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