I3 of Organic Polymeric Materials · ACOUSTO-OPTIC AND LINEAR ELECTRO-OPTIC PROPERTIES OF ORGANIC...

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Naval Research Laboratory Washington, DC 20375-5000 NRL Memorandum Report 6454 od I3 Acousto-Optic and Linear Electro-Optic Properties of Organic Polymeric Materials MICHAEL E. BOYLE AND ROBERT F. COZZENS Polymeric MaterialsBranch Chemistry Division April 27, 1989 DTIC S. ELECTE MAY 16 1989 e. Approved for public release, distribution unlimited.

Transcript of I3 of Organic Polymeric Materials · ACOUSTO-OPTIC AND LINEAR ELECTRO-OPTIC PROPERTIES OF ORGANIC...

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Naval Research LaboratoryWashington, DC 20375-5000

NRL Memorandum Report 6454

od

I3 Acousto-Optic and Linear Electro-Optic Propertiesof Organic Polymeric Materials

MICHAEL E. BOYLE AND ROBERT F. COZZENS

Polymeric Materials BranchChemistry Division

April 27, 1989

DTICS. ELECTE

MAY 16 1989e.

Approved for public release, distribution unlimited.

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SECURITY CLASSFCA7'ON 0; - 'S PAGE

REPORT~~~ DOU ETTINPGormr Approv~ed

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4 PERFORMING ORGANIZATION REPORT NUMBER(S) 5 MONITORING ORGANIZATION REPORT r.S2,

NRL Memorandum Report 64546a. NAME OF PERFORMING ORGANIZATION jbOFCE SOL

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I1I TITLE (Include Security Classification)

Acousto-Optic and Linear Electro-Optic Properties of Organic Polymeric Materials

12. PERSONAL AUTHOR(S)

Boyle, M.E. and Cozzens, R.F13a. TYPE OF REPORT 13b TIME COVERED 14 DATE OF REPORT (Year, Month Day) ;5 AI37''"4

1FROM TO 1989 April 27 3616 SUPPLEMENTARY NOTATION

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19 ABSTRACT (CoiV e on reverse if necessary and identify by block number)

The potential of organic polymeric materials for use inacousto4joptic and electro~bptic devices operating at GHzfrequencies is examined. >Tihis is accomplished by identifyingand briefly discussing the important molecular properties forenhanced acousto~optic and electro-Ooptic ef fects and thenrelating these to "current polymericr ,aterials..-)I -) -\

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DO Form 1473, JUN 86 Previous editions jre obsolete CR'A A

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CONTENTS

IN TR O D U CT IO N .1........................................................................................................... I

ACOUSTO-OPTIC MATERIALS AND DEVICES .....................................................................

LINEAR ELECTRO-OPTIC MATERIALS AND DEVICES ........................................................ 15

R EFE R E N C E S .......................................................................................... ................ 27

NTIS GRA&L[Access

on For

DTIC TAB iUnannounced 0li

Justiioationj

Distribution/

Availability Codes DIAvai, and/or DI

Dist Special INSPECED

4

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ACOUSTO-OPTIC AND LINEAR ELECTRO-OPTIC PROPERTIESOF ORGANIC POLYMERIC MATERIALS

INTRODUCTION

Currently, most acousto-optic and electro-optic devicesmake use of inorganic crystals as their active medium. 1-6 Thisis due, in part, to the historical development of deviceconcepts and the ease in theoretically modeling such effects incrystalline versus amorphous or polycrystalline materials.Although inorganic crystals are good acousto-optic and electro-optic device materials, the search for better materialscontinues. It is in this spirit that we assess the use oforganic polymeric materials in such devices operating in theGHz frequency range. The assessment is accomplished byidentifying and briefly discussing the important molecularproperties for enhanced acousto-optic and electro-optic effectsand then relating these to current polymeric materials.

ACOUSTO-OPTIC MATERIALS AND DEVICES

Acousto-optic devices are based on changes in the opticalproperties of materials due to traveling pressure disturbances(acoustic waves) which produce regions of compression andrarefaction in the material. Such density disturbances causemodifications of the refractive index of the material andthereby, cause the medium to respond as a phase grating. Tnthe thin grating limit,4 ,5 ,7 described by

2 7rA L<< 1 (1)

A2

where A is the light wavelength, A is the acoustic wavelengthand L is the width of the interaction region, diffraction inmany orders will occur with the probe light at normalincidence.4,5, 7 However, at large values of L, the higher orderdiffractions interfere with each other, completely destroyingthe diffraction pattern. To achieve constructive interferencein a thick gratin , the light beam must be set at the angleOB = sin -1 A/2A.Z ,5 ,7 The diffracted beam will be seen at thissame angle as indicated in Fig. 1. Note that there are not any

Mamucrit approved January 24, 1989.

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e(a) Thin Grating Limit "4 *v)t

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Figure 1: Schematic diagrams of the two limiting cases ofdiffraction from an acoustically generated phase grating. In (a)the thin limit grating is diagramed detailing the diffracted

waves' respective phases; w is the frequency of the light and v isthe acoustic frequency. In (b) the thick limit grating isdiagrammed showing the Bragg diffraction angle, OB. This figureis adapted from Ref. 5.

2

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higher-order diffracted beams. (This is commonly referred to asBragg diffraction due to its similarity to x-ray scatteringfrom atoms in crystals.)

