractionr33 = electro-optic coefficientL = interaction length= modal overlap integrald = electrode gap
The Mach Zehnder Interferometer
• Dalton, Steier, et al., “Polymeric waveguide prism based electro-optic beam deflector,” Opt. Eng., 40, 1217-22 (2001)• Dalton, Steier, et al., “Beam deflection with electro-optic polymer waveguide prism array,” Proc. SPIE, 3950, 108-116 (2000)• Dalton, Steier, et al., “Polymeric waveguide beam deflector for electro-optic switching,” Proc. SPIE, 4279, 37-44 (2001)
Spatial Light Modulator (SLM)Schematic Diagram
Literature Citations
= n3r33(V/h)(L/d)
= optical wavelength
n = index of refraction
r33 = electro-optic coefficient
L = interaction length (length of
base of cascaded prism)
= modal overlap integral
h = electrode gap
d = height of prism
High Bandwidth Optical Switches (The Electrical Problem)
Two bands approach:• DC-65 GHz direct modulation, use one modulator section;• 65-130 GHz using up-conversion scheme, RF applied to one modulator section, and LO applied to the other section.
Steier, Bechtel, Dalton et al., Proc. SPIE, 4114, 58-64 (2000).
POLYMER MICRO-PHOTONIC RING RESONATOR USING ELECTRO-OPTIC POLYMERS
5m
4.5m
3m
Si
UV15
CLD1 CLD1
SU-8
UFC 170
Au
Au Au
CROSSECTIONGND
Au upper modulation electrode
Complementary modulated output
Input Modulated output
Why Polymers?-Wide range of indices of refraction-Easy fabrication on multiple levels and integration with other devices-Voltage tunable filter or switch/ modulator using electro-optic polymers-Compact structure; size limited by index contrast-Temperature tuning, 0.1nm/C (use as an advantage or eliminate by athermal design in which thermal expansion of polymer substrate balances dn/dT of waveguide)
1, 2, 3
1 32
Laser1, 2, 3
Modulates 1 Modulates 2Modulates 3
Transmitter Receiver
1, 2, 3
1 32
1, 2, 3
1 32
Laser1, 2, 3
Modulates 1 Modulates 2Modulates 3
Laser1, 2, 3
Modulates 1 Modulates 2Modulates 3
Transmitter Receiver
Eye diagram10 Gb/s, Vpeak = 1 VDevice has ~15GHz BW
Au Electrode
SU-8
Gold ground
GND
Integrated WDM Transmitter Receiver
Dalton, Steier, et al., J. Lightwave Technology, 20, 1968-75 (2002)
Input waveguide
Output waveguide
T
Widely Tunable Polymer Double Micro-Ring
The wavelength of maximum transmission from input to output occurs when both micro-rings are resonant. If the wavelength of one of the rings is tuned by , the wavelength of maximum transmission tunes by M where
d1
d2
-70
-60
-50
-40
-30
-20
-10
0
1530 1540 1550 1560 1570
Wavelength (nm)
Pow
er (d
Bm
)
We have demonstrated both voltage (electro-optic) and thermally tuned polymer double micro-rings and Demonstrated an oscillator tuning across the band of the Erbium amplifier (1520-1560nm).d1 = 240mm, d2 = 246mm, M = 40 Side mode suppression >30 dBVoltage tuning 0.1nm/V, Thermal tuning 0.6nm/mW
Optical output spectra of thepolymer DR tuned laser
M = d1/(d1-d2) = DR/1
100 Gbit/sec Analog-to-Digital Converter(1 of 2 approaches)
• Dalton, Steier, Fetterman, et al. “Time stretching of 102 GHz millimeter waves using a novel 1.55 m polymer electrooptic modulator,” IEEE Photonics Technology Letters, 12, 537 (2000))
• Dalton, Steier, Fetterman, et al. “Photonic time-stretching of 102 GHz millimeter waves using 1.55 m polymer electro-optic modulator,” Proc SPIE, 4114, 44 (2000).
