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ONO2000Tutorial

INTRODUCTION

--The phenomena of optical nonlinearity

--Voltage dependent index of index of refraction

--Simple Devices

CHROMOPHORES

--Optimizing hyperpolarizability

-- Auxiliary Properties

MATERIALS

--Optimizing electro-optic activity

--Theory and optimized design of chromophores

--Optical Loss

--Lattice Hardening

PROCESSING

--Fabrication of buried channel wavguides

--Tapered and vertical transitions

--Fabrication of 3-D integrated circuits

DEVICES AND PERFORMANCE

--Prototype devices and performance evaluation

--Advanced devices (e.g., phased array radar)

FUTURE PROGNOSIS

REFERENCES

INTRODUCTION: Frequency

Dependence of Polarization

ONO2000TutorialFor a sinusoidal field,

E(z,t) = E0cos(wt-kz)

the polarization becomes:

INTRODUCTION: Useful

Relationships

ONO2000TutorialRelationship of phase shift to EO coefficient and applied field

Relationship of Vp voltage to EO coefficient

where l is the free-space wavelength, d is the thickness of the

waveguide core and cladding, L is the length of the electrode.

Applied electric field is now denoted by V rather than E.

Voltage Length Product

Figure of Merit

where e is the dielectric constant

INTRODUCTION: Comparison of

Organic and Inorganic Materials

ONO2000TutorialStability will vary depending how the final polymeric EO

material is prepared.

Trace 1, guest/host composite; Traces 2-4, chromophores in

hardened polymers. Trace 3 corresponds to DEC shown

below. Trace 5 corresponds to sol-gel glass.

Circles denote an IBM polymer with the DANS chromophore

covalent attached by one end to PMMA. DEC refers to a

double end crosslinked chromophore prepared by Dalton, et al.

INTRODUCTION: Simple Device

Configurations

ONO2000TutorialMach Zehnder Modulator

Birefringent Modulator

Directional Coupler

INTRODUCTION: Mach

Zehnder Modulator and Simple

Device Performance Comparison

ONO2000TutorialComparison of key features of simple devices

Mach Zehnder Birefringent Directional

InterferometerModulatorCoupler

reff r33 r33-r13 r33

Vp VpMZ 1.5 VpMZ 1.73 VpMZ

Mod. PMZ 2.75 PMZ 3 PMZ

Power

CHROMOPHORES: Charge-

Transfer (Dipolar Chromophores)

ONO2000TutorialWith the exception of octupolar chromophores (which we will

not discuss) electro-optic chromophores are dipolar charge-

transfer molecules consisting of donor, bridge, and acceptor

segments. They are by nature modular materials (see below).

CHROMOPHORES: Optimizing

Chromophore Hyperpolarizability

ONO2000TutorialThe two level model has provided useful guidance

in optimizing molecular hyperpolarizability, b.

where weg is the frequency of the optical transition, f is the

oscillator strength, Dm is the difference between the ground

and excited state dipole moments.

Through this relationship, b can be related to

material properties such as bond length alternation,

BLA, and to donor and acceptor strength.

CHROMOPHORES: Variation of

mb with Molecular Structure

ONO2000TutorialThe simple two level model and structure/function insight

gained from the model has permitted a dramatic improvement in

molecular hyperpolarizability (see below). In the limit of non-

interacting chromophores, electro-optic activity (induced by

electric field poling) scales as Nmb (where N is number density

and m is dipole moment); thus, we list mb instead of b. In

the 1990s, an improvement of a factor of 40 was achieved.

CHROMOPHORES: Auxiliary

Properties--Thermal Stability

ONO2000TutorialThermal stability depends on host matrix (see below) and

atmosphere (packaging). Typically defined as temperature at

which EO activity is first observed to decrease.

CHROMOPHORES: Auxiliary

Properties--Thermal Stability

ONO2000TutorialGood thermal stability and molecular hyperpolarizability are

not mutually exclusive (see example below)

CHROMOPHORES: Auxiliary

Properties--Purity

ONO2000TutorialIonic impurities can lead to ionic conductivity during

electric field poling. This can reduce the field felt by

chromophores and poling efficiency.

CHROMOPHORES: Summary

ONO2000TutorialChromophore Requirements:

•Large hyperpolarizability and large dipole

moment

•No absorption at operating wavelength

•Stability

--Thermal

--Chemical & Electrochemical

--Photochemical

•Solubility in spin casting solvents

•Compatibility with polymer hosts (particularly

for guest/host materials)

•Low volatility (particularly if used for guest/host

materials with high Tg polymer)

CHROMOPHORES: Dipole

Moments

ONO2000TutorialChromophore dipole moments will be very useful for

understanding the translation of microscopic optical non-

linearity to macroscopic electro-optic activity. Below we

show dipole moments calculated for representative EO

chromophores using SpartanTM

MATERIAL ISSUES: Translating

Molecular Optical Nonlinearity to

Macroscopic Electro-Optic Activity

ONO2000TutorialEO coefficient is not a simple linear function of chromophore

loading. Curves exhibit a maximum. Why?