Several figures of merit for Bragg diffracting acousto-optic devices have been derived for use in comparing potentialmaterials and are given below: n is the refractive index; p isthe material 's maximum photoelastic coefficient, the parameterof principle interest in device design; p is the density; and vis the acoustic velocity of the material. 3-5, 8, 9

Ml = (n7 p2/pv) (2)

M2 = (n6p2/pv3 ) (3)

M3 = (n7 p2/pv2 ) (4)

M2 is an experimentally measurable quantity which givesinformation on the diffraction efficiency. 3,41 9 The otherfigures of merit, M1 and M3 , involve considerations of thebandwidth due to the shift in the Bragg angle with acousticfrequency changes and the capacity (number of separablepositions) of the material due to the size of the interactionregion, respectively. 3,4,8,9 In general, it can be seen thatthe most important material properties for large acousto-opticeffects are a high refractive index and photoelastic constantand a low acoustic velocity.

There are two additional properties that must beconsidered in evaluating materials for acousto-opticapplications: optical and acoustic attenuation. Attenuationrefers to both absorption and scattering losses. Acousto-opticmaterials must have simultaneously, reduced scattering andabsorption of both optic and acoustic waves.

ORGANIC MATERIALS

While data on indices of refraction and density ofpolymeric materials abound, data on photoelastic coefficients,acoustic velocity and acoustic attenuation in the GHz regimeare very limited. Therefore, the evaluation of many polymericmaterials for GHz acousto-optic applications will depend onderiving appropriate theoretical values for these parameters.Such values will be based, whenever possible, on extrapolationsof lower frequency data. For the discussions that follow, itwill also be assumed that the materials of interest areisotropic.

Photoelastic Coefficients

Photoelastic coefficients (Pijkl) relate the strain tensor

(Skl) to the indicatrix (1i/n 2 ]ij) where n is the index of

3

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refraction. 3- 5 That is, they describe the coupling ofmechanical strain to a material 's optical properties.

A(l/n 2 )ij = . Pijkl Skl (5)

The tensor notation is required because of the directionaldependence of material properties. For an isotropic materialthere are only two independent photoelastic coefficients. Theyare related to the material properties along (longitudinal) andperpendicular (transverse) to the direction of propagation ofthe acoustic wave. The longitudinal photoelastic coefficientis the one of interest in device design; it is much larger thanthat in the transverse direction and, therefore, is the onlyone discussed in the sections that follow. 3- 6 ,8 ,9

Determination of material photoelastic coefficients isstill essentially an empirical science and unfortunately, veryfew polymeric material photoelastic coefficients have beenreported (see Table 1) . 1-5 The photoelastic constants ofinorganic crystals and glasses have been extensively investi-gated and, in general, vary between 0.2 and 0.6. 2 - 6,8- 10 Thelatter is the theoretical value of the photoelastic constantfor an ideal material with a refractive index of 1.5,4, 5 therefractive index value for nearly all carbon based polymericmaterials.11

Theoretical discussions of the photoelastic effect oftencite Mueller's microscopic description. 12 This theory suggeststhat photoelastic effects arise from refractive index changesrelated to alterations in the material density and polarizabil-ity under strain. For most solids, a decrease inpolarizability occurs when the material is compressed becauseof the increased orbital overlap and subsequent localization ofthe optical electrons. However, interatomic repulsions arealso very important and can overwhelm the above coulombicattraction effects. That is, the density and polarizabilityeffects may interfere constructively or destructively and it isnot possible to predict the exact magnitudes of the respectiveeffects. 4 In other words, neither the magnitude nor the signof the photoelastic coefficient can be predicted and nospecific enhancing molecular properties can be identified.

Acoustic Velocities

The majority of work reported on acoustic velocities inpolymeric materials has involved MHz frequencies. However, anumber of GHz studies have also been published. A briefcompilation of reported longitudinal acoustic velocities isgiven in Table 2.

The measured acoustic velocities in polymeric materil sfall between those of metals (3000-6000 m/s) and liquids (900-1500 m/s) 13 and it has been qualitatively determined that, over

4

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Table 2: Acoustic Velocities in Some Common Organic Materialsat Room Temperature

MATERIAL DENS ITY LONGITUDINAL FREQUENCY RE F(g/cm ) VELOCITY

(10 m/s)

P4M1P 0.835 2.2 1.8 MHz 402.2 5-12 GHz 19

PMMA 1.191 2.7 0.6 MHz 382.7 1 MHz 412.7 6-30 MHz 392.7 10-90 MHz 172.7 11 GHz 17

PVC 1.386 2.3 1 MHz 412.3 10-90 MHz 172.4 10 GHz 17

ABS 1.022 2.1 0.6 MHz 382.0 1.8 MHz 40

Poly(styrene) 1.102 2.4 0.6 MHz 382.3 1 MHz 412.4 31 GHz 20

Poly(carbonate) 1.1,4 2.2 1 MHz 152.2 1 MHz 412.3 5 MHz 44

Poly(sulfone) 1.240 2.3 1 MHz 412.3 5 MHz 44

Poly (ethersulfone) 1.373 2.2 5 MHz 44

OMTS-toluene 1.7-1.9 1.8-2.2 70 MHz 35HMDS-styrene

EPON 828-Z 1.202 2.7 1-3 MHz 43

P4MIP Poly(4-methyl-l-pentene); PMMA = Poly(methylmethacrylate) ; PVC = Poly(vinyl chloride); AE2 = Poly(acrylonitrilebutadiene sytrene); OMTS - Cctamethytrisiloxan= chin film; HMDS =Hp.xamethyldisilozane thin film.

6

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wide ranges, as frequency increases the acoustic velocityincreases. 13 - 1 5 However, a brief examination of Table 2 showsthat for materials below their glass transition temperature,the acoustic velocity dispersion is small over the MHz to GHzfrequency regimes.

