Materials for Nonlinear Optics - American Chemical Society

Chapter 32

Ferroelectric Liquid Crystals Designed for Electronic Nonlinear Optical Applications 1

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David M. Walba , M. Blanca Ros , Noel A. Clark , Renfan Shao , Kristina M. Johnson , Michael G. Robinson , J. Y. Liu , and David Doroski 3

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Downloaded by IMPERIAL COLLEGE LONDON on February 18, 2015 | http://pubs.acs.org Publication Date: March 11, 1991 | doi: 10.1021/bk-1991-0455.ch032

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Department of Chemistry and Biochemistry, Department of Physics, and Department of Electrical and Computer Engineering and Optoelectronic Computing Systems Center, University of Colorado, Boulder, CO 80309 3

The first ferroelectric liquid crystal (FLC) designed specifically for electronic second order nonlinear optics (NLO) applications, and possessing|d ff|> 0.074 pm/V for frequency doubling from 1.064 μm Nd:YAG laser light, is described. This value (a conservative lower limit) is about 7 times larger than d ff of the commercial FLC mixture SCE9, until now the FLC material with the largest measured second order susceptibility. Our approach to the design of FLCs for NLO, a description of the polar order expected for the C* phase for the newly designed materials, and data on the macroscopic spontaneous electric polarization exhibited by these materials is given in addition to the NLO data. e

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Ferroelectric liquid crystals (FLCs) are true fluids possessing thermodynamically stable polar order. When incorporated into devices in the Clark-Lagerwall surface stabilized ferroelectric liquid crystal (SSFLC) geometry (1), electro-optic and photo-optic light valves with very large interaction strength (i.e. half-wave interaction lengths on the order of 2 μιη) are easily fabricated. However, since switching in the SSFLC light valve involves large nuclear motions, the switching speeds are relatively slow (for currently available materials rise times are = 20 μsec in response to 15 V/μιη driving field). Even given the slow switching, SSFLC light valves are typically several orders of magnitude faster than other liquid crystal switches. This coupled with low switching energy, bistable memory, the strong interaction with light, and relative ease of obtaining "single crystal" thin films on diverse substrates (glass, amorphous silicon films, silicon integrated circuits), has led SSFLC devices to emerge as a potentially important solution for many optoelectronic applications, ranging from spatial light modulators for flat panel T V and optical computing to phase conjugating mirrors (2). The advantages of SSFLC devices derive to a large extent from the spontaneous macroscopic polarization Ρ of the phase. For example the electrcoptic rise time of a prototypical SSFLC light valve is inversely proportional to the magnitude of the polarization. In order to design new F L C materials with large Ρ in a directed way, we 0097-6156/91/0455-O484$06.00/0 © 1991 American Chemical Society In Materials for Nonlinear Optics; Marder, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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have developed a simple stereochemical model for the molecular origins of P, and have been testing and refining the model in a program of synthesis and evaluation of new F L C materials (3-13). Based upon this work, we feel it should be possible to orient virtually any organic functional array along the polar axis of an F L C thin film. Naturally, this proposal suggests another application of F L C materials: Electronic second order nonlinear optics (NLO). For example, F L C films possessing large second order hyperpolarizability χ( ) should provide the basis of a light switching technology where the optical interaction strength is much weaker than in SSFLC devices, but the switching speeds are much faster. Of course, for many applications, in general those where large information density is temporal rather than spatial, this tradeoff of interaction strength for speed is desirable. In this paper, we describe initial experiments of a project aimed at the development of F L C materials with large χ( ).


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Background—FLCs for NLO F L C phases in the surface stabilized geometry possess a single C2 axis of symmetry, and therefore polar order and non-zero χ( > in the simple electronic dipolar model. Thus, it is not surprising that experiments aimed at measuring this property were first reported shortly after the Clark-Lagerwall invention. Early studies (14-15) described second harmonic generation in (S)-2-Methylbutyl 4-(4-decyloxybenzylideneamino)cinnamate, the first ferroelectric liquid crystal, also known as D O B A M B C (1). 2