High Bandwidth Oscillators (Signal Generators)
• Dalton, Steier, Fetterman, et al., “Photonic control of terahertz systems,” Terahertz Electronic Proceedings, 102-5 (1998)
• Dalton, Steier, Fetterman, et al., “Electro-optic applications,” in Encyclopedia of Polymer Science and Technology (J. Kroschwitz, ed) Wiley & Sons, NY, 2001
Diode PumpedNd:YAG Laser
(1.3 µm)
OpticalIsolator
/2Plate
PolarizingBeamsplitting
Cube
CollimatingLens
2x2Coupler
OpticalSpectrumAnalyzer
PD
Low NoiseAmplifier
YIG TunedBandpass Filter
20 dBCoupler
SpectrumAnalyzer
Phased Array Radar with Photonic Phase Shifter (1 of 3 approaches)
Dalton, Steier, Fetterman, et al., IEEE W & Guided Wave Lett., 9, 357 (1999)
Many Other Applications
•Optical gyroscopes (China Lake Naval Weapons Lab and Redstone Arsenal)
•Acoustic Spectrum Analyzers (IEEE J. Sel. Topics in Quantum Electronics, 6, 810-6 (2000)
•Various Sensors and Test Equipment (e.g., printed circuit board testers)
•Antenna Structures, e.g., for Land Mine Detection
•Terahertz signal generation and detection for a variety of applications
Electro-Optic Devices: The on-ramps & interchanges of the information superhighway
(The Metro Loop and Fiber to the Home)
Critical to Next Generation Computing•Semiconductor Research Corporation Workshop on Optical Interconnectshttp://www.src.org/member/sa/nis/E002117_Opto_wksp.asp
•British House of Lords Select Committee on Science & Technology Study of Innovations in Computer Processors
•IEEE Computer magazine (“Data at the speed of light”, July 2002, p. 24)
•High frequency, ultra high stability clocks
•On-chip signal distribution (not necessarily Fiber to the Processor)—IEEE Spectrum
•Chip-to-chip interconnection
•Module-to-module interconnection
Copper Versus Fiber(Analysis Courtesy of IBM - private communication from Alan
Benner)Ratio of Optical to Electrical Performance
Key Metrics
Cabled, 10 meters Backplane, <1 meter
2002 2005 20080.001
0.01
0.1
1
10
100
1000
2002 2005 20080.001
0.01
0.1
1
10
100
1000
Cost
Power
Edge density
Areal density
Cable density
Use Optics for Performance
Use Optics forPerformance
Use Optics for Perf & Cost
Optical better
conservative optimistic
Ra
tio
Op
tic
al
to E
lec
tric
al
Pe
rfo
rma
nc
e E
lectrical better
Critical to Aerospace IndustryCritical to Aerospace Industry
U WashingtonU WashingtonCaltechCaltech
(Courtesy of Boeing)
Motivation for use of photonics in RF systems:Motivation for use of photonics in RF systems:
••Performance Performance (Bandwidth, (Bandwidth, arrayabilityarrayability, voltage requirements, , voltage requirements, immunity to EMI)immunity to EMI)
••Weight savings Weight savings (Replacement of coaxial cable with fiber) (Replacement of coaxial cable with fiber)
••Cost savings Cost savings (compatible with semiconductor VLSI fabrication)(compatible with semiconductor VLSI fabrication)
Electro-Optic Polymers for RF Photonics – a Key TechnologyElectro-Optic Polymers for RF Photonics – a Key Technology(Courtesy of Dr. Susan Ermer of Lockheed Martin Palo Alto)
Why Organic Electro-Optic Materials (Devices)?
.
•Intrinsic material bandwidths of several hundred gigahertz. The response time (phase relaxation time) of -electrons in
organic materials to electric field perturbation is on the order of femtoseconds. Operational 3 dB bandwidths of 200 GHz have been demonstrated for modulators & switches•Organic electro-optic coefficients are currently 2-4 times higher than lithium niobate and getting larger. Theoretically-inspired rational design of materials will keep electro-optic activity improving for several years. Device operational voltages of less than 1 volt can be routine.•Organic EO materials are highly processable into 3-D circuits and can be easily integrated with semiconductor VLSI electronics and silica fiber optics. Low loss coupling structures can be straightforwardly fabricated.•Cleaner and Greener (Environmental Health Perspectives, vol. 111, no. 5, May 2003, pp. A288-291)
The Bandwidth Potential
Abstract of an article published in Science by Lucent researchers
“A major challenge to increasing bandwidth in optical telecommunications is to encode electronic signals onto a lightwave carrier by modulating the amplitude of the light up to a very fast rate. Polymer electro-optic materials have the physical properties necessary to function in photonic devices beyond the current 40 GHz state-of-the-art bandwidth. We show that an appropriate choice of polymer materials can effectively eliminate all dielectric factors contributing to the decay of an optical modulator’s response at high frequencies. The resulting device modulates light with a bandwidth between 150 to 200 GHz and is capable of producing detectable modulation signal at 1.6 THz. These rates are faster than commercial bandwidth requirements for the foreseeable future.”