Material properties that affect the acoustic velocity havebeen investigated in the lower frequency range (< GHz) and astrong correlation with the volume of the material has beenestablished.1 3 - 15 This relationship arises because of thevolume dependence of the intermolecular potential. Forexample, replacing hydrogen atoms with fluorine atoms has theeffect of lowering the acoustic velocity since the largerfluorine atoms will decrease the intermolecular attraction andresult in an increase in the material's volume. Using a morecommon physical property to describe this effect, as densityincreases, acoustic velocity will increase. This concept hasbeen successfully extended to copolymer and compositesystems.15

Experimental studies have also shown that increasing therigidity of the mainchain backbone of the polymer by theinclusion of phenylene groups results in increased acousticvelocities. 15 This is due to the restricted conformationalchanges that can occur. Thus, to minimize the acousticvelocity and thereby maximize the figure of merit (M2 ), a lowdensity, elastic material is desired.

Acoustic Attenuation

The acoustic attenuation of polymeric materials, whichinvolves both absorption and scattering, has received a greatdeal of attention because of thepotential use of thesematerials in sonar devices. 13- I1 The attenuation coefficient,1, is defined by

= (1/x) log(A0 /A) (6)

where x is the distance traversed and A0 and A are the initialand final wave amplitudes, respectively, a is measured innepers/centimeter but commonly found expressed in terms ofdB/cm. Experimentally, it has been found that absorptions inpolymeric materials are either linearly or quadraticallydependent on frequency. 13- 15 Linearly dependent absorption isalso known as hysteresis absorption and is expressed as a/ =constant (v = frequency). Hysteresis absorption has beenobsenred in the GHz frequency regime. 15-17

The microscopic description of the absorption processesoccurring at sub-GHz frequencies involves contributions frommolecular separations (a volume or density dependence) andstructural rearrangements. 13 - 1 5 For acoustic frequencies inthe GHz regime, only limited structural rearrangements can

7

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contribute to absorption as most materials can be regarded asbeing in the glassy state. (The glassy state corresponds to alarge tensile modulus for the material.) In such cases, theabsorption mechanism can be viewed as amplitude losses throughthe excitation of librations and/or rotations of mainchain sidegroups and limited partial seqmental motions and reorientationsof the mainchain backbone. 15- 7

A limited number of results have been reported on theacoustic absorption of polymeric materials in the GHz regime.This is due, in part, to the experimental difficulties inmaking these types of measurements: because of the highfrequencies involved, Brillouin scattering experiments (lightscattering from thermal phonons) are required. 17-24 Table 3lists the reported attenuation coefficients of severalpolymeric materials measured at a number of acousticfrequencies.

The theoretical explanation for acoustic absorption andthe results of experimental measurements (Table 3), indicatethat highly crosslinked or crystalline type polymers are bestat minimizing acoustic absorption. Note that this is the exactopposite of the type of material properties desired for a highfigure of merit (M2 ). As is so often the case in materialsscience, a compromise between competing material parametersmust be reached and depends on the specific nature of thedevice design and application.

Scattering of acoustic waves is exactly like scattering oflight waves; it depends on the size of the inhomogeneity.Therefore, choosing an acceptable optical material should go along way to insuring an acceptable acoustic material.

Organic Acousto-Optic Material Evaluation

in evaluating organic materials for possible use inacousto-optic devices, the first issue to be considered is oneof optical transparency. Polymeric materials can have excellentoptical properties in the visible and near infrared spectralregions but will be absorbing in the infrared due to theirchemical composition. This is an inescapable feature oforganic materials, but ona that can be mitigated by the use ofunique molecular designs such as fluoridation and also by theuse of thin films. The optical properties of polymers are welldocumented. 11,15 For the purposes of this evaluation, onlymaterials that are transparent in the visible spectral regionwill be considered.

Table 4 is a listing of several optically transparentpolymeric materials and their important acousto-opticparameters. The density and refractive index entries arerepresentative literature values. 11,15,25 The table entriesfor the acoustic parameters are either the reported GHz valuesfound in Tables 2 and 3 or have been estimated as discussed

8

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Table 3: Acoustic Attenuation Factors for Some Common OrganicMaterials at Room Temperature

aMATERIALS LONGITUDINAL FREQUENCY af/ ** REF

ATTENUATION v (dB-s/5m)(dB/cm) (10-

P4M1P 1.4 2 MHz 7.0 151.4 1.8 MHz 7.8 40

PMMA 1.4 2 MHz 7.0 151.8 2.2 MHz 8.1 163.7 5.5 MHz 6.7 165.0 6 MHz 8.3 397.7 10 MHz 7.7 29

12.7 18 MHz 7.1 3912.6 20 MHz 6.3 3919.6 30 MHz 6.5 39

2.1 x 104 11.3 GHz 18.5 17

PVC 5.7 1.5 MHz 37.8 1711.3 5 MHz 23 4716.1 10 MHz 16 47

3.4 x 104 10.2 GHz 33.3 17

ABS 1.8 2 MHz 9.0 40

Poly(styrene) 0.4 0.5 MHz 8 451.9 5 MHz 3.8 473.8 10 MHz 3.8 475.6 15 MHz 3.7 475.8 20 MHz 2.9 47

Poly(carbonate) 21.7 5 MHz 43.4 4452.1 15 MHz 34.7 4486.9 25 MHz 34.7 44

Poly(sulfone) 8.7 5 MHz 17.4 4421.7 15 MHz 14.4 4434.7 25 MHz 13.9 4460.8 35 MHz 17.4 44

Poly(ethersulfone) 8.7 5 MHz 17.4 44

26.0 15 MHz 17.3 4452.1 25 MHz 20.8 4465.0 35 MHz 18.6 44

Poly(ethylene) 3.3 2 MHz 16.5 1520 10 MHz 20 16

9

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Table 3: Acoustic Attenuation Factors for Some Common OrganicMaterials at Room Temperature (cont.)