D O B A M B C (1) Α-priori, given that F L C phases possess polar order, and the D O B A M B C structure should possess a substantial molecular second order hyperpolarizability β, it seems reasonable that χ( ) of a material such as D O B A M B C in the F L C phase might be large. Measured values of the second harmonic generation efficiencies of D O B A M B C and of several other F L C materials, however, indicate that in fact the dcff of FLCs is very small relative to LiNb03. For example, prior to 1990 the commercial F L C mixture ZLI 3654 was shown by Taguchi, Ouchi, Fukuda, and Takazoe to possess the largest known second order susceptibility for an F L C material (16-17). Using type I eeo phase matching, a value of Ideffl = 0.0025 time Id22' ( L 1 N D O 3 ) was measured for ZLI 3654 when phase matched. Recently, Johnson et al have more fully characterized the commercial F L C material SCE9, which possessed double the d ff of ZLI 3654 (18). Using measured values of the refractive indices (the sample behaves uniaxially), Maker fringe measurements on wedge cells, type I eeo angular phase matching, and some assumptions regarding the ratio of d-coefficients, it was possible to obtain an excellent fit to the observed phase matching curve, and to extract the absolute values of all the non-zero components of the d tensor for the C2 symmetrical sample of SCE9. The values so obtained are given in Table I. The coordinate system used for this work has y along the polar axis and ζ along the liquid crystal director. 2

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In Materials for Nonlinear Optics; Marder, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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MATERIALS FOR NONLINEAR OPTICS: CHEMICAL PERSPECTIVES

Table I. Absolute values of the components of the d tensor for the commercial F L C material SCE9 d

= d34 = 0.073(2) d 2 = 0.027(1) d i = d i = 0.0026(1) d?,s = d u = d 6 = 0.0009(1) 2 3

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pm/V pm/V pm/V pm/V

Two points of special interest concerning the data given in Table 1 are as follows. First, the largest component is d 3, that reflecting the second order N L O response of the sample along the polar axis when driven along the F L C director. The component reflecting the response along the polar axis when driven along the polar axis (d ) is a factor of two smaller. This seems reasonable in light of the fact that the axis of maximum linear polarizability (maximum refractive index) is along the director, normal to the polar axis in known FLCs. Second, given that for L1NDO3 dl5 = 5 pm/V and d = 3.2 pm/V, the values for SCE9, with the largest second order susceptibility reported to date for an FLC, are small. In the context of organic second order NLO materials, where electronic hyperpolarizabilities can be very large relative to inorganics such as LiNb03, the small measured χ( ) for FLCs seems discouraging and has led some workers in the field to dismiss these materials as a possible solution to the general problem of obtaining thin films with large χ( ). The processibility and stability advantages of FLCs and F L C polymers, however, would make them potentially very useful if materials with large χ( ) could be obtained. A description of our approach to solving this fascinating problem in materials synthesis follows.


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Geometry of the F L C self assembly Figure 1 illustrates the relationship between the laboratory frame (glass bounding plates), smectic layers, the layer normal z, and the F L C director η (the long axis of the molecules = the optic axis of the sample), in a parallel-aligned SSFLC cell. Generally, parallel alignment is favored by standard rubbed polymer alignment layers (films of polymer about 300A thick spun onto the glass plates and rubbed — when filled with F L C , such surface treatment induces a "crystal" orientation where the layers are normal to the rubbing direction). With this geometry of the F L C relative to the bounding plates, the C axis of the crystal is oriented normal to the plates, and the director aligns parallel to the plates. Under these conditions the tilt plane, that plane containing both ζ and n, is parallel to the plates with the following caveat. Elegant studies of defects commonly occurring in SSFLC cells, and termed zig zag walls, have shown that the layers are in fact tilted with respect to the bounding plates, taking on a chevron structure as indicated in Figure 1 (19-20). Thus, the polar axis of the crystal and the tilt plane actually intersect the bounding plates at an angle. It is important to note, however, that the chevron layer structure itself introduces no problematic defects, but rather defects occur where two chevron domains intersect. Since it is possible to obtain a monodomain of chevrons, essentially "single crystal" F L C films may be relatively easily fabricated over large areas. In addition, in the absence of the bounding plates, a spontaneous helix develops in the director field about the layer normal along the tilt cone (indicated in the Figure) 2



Figure 1. The geometry of a parallel-aligned SSFLC cell. Note that the spacing between the glass bounding plates (= 1.5 μm) and the smectic layer spacing (= 35 A) are not to scale.