MATERIALS ISSUES: Optimizing

Material Electro-Optic Activity--

Dependence on Chromophore Shape

ONO2000TutorialData are shown for two different structures of the same

chromophore: With isophorone groups (circles) and without

isophorone protection of the polyene bridge (diamonds)

MATERIAL ISSUES: Optimizing

Electro-Optic Activity--Variation

with Chromophore Structure

ONO2000TutorialElectro-optic coefficients for 4 different chromophores (FTC,

squares; CLD,diamonds; GLD, circles; and CWC, crosses) are

shown as a function of chromophore number density in

PMMA. The dipole moments for these chromophores were

shown in a previous overhead.

1

r33

pm/V

1

MATERIAL ISSUES: Optimizing

Electro-Optic Activity: Theory--

Equilibrium Statistical Mechanics

ONO2000TutorialElectro-optic activity can be calculated according to

r33 = 2NbF(w)<cos3q>/n4 The order parameter is

where U = U1 + U2 is the potential energy describing the interaction of chromophores with the poling field (U1) and with each. For non-interacting chromophores, U = -mFcosq where F is the poling field felt by the chromophore. For this case, <cosnq>is

Ln is the nth order Langevin function and f = |mF/kT|

Consider chromophores interacting through a mean distance, r, which is related to number density by N = r-3. Let us follow Piekara and write the effective field at a given chromophore from surrounding chromophores as U2 = -Wcos(q2). The position w.r.t. the poling field is defined by Euler angles, W1 = {q1,f1} and the angles. or

MATERIAL ISSUES: Optimizing

Electro-Optic Activity: Theory--

Equilibrium Statistical Mechanics

ONO2000TutorialAveraging is done over the two variables W and W2. Explicitly,

The total potential is taken as -fcos(q) -Wcos(q2). In the

high temperature approximation, exp(-U1/kT) = 1-fcos(q1).

These integrals can be done analytically with the result

MATERIAL ISSUES: Optimizing

Electro-Optic Activity

ONO2000TutorialEquilibrium statistical mechanical calculations are easily

modified to take into account nuclear repulsive effects (by

simply adjusting the integration limits). Below we show

simulation data for a typical chromophore separating nuclear

(shape) and intermolecular electronic effects.

MATERIAL ISSUES: Optimizing

Electro-Optic Activity--Theory

ONO2000TutorialCritical Conclusion: Chromophore shape is very

important. Need to try to make chromophores more

spherical.

(Independent particle model)

Comparison of Theory and

Experiment for FTC

MATERIAL ISSUES: Optimizing

Electro-Optic Activity--Theory &

Practice

ONO2000TutorialAn example of modification of chromophore shape

(CWC) to improve electro-optic activity is shown.

MATERIAL ISSUES: Optimizing

Electro-Optic Activity--Theory:

Monte Carlo Methods

ONO2000TutorialInitially: No applied poling field, no intermolecular

interactions

Steps 1-400: Poling field on, no interactions

Steps 400-800: Poling field and full interactions

MATERIAL ISSUES: Optimizing

Electro-Optic Activity--Theory:

Monte Carlo Methods--Chromophore

Distributions with Increasing Interactions

ONO2000TutorialChromophore distributions are shown as a function of increasing

chromophore concentration for concentrations of 1 x 1017/cc,

5 x 1020/cc, and 1.5 x 1021/cc.

MATERIAL ISSUES: Optimizing

Electro-Optic Activity--Theory:

Monte Carlo Methods

ONO2000TutorialVariation of calculated electro-optic activity with number

density is shown for different values of chromophore dipole

moment.

MATERIAL ISSUES: Optimizing

Electro-Optic Activity--Theory:

Comparison of Methods

ONO2000TutorialComparison of Monte Carlo and equilibrium

statistical mechanical (smooth and dashed lines)

methods. Methods is shown below. Both

methods predict same functional dependence on

number density.

MATERIAL ISSUES: Optimizing

Electro-Optic Activity--Theory:

Monte Carlo Methods

ONO2000TutorialThe effect of chromophore shape on electro-

optic activity is shown.

MATERIAL ISSUES: Optimizing

Electro-Optic Activity--Theory:

Phase Separation

ONO2000TutorialTheory can also be used to identify the conditions

where phase separation (chromophore aggregation)

occur. Phase separation will depend on applied

electric field, chromophore concentration, and host

dielectric constant. Curves 1-5 correspond to phase

boundary lines for host dielectric constants of 2,3,

5,7, and 10. To the left is the homogeneous phase.

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