MATERIALS LONGITUDINAL FREQUENCY a/i REFATTENUATION (dB-s/5m)

(dB/cm) (10

OMTS-tolueneHDTS-styrene 0.4 70 MHz 0.06 35

EPON 828-Z 4.1 1 MHz 41.0 437.1 2 MHz 35.5 43(EPOXY) 10.5 3 MHz 35.0 43

P4M1P - Poly(4-methyl-l-pentene); PMMA = Poly(methyl

methacrylate); PVC = Poly(vinyl chloride); ABS = Poly(acrylonitrilebutadiene sytrene); OMTS = Octamethytrisiloxane thin film; HMDS =Hexamethyldisilozane thin film.

**Calculated using table entries.

10

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below. The dispersion of acoustic velocities between MHz andGHz frequency regimes has been experimentally shown to be verysmall1 7 (see Table 2) and therefore, in the absence of areported GHz value, the highest MHz value is used. With regardto acoustic attenuation, table values are calculated assuminghysteresis absorption using the constants found in Table 3.Based on the results reported for PMMA and pVC, 17 such valuesare certainly correct to within an order of magnitude.Unfortunately, only a very limited number of photoelasticcoefficients have been reported for polymeric materials and noclear method for estimating such values is available. Reportedvalues (Table 1) are used where appropriate and an assumedvalue of 0.30 is used in the remaining cases. The assumedvalue choice is based on the observation that although thematerials in Table 1 are widely different, p, the photoelasticcoefficient, is essentially constant.

The figures of merit listed in Table 4 are calculatedbased on the corresponding table entries.

Figure 2 is a plot of the figure of merit, M2 , versusattenuation, for a variety of inorganic materials and those inTable 4. From this plot we see that polymeric materials,although having a high figure of merit, also possess a verylarge acoustic attenuation in comparison to inorganic crystals.That is, present organic materials cannot compete with currentinorganic materials.

Although the potential for polymeric materials looksbleak, new materials with novel properties are constantly beingdeveloped. One area that has yielded unique materials is thestudy and development of materials having large nonlinearoptical properties.2 6 Several materials with large delocalizedx electron systems have been synthesized. Such materials havelarge polarizabilities and therefore may yield enhancedacousto-optic properties. One group of these nonlinearoptical polymers, known as rigid-rod or ladder polymers, isespecially interesting. These polymers (Fig. 3) are composedof interlocking phenylene rings with a limited number of sidegroups; features which should lead to reduced acousticattenuation but relatively high acoustic velocity. Couplingthe reduced attenuation with the increased polarizability (ther electron conjugation) suggests that these materials m haveinteresting acousto-optic potential. Unfortunately, littlemore can be said due to the lack of a means for estimating thephotoelastic coefficient: material properties need to bemeasured.

With regard to other types of polymeric materials such asliquid crystals, acousto-optic measurements have been reportedbut involve complex effects due to shear induced structuralreorientations (transverse wave effects) .27

- 33

12

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Finally, polymeric materials are finding increased use insurface acoustic wave (SAW) devices as thin films. 34 ,3 5 SAWconcepts3 6 ,37 may prove useful in a variety of applications andshould be investigated. However, that is beyond the scope ofthis work.

LINEAR ELECTRO-OPTIC MATERIALS AND DEVICES

The application of an electric field (E) induces a changein the polarization (P) of all materials and can be describedby the following tensor equation. 48 (The tensor coefficientsubscripts are understood.)

P = P0 + x(1)eE + x(2 ):EE + X(3 ):EEE + ... (7)0

P0 is the material's intrinsic polarization and the x's areknown as the material susceptibilities. The first term,(is used to describe usual linear optical phenomena such asabsorption reflection and refraction. The latter two terms,x(2 ) and x(3), are known as the second-order and third-ordersusceptibilities, respectively, and are used to describenonlinear optical effects such as the electro-optic effect andthird harmonic generation.

Another notation for describing the effect of an electricfield (E) on the optical properties of a material makes use ofthe index of refraction (n) directly. 1,4

(1/n2 ) = (1/n02 ) + r.E + h:EE + ... (8)

no is the index of refraction at zero field and r and h areknown as the linear and quadratic electro-optic coefficients.This type of notation is popular for describing effects ininorganic crystals.

The linear electro-optic effect, also known as the Pockelseffect, is characterized by the second-orderhyperpolarizability, x(2), or the linear electro-opticcoefficient, r, and is only possible in noncentrosymmetricmaterials, i.e., materials which do not have centers ofinversion. This also happens to be the symmetry requirementfor the linear piezoelectric effect.1, 4 ,5 (The piezoelectriceffect is the production of an electric field in a material bythe application of a stress or strain.) The majority of workconducted on linear electro-optic materials has focussed oninorganic crystals. 1,4 However, organic/polymeric materialshave been receiving increased attention because of thepotential for large electro-optic effects coupled with bettermechanical processing and design flexibility. Below, a briefdescription of the important molecular and material propertiesfor enhanced electro-optic effects in organic materials is

15

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presented. This is followed by a review of recent researchefforts in material development with respect to applications athigh modulation frequencies.