(private communication from Howard Katz and Mark Lee)M. Lee and co-workers, Science, 298, 1404 (2002).
What are the critical requirements for EO materials and devices?
Low halfwave voltage is a critical requirement in externally modulated photonic systems:
Analog systems: For RF transparency:
Link gain 1/V2
For high dynamic range:NF V
2
(low level signal detection limited by noise floor)
Digital systems:High speed digital circuits have low output voltage
Digital amplifiers very costly
Bandwidth is the other critical requirement!
Table. Comparison of Electrooptic Modulator Performance Parameters ofNLO Materialsa
Parameter LiNbO3 NLO Polymer NLO Polymer NLO Polymer
(2001-2002) (current) (in 18 months)______________________________________________________________________________________________________ reff, pm/V 30 50 130 300
n3(reff)
b 330 225 584 1,348
30 3 3 3 n3(reff)/ b 10 75 195 450
lengthbandwidth product, GHzcm 7 >100 >100 >100 VL, Vcm 5 2.5 0.9 0.4
optical loss, dB/cm 0.2 0.2-1.0 0.2-1.0 0.2-1.0 _____________________________________________________________________________________________________aAt optical wavelength of 1.3 microns.bValues given are figures of merit.
Optimization of Molecular Hyperpolarizability
Quantum Mechanics is the Key!
How far have we come?
How far can we go?
Take home observation: Molecular electro-optic activity is enormous and getting larger. If molecular electro-optic activity could be translated to macroscopic (material) activity, electro-optic coefficients greater than 1000 pm/V would be achieved.
N NO2R
R
N NR
R
N NO2
N
R
R
SN
OO
Ph
ISX
N
R
R
S CN
NC
CF2(CF2)5CF3
N
R
R
NO
O
Ph
FCN
APTEI
N
R
R
S
NC
CN
NC
CN
TCI
N
R
R
S CN
NC
CN
N
R
R
S CN
NC
CN
TCV
N
R
R
S SO2
NC
CNTCVIP
SDS
N
R
R
SO
NCCN
NC
N
R
R
O
NCCNNC
R'
NA
DR, 30 wt%, r33 = 13 pm/V
FTC, 20 wt%, r33 = 55 pm/V
CLD
(x10-48 esu)
80
580
2,000
3,300
4,000
6,100
(x10-48 esu)
9,800
13,000
15,000
18,000
30,000
Systematic Improvement in Molecular Electro-Optic Activity: Variation of (r33 = N<cos3>f ~ Nf/5kT (at low N and )
N O
CN
NCNC
1
N O
CN
NCNC
CF3
2
N NH
NCCN
NC
O10
µ = 11.8 Dßo = 23.8 x 10 -30 esuß1907 nm = 31.6 x 10 -30 esuµßo = 280.8 x 10 -48 esuµß1907 nm = 372.9 x 10 -48 esumax = 390 nm
µ = 11.9 Dßo = 34.4 x 10 -30 esuß1907 nm = 46.6 x 10 -30 esuµßo = 409.4 x 10 -48 esuµß1907 nm = 554.5 x 10 -48 esumax = 403 nm
µ = 11.7 Dßo = 43.0 x 10 -30 esuß1907 nm = 62.4 x 10 -30 esuµßo = 503.1 x 10 -48 esuµß1907 nm = 730.1 x 10 -48 esumax = 430 nm
N
NCCNNC
O
O
OO
FF
F
FF
F
FF
F
F
S
N
Chromophore content: 20 wt% in APCBaking condition: 85 C overnight;Poling condition: 110 C and 65 V/m for 10 min. r33 = 101 pm/V at 1.55 m
Dr. Sei-Hun JangPostdoctoral Fellow (UW)
Comparison of Microwave & RefluxSynthesis of CF3-TCF acceptor
CF3
Oi,
ii, dilute HCl
OEt
Li
O
CF3
OH
70%
O
CF3
OH
2CN
CN
O
CN
F3C
CN
CN
Condition Base Reaction time Yield (%)
Reflux
Microwave
LiOEt
NaOEt
48 h
20 min
30
55
CF3-TCF
-Hydroxyketone
Table. Comparison of conventional andmicrowave methodologies
Condensation
Breakthroughs in Organic Synthesis are Important
Focused Microwave-assisted Synthesis of Dihydrofuran Acceptors with Tunable Electron-withdrawing Strength
OOH CN
CN O
CN
NH
CN
NO2
O
CNCN
NO2
O
CN
NH O
CNN
NO
O
S
Et
Et
N
NO
O
Et
EtS
OOH CN
O NHN
N
CNCN O
NCN
CN
CN
COOEtO
CNCN
COOEt
Microwave accelerated step-wise control of imine intermediate
Electron withdrawing strength tunable dihydrofuran acceptors
Ele
ctro
n w
ith
-dra
win
g s
tren
gth Derivative with electron
deficient heterocyclic group
+
+
+
Optimization of Macroscopic Electro-Optic Activity
(Currently, we can utilize only 1 to 9 percent of the electro-optic activity of a chromophore molecule—Statistical mechanics is the
answer to improving this figure)
Statistical Mechanics is the answer!