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with a pitch of about 1 μιη. If the layer thickness (spacing between the plates) is small relative to the helix pitch, then the helix is spontaneously unwound. If, however, the layer is thick relative to the helix pitch, then the sample far from the plates will exhibit the helix parallel to the surface of the plates. Application of an electric field normal to the plates (typically the plates are coated with thin films of conducting glass such as indium-tin oxide) unwinds the helix if there is one, and also may cause the polar axis to orient normal to the plates (along the field), or even flatten the chevrons. It should be stressed that any added orientation of molecular dipoles along the field direction should be a weak secondary effect — the polar order occurring in the F L C phase is a thermodynamic property of the phase and not dependent upon applied fields. Molecular Origins of Ρ in FLCs The existence of the layers and director tilt in the achiral smectic C liquid crystal phase are experimental facts. Given these, the maximum possible symmetry of the phase would be Ci, with a C2 axis normal to the tilt plane, and a σ plane congruent with the tilt plane. In fact, there is no fundamental reason why a given C phase must possess either of these symmetry elements. But, breaking of either of the symmetry elements would afford polar symmetry, and no C phase has ever been shown to possess any property associated with polar symmetry (e.g. pyroelectricity). Therefore, we can say that all known C phases indeed possess the maximum possible symmetry consistent with the layers and tilt. As first realized by Meyer in 1974, when the molecules making up the C phase are non-racemic, the resulting chiral C* phase can possess no reflection symmetry. Thus, the maximum possible symmetry of a C* phase is C2, and the phase must possess polar order (21). One of the macroscopic manifestations of polar order can be a macroscopic electric dipole moment (the polarization P) associated with orientation of molecular dipoles along the polar axis. While the existence of polar order is not sufficient to assure an observable polarization (just as chirality does not assure optical activity), in fact many F L C materials do possess an observable P. This polarization has a sign, which for all known materials is related directly to the chirality of the molecules. Thus, enantiomers possess equal magnitude but opposite sign of P. By convention, Ρ (pointing from the negative end to the positive end of the macroscopic dipole following the physics convention) is positive when in the direction of the unit vector ζ χ η. Ρ is negative if opposed to ζ χ ft. While the symmetry argument is rigorous, it gives no insights into which functional groups are actually oriented along the polar axis. For example, (S)D O B A M B C possess an experimental polarization of -0.009 Debye/molecule. Given the large dipoles present in the molecule, it is not clear at all exactly which of those dipoles is oriented along the polar axis, or how strong the orientation is. In order to design new F L C materials possessing large Ρ or χ( ), it is necessary to have some insight into the molecular origins of the functional group orientation occurring in the phase. Much of our recent research has focussed upon the development of such a model, the basics of which have been published (1-13). In essence, the molecular orientation occurring in the phase derives from intermolecular interactions between neighboring molecules (the "crystal" lattice). On the time average, this interaction may be considered to take the form of a "binding site" with the shape of a bent cylinder oriented in a specific way relative to the tilt plane. The C phase order results when the molecules "dock" into this binding site. 2



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Currently, we feel that the existing data may be interpreted by assuming that the shape of this binding site is the same for known C phases or C* phases, regardless of the structure, chirality or achirality of the actual molecules comprising the phase. Thus, the molecular order occurring in the phase is the same whether the molecules are chiral or achiral. If the molecules are achiral or racemic, this order gives rise to no macroscopic polarization, while a polarization results when the molecules are nonracemic. As indicated in Figure 1, the binding site shape is such that the molecules are oriented with the tails less tilted than the core relative to the layer normal. The macroscopic polarization of the phase is given by equations 1 and 2, where Dj is the number density of the ith conformation, is the component of the molecular dipole normal to the tilt plane when the ith conformation of the molecule is oriented in the rotational minimum in the binding site, ROF, is the "rotational orientation factor", a number from zero to one reflecting the degree of rotational order for the ith conformation, and ε is a complex and unmeasured dielectric constant of the medium (local field correction).

p=E i*i D

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over all i conformations

Pj = f l j

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OF

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For this discussion, several points should be stressed here. Most importantly, there is no polar order along the director in any known liquid crystal phase, including the C* phase. Thus, functional arrays with large β along the director are not oriented along a polar axis in the F L C phase. This is our interpretation of the small χ( ) of D O B A M B C and other F L C materials. There are other possible problems as well, however. For example, though D O B A M B C possesses substantial dipoles oriented normal to the director, it's observed macroscopic polarization (-0.009 D/molecule) is very small. This could be due to poor molecular orientation in the F L C phase, which in turn could represent a fundamental problem in design of FLCs for χ( ). We feel, however, that the data suggests molecular order in FLCs is in fact high. That is, when the F L C compounds are properly designed, molecular dipoles are oriented along the polar axis to a high degree. Thus, consider the 1-fluorononylbenzoate 2 shown in Figure 1. According to our model, the only molecular dipole expected to orient anisotropically along the polar axis is the C-F dipole, with the orientation as indicated in the Figure. Thus, assuming a dipole moment of 2 D for the C-F bond, if the orientation were perfect (as in a crystal), then the component of the C-F dipole along the polar axis (jljj would be about 1.6 D, and assuming ε = 3, and of course ROF=l, then the macroscopic polarization should be about 0.5 D/molecule according to equ 2. In fact, the experimental polarization of compound 2 = +44 nC/cm = +0.15 D/molecule assuming a density of 0.8 gm/cm , or fully 1/3 of that expected for the corresponding perfectly oriented solid (7). While this calculation is very crude, and depends upon estimation of a dielectric constant which cannot be measured, it still seems that good functional group orientation typically can occur in F L C phases (i.e. ROF>0.1). 2