Organic Material Development

To achieve response times on the order of GHz, the opticalprocesses occurring in solid materials must be mainlyelectronic in nature, i.e., vibrational and rotationaly motionsare too slow. Thus, from a molecular point of view, enhancedelectro-optic effects will be demonstrated bynoncentrosymmetric molecules with easily polarized molecularelectric fields. This latter condition is readily met inunsaturated organic molecules with highly conjugated w electronsystems. The addition of substituent groups that induce acharge asymmetry, i.e., strong electron donor and acceptorgroups, will also greatly enhance the electro-optic effect bymaking it easier to polarize such molecules. Great success hasbeen achieved in synthesizing molecules of this type (second-order nonlinear optical molecules) using the abovecriteria.49-53

The incorporation of the active species into a macroscopicsample is also an area of current research; noncentrosymmetrywith respect to molecular orientation must be maintained inorder to observe electro-optic effects. Additionally, the moreordered the material, the greater the electro-optic effect. Thetwo most popular and successful methods for ordering moleculesin materials are electric field poling and Langmuir-Blodgettfilm deposition. Detailed descriptions of both of theseordering methods can be found elsewhere.5 4 - 5 7

Most of the recent research involving second-order organicmaterials for use in electro-optic devices has focussed onliquid crystal side chain polymers and guest/host typematerials. Guest/host materials are prepared by dispersing theactive molecules in a polymeric host material while liquidcrystal side chain polymers are formed by attaching the activemolecule to the polymer mainchain using spacer groups todecouple motion of the side chain from the backbone (See Fig.4). In both cases, electric field poling is used to orient thematerial.

Comparison with Inorganic Materials

Table 5 contains several organic and inorganic electro-optic materials and parameters describing the magnitude of thereported electro-optic effect. The most common techniques formeasuring these parameters are second harmonic generation(x(2)) and modulated phase waveguiding (r). When comparingvalues, it is important to note that second harmonic generationcan only arise from electronic contributions while electro-optic measurements can include contributions from molecular

16

Page 20: I3 of Organic Polymeric Materials · ACOUSTO-OPTIC AND LINEAR ELECTRO-OPTIC PROPERTIES OF ORGANIC POLYMERIC MATERIALS INTRODUCTION Currently, most acousto-optic and electro-optic

CI) Z 0

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Page 21: I3 of Organic Polymeric Materials · ACOUSTO-OPTIC AND LINEAR ELECTRO-OPTIC PROPERTIES OF ORGANIC POLYMERIC MATERIALS INTRODUCTION Currently, most acousto-optic and electro-optic

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18

Page 22: I3 of Organic Polymeric Materials · ACOUSTO-OPTIC AND LINEAR ELECTRO-OPTIC PROPERTIES OF ORGANIC POLYMERIC MATERIALS INTRODUCTION Currently, most acousto-optic and electro-optic

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19

Page 23: I3 of Organic Polymeric Materials · ACOUSTO-OPTIC AND LINEAR ELECTRO-OPTIC PROPERTIES OF ORGANIC POLYMERIC MATERIALS INTRODUCTION Currently, most acousto-optic and electro-optic

vibrations and rotations. 58,59 Therefore, an organic materialwith the same x(2) as an inorganic material (as measured bysecond harmonic generation) will probably have a smallerelectro-optic coefficient (r), a consequence of the importantcontributions of lattice phonons to the electro-opticcoefficient of inorganic crystals. In other words, theelectro-optic effect in organic materials arises mainly fromelectronic contributions (polarizations of the loosely boundelectrons) while in inorganics the main contribution is fromphonons (distortions of the nuclear framework and it's tightlybound electrons) .60-62

The delocalized electronic nature of organic materialsresults in other material properties that are very important toelectro-optic effects. One of the most important involvesmodulation frequency. 55,57,58

The response time of electro-optic devices depends on thedielectric properties of the material and favors lower values:the bandwidth per modulating power (Av/P) of a guided-wavemodulator is given by 55 ,5 7

AV/P a /( + (e/0))] (9)

while for a traveling wave modulator the rise time is describedby 5 5 ,5 7

tr 0 (/eo) - n (10)

where e is the static dielectric constant. The dielectricconstants of organic materials are, in general, substantiallylower than those of inorganic materials and arenondispersive.6 3 The dielectric losses (tan 6) in organics arealso generally low and nondispersive (See Table 6). Therefore,organic materials should allow higher modulation frequencies(than inorganics) for the same voltage (Fig. 5). This is aconsequence of the "disassociation" of polarizable electronsfrom nuclei in organic materials.

Experimentally, a variety of device designs are used tomeasure the electro-optic properties of materials; the two mostcommon are diagrammed on the following pages. MHz modulationof a variety of organic materials has been amply demonstratedusing these types of devices. 64 ,6 5 With regard to higherfrequency modulation, phase modulation of a liquid crystal sidechain polymer, in one arm of a Mach-Zender interferometer, atone GHz has been reported but not published. 6 4 A theoreticalcalculation has also been reported, but not published, thatindicates polymeric materials could be modulated up to 20 GHz(See Fig. 8).