Simple chromophore/polymer composites.
Covalently coupled systems (dendrimers, dendronized polymers, “new” side chain polymers) where we need to worry about restrictions placed on motion by covalent bonds and non-bonding (secondary, tertiary) interactions.
MATERIAL ISSUES: Translating Molecular Optical Nonlinearity into Macroscopic Electro-Optic Activity
EO coefficient is not a simple linear function of chromophore loading. Curves exhibit a maximum. Why?.
1.0
0.8
0.6
0.4
0.2
0.0
3 3
403020100
Chromophore loading wt%
TCI =6000x10-48 esu
ISX =2200x10-48 esu
DR19 = 550x10-48 esu
NN
Me2N
NO2
N
SAcON
OO
Ph
N
SCN
CN
CF2(CF
2)5CF
3
AcO
AcO
N O
O
Ph
N
APTEI
FCN
ISX
N
OH
HO NO
Ph
O
APII
DRN
SAcO
AcO
CN
CN
CN
CN
NS
CN
NC
CN
N
S
CN
NC
CN
Bu2N S SO
2
CN
NC
N S
Bu Bu
AcO
AcO
O
NC CNNC
TCI
SDS
FTC
Centric Ordering
E
Chromophore-polingField Interaction Thermal Randomization
Chromophore-ChromophoreElectrostatic Interaction
Acentric Ordering Isotropic
<cos3> = F/5kT = f(0)Ep/5kT
<cos 3> =
(F/5kT)[1-L2(W/kT)]
34
cosn
NFreff
Translating Microscopic to Macroscopic Electro-Optic Activity
Comparison of Theory & Experiment
Experiment—SolidDiamonds
2max 2 2
0.48 0.28 4.8 kT kT
N f
EO Activity DependsOn Shape!
Dendrimers—3 D Organization of Chromophores
Statistical Mechanical Theory explains the improved performance of dendritic chromophores.
By choosing a tilt
angle for the three chromophores (~60°) the experimental enhancement (of ~ 2 fold) was realized.
O
O
OO
O
ON
S
CNNC
NC
NC
O
O
O O
FF
OF
O
O
FF
OF
O
N
S
NCCN
NC CNO
O
O
OF
F
O
FO
OF
F
O
FO
NS
NCCN
CN
CN
O
O
OO
F F
O F
O
O
F F
O F
O
O
O
O
Full Dendrimer (AJ3) Three chromophores -- 20 Debye dipole each Best of Set of accepted M.C. moves. Near perfect order of 3 Chromophores Very Large Field ( 3000 MV/m) Points on Z (up)
Chromophore
Magic Angle
Center
.
Various Dendritic Structures
: Dendritic moiety
: Polymer backboneCore moiety
: NLO chromophore moiety
: Crosslinkable moiety:
x yx y
Side-Chain dendronized NLO polymer
Dendritic NLO chromophore
NLO dendrimer
Strategy to Achieve Cylindrical Shape Conformation in
NLO Synthetic Supramolecular Systems
tobacco mosaic virus
Dendritic Crosslinker
Dendritic NLOChromophore
OO
O F
F F
OO
O
O
N
O
NC
CNNC
O
O
O
O
O
FF
F
OF
F
F
( )3
( )1-x ( )x
OO
O O
O F
F F
O F
F F
OO
OO
NCCN
CN
NOO
OO
OO
FF
F
FF
F
( )x ( )y
O
0 30 60 90 120
Electro-opticCoefficientDensity of NLOActive Species
30 pm/V
Lithium Niobate Crystal, 100 wt%
58 pm/V
30 wt%
97 pm/V
20 wt%
111 pm/V
20 wt%
Guest-Host Polymeric System
Side-Chain DendronizedNLO Polymer
Dendronized NLO Cylindrical polymer
Performance Comparison of Various NLO Materials
Dendrimers permit great synthetic flexibility—e.g., Fluorination
.