6 5

20

Page 24: I3 of Organic Polymeric Materials · ACOUSTO-OPTIC AND LINEAR ELECTRO-OPTIC PROPERTIES OF ORGANIC POLYMERIC MATERIALS INTRODUCTION Currently, most acousto-optic and electro-optic

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21

Page 25: I3 of Organic Polymeric Materials · ACOUSTO-OPTIC AND LINEAR ELECTRO-OPTIC PROPERTIES OF ORGANIC POLYMERIC MATERIALS INTRODUCTION Currently, most acousto-optic and electro-optic

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Page 26: I3 of Organic Polymeric Materials · ACOUSTO-OPTIC AND LINEAR ELECTRO-OPTIC PROPERTIES OF ORGANIC POLYMERIC MATERIALS INTRODUCTION Currently, most acousto-optic and electro-optic

-4

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23

Page 27: I3 of Organic Polymeric Materials · ACOUSTO-OPTIC AND LINEAR ELECTRO-OPTIC PROPERTIES OF ORGANIC POLYMERIC MATERIALS INTRODUCTION Currently, most acousto-optic and electro-optic

>--

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Page 28: I3 of Organic Polymeric Materials · ACOUSTO-OPTIC AND LINEAR ELECTRO-OPTIC PROPERTIES OF ORGANIC POLYMERIC MATERIALS INTRODUCTION Currently, most acousto-optic and electro-optic

-- ..- 44i

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o 0 -

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25

Page 29: I3 of Organic Polymeric Materials · ACOUSTO-OPTIC AND LINEAR ELECTRO-OPTIC PROPERTIES OF ORGANIC POLYMERIC MATERIALS INTRODUCTION Currently, most acousto-optic and electro-optic

In conclusion, organic and polymeric materials haveseveral advantages over inorganic materials for use in electro-optic devices: ease and flexibility in mechanical processing,high optical damage thresholds, low modulating voltages, lowdielectric constants and large electro-optic coefficients. (Thelargest known electro-optic coefficient was measured for theorganic crystal methylnitroaniline, 67 pm/V) .66,67 GHzmodulation in organic polymeric materials has been observed andresearch is underway to extend this to higher frequencies.

26

Page 30: I3 of Organic Polymeric Materials · ACOUSTO-OPTIC AND LINEAR ELECTRO-OPTIC PROPERTIES OF ORGANIC POLYMERIC MATERIALS INTRODUCTION Currently, most acousto-optic and electro-optic

REFERENCES

[1] Pinson, L.J., Electro-Optics, John Wiley & Sons:New York,1985.

(2] Houston, J.B., SPIE Acousto-Optics: DeviceDevelopment/Instrumentation/Applications, Vol. 90, 1976.

[3] Sapriel, J., Acousto-Optics, John Wiley & Sons:New York,1979.

[4] Gottlieb, M., Ireland, C.L.M., Ley, J.M., Electro-Opticand Acousto-Optic Scanning and Deflection, Marcel Dekker,Inc:New York, 1983.

[5] Gottlieb, M., "Elastooptic Materials," in CRC Handbookof Laser Science and Technology, Volume 4:OpticalMaterials, M. Weber (ed.), CRC Press:Boca Raton, FL., 1986.

[6] Chang, I.C., "I. Acoustooptic Devices and Application," IEEETransactions on Sonics and Ultrasonics, SU-23 (1976) 2-21.

[7] Born, M., Wolf, E., Principle of Optics, Pergamon Press:NewYork, 1975, pp. 593-610.

[8] Pinnow, D.A., "Guide Lines for the Selection of AcoustoopticMaterials," IEEE Journal of Quantum Electronics QE-6 (1970)223-238.

(9] Uchida, N., Niizeki, N., "Acoustooptic Deflection Materialsand Techniques," Proceedings of the IEEE 61 (1973) 1073-1092.

(10) Dixon, R.W., "Photoelastic Properties of Selected Materialsand Their Relevance for Applications to Acoustic LightModulators and Scanners," J. Appl. Phys. 38 (1967) 5149-5153.

£11] Bikales, N.M. (ed), Encyclopedia of Polymer Science andTechnology: Plastics, Resins, Rubbers, Fibers, Vol. 9, JohnWiley & Sons:New York, 1968.

[12] Mueller, H., "Theory of the Photoelastic Effect of CubicCrystals," Phys. Rev. 47 (1935) 947-957.

(13] Hartman, B., "Further Mechanical Techniques," in Methodsof Experimental Physics, Vol 16c, R.A. Fava, (ed), AcademicPress:New York, 1980, pp. 59-91.

[14] Szilard, J., "Physical Principles of Ultrasonic Testing,"in Ultrasonic Testing, J. Silzard (ed), John Wiley &Sons:New York, 1982, pp. 1-23.

27

Page 31: I3 of Organic Polymeric Materials · ACOUSTO-OPTIC AND LINEAR ELECTRO-OPTIC PROPERTIES OF ORGANIC POLYMERIC MATERIALS INTRODUCTION Currently, most acousto-optic and electro-optic

[15] Hartmann, B., "Acoustic Properties," in Encyclopedia ofPolymer Science and Engineering, Vol. 1, Second Edition,Mark-Bikales-Overberger-Menges (ed), John Wiley & Sons,1985, pp. 131-160.

[16] Hartmann, B., Jarzynski, J., "Ultrasonic HysteresisAbsorption in Polymers," J. Appl. Phys. 43 (1972) 4304-4312.

[17] Jackson, D.A., Pentecost, H.T.A., Powles, J.G., "HypersonicAbsorption in Amorphous Polymers by Light Scattering," Mol.Phys. 23 (1972) 425-432.

[18] Hu, Z., Vanderwal, J.J., Walton, D., "Brillouin-scatteringStudy of Propylene Glycol," Phys. Rev. A37 (1988) 4507-4509.

[19] Kruger, J., Peetz, L., Pietralla, M., "Brillouin Scatteringof Semicrystalline Poly(4-methyl-l-pentene): Study ofSurface Effects of Bulk and Film Material," Polymer 19(1978) 1397-1404.

[20] Brody, E.M., Lubell, C.J., Beatty, C.L., "Measurement of theGruneisen Number in Polystyrene by Brillouin LightScattering," J. Poly. Sci:Poly. Phys. 13 (1975) 295-301.

[21] Patterson, G.D., "Brillouin Scattering from Polymer Films,"J. Poly. Sci.:Poly. Phys. 14 (1975) 143-149.

[22] Patterson, G.D., "Brilluoin Scattering and HypersonicRelaxation in Amorphous Polymers," J. Poly. Sci:Poly. Phys.15 (1977) 455-464.

[23) Quate, C.F., Wilkinson, C.D.W., Winslow, D.K., "Interactionof Light and Microwave Sound," Proceedings of the IEEE 53(1965) 1604-1622.

[24] Wang, C.H., Liu, Q-L., "Rayleigh-Brillouin Scattering ofTransverse and Longitudinal Acoustic Waves in a SupercooledViscoelastic Liquid," J. Chem. Phys. 78 (1983) 4363-4373.

[25] Kline, G.M. (ed.) , Analytical Chemistry of Polymers: PartIII. Identification Procedures and Chemical Analysis,Interscience Publishers:New York, 1962.

[26] Boyle, M.E., Cozzens, R.F., "Eye/Sensor Protection AgainstLaser Irradiation: Organic Nonlinear Optical Materials,"NRL Memorandum Report - in press.

[27] Khoo, I.C., Normandin, R., "Nanosecond-laser-induced OpticalWave Mixing and Ultrasonic Wave Generation in the NematicPhase of Liquid Crystals," Optics Letters 9 (1984) 285-287.

28

Page 32: I3 of Organic Polymeric Materials · ACOUSTO-OPTIC AND LINEAR ELECTRO-OPTIC PROPERTIES OF ORGANIC POLYMERIC MATERIALS INTRODUCTION Currently, most acousto-optic and electro-optic

[28] Kessler, L.W., Sawyer, S.P., "Ultrasonic Stimulation ofOptical Scattering in Nematic Liquid Crystals," Appl. Phys.Lett. 17 (1970) 440-441.

[29] Bader, M., Martinoty, P., "Birefringence Induced byUltrasonic Waves in the Isotropic Phase of PCB," Mol.Cryst. Liq. Cryst. 76 (1981) 269-277.

[30] Belova, G.N., Remizova, E.I., "Acoustooptic Effect in anUnoriented Layer of Nematic Liquid Crystal Under PeriodicShear Deformation," Sov. Phys. Crystallogr. 31 (1986) 304-307.

[31] Martinoty, P., Bader, M., "Pulse Acousto-optic Modulatorbsing a Nematic Liquid Crystal in its Isotropic Phase,"Appl. Phys. Lett. 37 (1980) 33-34.

[32] Bertolotti, M., Scudieri, F., Sturla, E., "Ultrasound-Produced Rolls in Nematic Liquid Crystals as Studied Throughthe Spatial Coherence of Scattered Light," J. Appl. Phys.49 (1978) 3922-3926.

[33] Popov, A.I., Pasenchnik, S.V., Balandin, V.A., Nozdrev,V.F., "Diffraction of Light by Periodic Structures Inducedby Ultrasound in a Layer of Nematic Liquid Crystal," Sov.Phys. Crystallogr. 29 (1984) 74-76; Popov, A.I., Pasenchnik,S.V., Nozdrev, V.F., Balandin, V.A., "Dynamics ofUltrasound-Induced Diffraction Effects in a Nematic LiquidCrystal Oriented with a Magnetic Field," Sov. Tech. Phys.Lett. 8 (1982) 431.

[34] Osadchev, L.A., "Efficiency of Acoustooptic Interactions inPolystyrene Optical Waveguides," Sov. Phys. Tech. Phys. 25(1980) 106-108.

[35] Andrushko, L.M., Voznesenskii, V.A., "AcoustoopticInteractions in Polymer Films," Opt. Spectrosc. 56 (1984)649-650.

[36] Schmidt, R.V., "II. Acoustooptic Interaction Between GuidedOptical Waves and Acoustic Surface Waves," IEEETransactions on Sonics and Ultrasonic, SU-23 (1976) 22-32.

[37] Stegeman, G.I., "III. Optical Probing of Surface Waves andSurface Wave Devices," IEEE Transactions on Sonics andUltrasonics SU-23 (1976) 33-63.

[38] Hartmann, B., Jarzynski, J., "Immersion Apparatus forUltrasonic Measurements in Polymers," J. Acoust. Soc. Am.56 (1974) 1469-1477.

29

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[39] Asay, J.R., Lamberson, D.L., Guenther, A.H., "Pressure andTemperature Dependence of the Acoustic Velocities inPolymethylmethacrylate," J. Appl. Phys. 40 (1969) 1768-1783.

[40] Hartmann, B., "Ultrasonic Properties of Poly(4-methylpentene-l)," J. Appl. Phys. 51 (1980) 310-314.

[41] Lagakos, N., Jarzynski, J., Cole, J.H., Bucaro, J.A.,"Frequency and Temperature Dependence of Elastic Moduli ofPolymers," J. Appl. Phys. 59 (1986) 4017-4031.

[42] Smith, T.M., Korpel, A., "Measurement of Light-SoundInteraction Efficiencies in Solids," IEEE Journal ofQuantum Electronics, September, 1965.