Dendronized chromophore yields 3 times the electro-optic activity and reduced optical loss (next figure). Optical loss comes from protons
N
S
NC CN
NC
NCO
O O
O O
O
O
O
O
O
O
O
FF
F
FF
F
FF
F
F
F
FF
FF
F
FF
FF
F
FF
FF
F
FF
F
F
N
S
NC CN
NC
NC
FLDRTCBD
0.0
0.1
0.2
0.3
0.4
0.5
0.6
1470 1490 1510 1530 1550 1570 1590
Wavelength (nm)
Loss
(dB
/cm
)Auxiliary Properties: Optical Loss including both
absorption and scattering loss
TE Mode
THERMAL STABILITY—The Need to Lock-In Poling Induced Acentric Order: Intermolecular Crosslinking
HO OH HO OH
OH
1. spin cast with diisocyanate crosslinker
2. electric field poling 3. thermal crosslinking
x y z
free-radical copolymerization with methyl methacrylate and
hydroxyethylmethacrylate
3-D crosslinked network
Crosslinkable PFCB Thermoset Systems
• Good thermal stability (Td~470 °C)• Excellent optical transparency (< 0.25 dB/cm at 1.55 m)• Low dielectric constant (k~2.35, 10 kHz)• Low moisture absorption • High glass-transition temperature (Tg~380 °C) (0.021% after 24 hrs soak in water)• Low birefringence
New method avoidsSensitivity to atmosphericmoisture!
CH 3
O
O
O
F
F
F
F
F F
FF
F
CH 3
O
O
O
CH 3
O
O
O
F
F
F F
F
F
H 3C O
O
OF
F F
F
FF
CH 3
O
O
O
F
F
FF
F
F
F
F
F
F
F
F
F
F
F
F
F
F
FF
F
F
F
F
Ultra low optical loss high temperature polymer: (< 0.2 dB/cm @ 1.55 m)
Processability: An Advantage of Organic Electro-Optic Materials
•The tailorability of organic materials and particularly of dendrimers permits integration of organic EO materials with virtually any material (silicon, silicon dioxide, Mylar, III-V semiconductors, metals, etc.)•Hardened organic EO materials are amenable to reactive ion etching (RIE) and to various photolithographic processes. Processing is very compatible with semiconductor processing techniques.•Organic materials are quite robust (high dielectric breakdown, good thermal stability at most processing temperatures, high radiation (gamma, high energy particle) damage thresholds, etc.•Likely amenable to high volume manufacturing using processing techniques such as spin casting and dry etching.•Straightforward fabrication of an array of prototype devices.
Electrooptic Modulator Fabrication
UV Exposure
Photolithography
Lithography mask
Photoresist (1400-27)
PU-DR19 active polymer
Polyurethane cladding
Gold elctrode
Quartz substrate
Develop
Dry etching
RIE O2
SpinUpper Cladding
Pattern electrodeGold plate
HV+
Corona poling
Remove photoresistCheck nonlinearity
T= 140° C
Reactive Ion Etching of 3-D Optical Circuits
Variable PhotoresistExposure
RIE SlopeTransfer
WaveguideCompletion
UV
OxygenIons
Cladding
Substrate
Photoresist
Core
Spin-CastingPreserves
Surface Contour
Cladding
Fabrication: Shadow Etch
• Shadow Masking of Ions– Angle RF Power, Gas Pressure,
Time, Mask Dimensions
– Angles: 0.1-3°
– Heights: 1-9m
– Lengths: 200-2,000m
• Fast Prototyping– Various Angles From Single Mask
– No Extensive Fabrication Steps
• Repeatable Quality
4
2
0
0 400 800 1200 1600
Hei
ght (m
)
Length (m)
Mask
Polymer
Offset
6
Oxygen Ions
n(active) > n(passive)
Length
Tapered Transitions: Minimization of Coupling Loss
small length material loss large length radiation loss
Fabrication of 3-D IntegratedActive/Passive Optical Circuits
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