[43] Sutherland, H.J., "Acoustical Determination of the ShearRelaxation Functions for Polymethyl methacrylate and Epon828-Z," J. Appl. Phys. 49 (1978) 3941-3945.

[44] Phillips, D.W., North, A.M., Pethrick, R.A., "UltrasonicStudies of Polycarbonate, Polysulfone, and PolyetherSulfone," J. Appl. Poly. Sci. 21 (1977) 1859-1867.

[45] Folds, D.L., "Experimental Determination of Ultrasonic WaveVelocities in Plastics, Elastomers and Syntactic Foam as aFunction of Temperature," J. Acoust. Soc. Am. 52 (1972)426-427.

[46] Froelich, B., Neol, C., Monnerie, L., "Dynamics of MolecularChains. Ultrasonic Relaxation of MonosubstitutedPolystyrenes," Polymer 20 (1979) 990-994.

[47] Private communication with Dr. N. Lagakos, Naval ResearchLaboratory, Code 5134, Washington, D.C., 20375-5000.

[48] The majority of material found in this section has beentaken from Ref. 26, a review of organic nonlinear opticalmaterials. Readers are referred to this study foradditional information and references.

[49] Williams, D. (ed.), Nonlinear Optical Properties of Organicand Polymeric Materials, ACS Symp. Ser. No. 233,ACS:Washington, D.C., 1983.

[50] Prasad, P., Ulrich D. (eds.), Nonlinear Optical andElectroactive Polymers, Plenum Press:New York, 1988.

[51] Chemla, D., Zyss, J (eds.), Nonlinear Optical Properties ofOrganic Molecules and Crystals, Volumes 1 and 2, AcademicPress:New York, 1987.

30

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[52] Khanarian, G. (ed.), Molecular and Polymeric OptoelectronicMaterials: Fundamentals and Applications, Proceedings ofSPIE:Washington, Volume 682, 1985.

[53] Musikant, S. (ed.), Advances in Materials for Active Optics,Proceedings of SPIE, Volume 567, SPIE:Washington, 1985.

[54] Willand, C.S., Feth, S.E., Scozzafava, M., Williams, D.J.,Green, G.D., Weinshenck, J.I., Hall, H.K., Mulvaney, J.E.,"Electric-Field Poling of Nonlinear Optical Polymers," inNonlinear and Electroactive Polymers, P. Prasad and D.Ulrich(eds.), Plenum Press:New York, 1988.

[55] Singer, K.D., Kuzyk, M.G., Sohn, J.E., "OrientationallyOrdered Electro-Optic Materials," in Nonlinear andElectroactive Polymers, P. Prasad and D.Ulrich (eds.),Plenum Press:New York, 1988.

[56] Barraud, A., Vandevyer, M., "Chapter 11-5. Growth andCharacterization of Organic Thin Films (Langmuir-BLodgettFilms) ," in Nonlinear Optical Properties of OrganicMolecules and Crystals, Volume 1, Academic Press:New York,1987.

[57] Small, R.D., Singer, K.D., Sohn, J.E., Kuzyk, M.G., Lalama,S.J., "Thin Film Processing of Polymers for NonlinearOptics," in SPIE Molecular and Polymeric OptoelectronicMaterials: Fundamentals and Applications 682 (1986) 160-169.

[58] Lytel, R., Lipscomb, G.F., Thackara, J., Altman, J.,Elizondo, P., Stiller, M., Sullivan, B., "Nonlinear andElectro-Optic Organic Devices," in Nonlinear andElectroactive Polymers, P. Prasad and D.Ulrich (eds.),Plenum Press:New York, 1988.

[59] Stamatoff, J.B., Buckley, A., Calundann, G., Choe, E.W.,DeMartino, R., Khanarian, G., Leslie, T., Nelson, G.,Stuetz, D., Teng, C.C., Yoon, H.N., "Development ofPolymeric Nonlinear Optical Materials," in SPIE Molecularand Polymeric Optoelectronic Materials: Fundamentals andApplications 682 (1986) 85-92.

[60] Garito, A., Government Report AD-A172 024, 1986.

:61] Garito, A., Teng, C., "Nonlinear optical processes in

organic media: large non-resonant third order electronicresponses in high performance liquid crystal polymerstructures," in SPIE Molecular and PolymericOptoelectronic Materials: Fundamental and Applications 682(i986) 85-92.

31

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£621 Garito, A., Wong, K.Y., Cai, Y.M., Man, H.T.,Zamani-Khamiri, 0., "Fundamental nonlinear optics issues inorganic and polymer systems," in SPIE Molecular andPolymeric Materials: Fundamentals and Applications 682(1986) 2-11.

[63] Brandrup, J., Immergut, E.H. (eds.), Polymer Handbook,Second Edition, John Wiley & Sons:New York, 1975.

£641 Lytel, R., "Nonlinear Optical Polymers Contractors Meeting,"Air Force Office of Scientific Research:National Academy ofSciences, Washington, D.C., April, 1988.

£65] Stamatoff, J., "Nonlinear Optical Polymers ContractorsMeeting," Air Force Office of Scientific Research:NationalAcademy of Sciences, Washington, D.C., April, 1988.

£661 Lipscomb, G.F., Garito, A.F., Narang, R.S., "Anexceptionally large linear electro-optic effect in theorganic solid MNA," J. Chem. Phys. 75 (1981) 1509-1516.

[671 Lipscomb, G.F., Garito, A.F., Narang, R.S., "A large linearelectro-optic effect in a polar organic crystal 2-methyl-4-nitroaniline," Appl. Phys. Lett. 38 (1981) 663-665.

32