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Design of FLCs for N L O While typical high polarization FLCs may show good functional group orientation along the polar axis, these groups are normally carbon-halogen, epoxy, or ester units expected to produce only a small bulk second order hyperpolarizability. This is our interpretation of the small χ< ) observed in FLCs to date. In considering design of FLCs with large χ( ), the simplest approach would be to achieve good orientation of functionalized aromatic rings along the polar axis in an F L C film. In fact, recent results strongly suggest that such orientation does occur, and can be easily understood in terms of the stereochemical model. Thus, as first demonstrated experimentally by the F L C chemistry group at Chisso (22), and later by us (11). FLCs possessing an aromatic ring with the 1-methylheptyloxy chiral tail, and substituted on the ring ortho to this tail, exhibit sign and magnitude of Ρ consistent with good orientation of the functionalized ring along the polar axis. We felt that the o-nitro-l-methylheptyloxy aromatic system should afford appropriate geometry for orientation of the nitroalkoxy β along the polar axis, and therefore prepared the series of compounds 3 - 5 . Compounds 3 and 4 represent our first generation of FLCs designed specifically for χ( ), while structure 5 serves as a control. To our knowledge, no compounds possessing an o-nitroalkoxy array similar to compounds 3 and 4 have been previously reported in the literature. 2


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Several empirical design criteria for these materials were applied. Specifically, it is well known that the 3-ring biphenylbenzoate core system, first explored by Gray and Goodby (23). is among the best for obtaining broad temperature range smectic C phases. However, these materials also typically possess large orientational viscosity, affording slow switching speeds in SSFLC devices. A major advantage of N L O from the point of view of F L C design is that this orientational viscosity is of only secondary importance for electronic NLO since switching does not require nuclear motion. In addition, the 3-ring biphenylbenzoates tend to show larger polarizations (better orientation) for a given chiral tail than a corresponding two ring system. Finally, in general they are relatively easily synthesized in a convergent manner. Most importantly, of course, is the expectation that the nitroalkoxy functional array in compounds 3 and 4, but not compound 5, should orient along the polar axis in a geometry leading to good orientation of molecular $s for large χ( > in the F L C phase. The rationale for this expectation was developed to interpret the sign and magnitude of Ρ observed for the unsubstituted 1-methylheptyloxy FLCs prior to any of the experimental reports on o-substitution. As illustrated in Figure 2 for the nitroalkoxy system found in compound 3, according to the model the two conformers A and Β should predominate in the C* phase. In these drawings, the polar axis of the phase is (almost) parallel to the plane of the page (normal to the tilt pane), and the conformers 2



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Figure 2. Preferred conformational and rotational orientation relative to the tilt plane for compound 3 in the C* phase according to the Boulder Model.


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are oriented relative to the tilt plane by our expectation of how they would dock into the bent cylinder binding site. Note that there must be an equal number density of molecules in an orientation flipped by 180° about the polar axis (the C axis present in all F L C phases), and also that these structures really represent large families of conformations, since many conformers differing in bonds past the stereocenter should also orient as indicated. 2


Experimental Evidence of Polar Orientation of the o-Nitroalkoxy System in the C* Phase The drawings in Figure 2 illustrate that a large, negative polarization is expected for both compounds 3 and 4 for the (S) absolute configuration at the stereocenter. In agreement with the Chisso data on similar o-halo and o-cyano substituted FLCs (22), large negative Ρ is indeed observed, as shown by the data in Figure 3. Specifically, at 34°C compound 3 shows a macroscopic polarization of -557 nC/cm , or about -2.1 D/molecule assuming a density of 0.8 gm/cm . This value is consistent with the model shown in Figure 2, though it seems quite large. Thus, the dipole moment of the nitroalkoxy system should be about 4.8 Debye, and this dipole should be oriented almost directly along the polar axis. Assuming a dielectric constant of 3, a macroscopic polarization of about 1.6 D/molecule should derive from perfect orientation of conformer A . The large observed value certainly suggests that conformer A is preferred over conformer Β, that the rotational orientation is excellent, and that perhaps the appropriate dielectric constant of the medium is smaller than 3 and/or the appropriate dipole moment is larger than 4.8 D. While compound 5 should also possess a large dipole associated with the onitroacyloxy grouping, as indicated in Figure 4 that dipole should not orient relative to the tilt plane, since conformers A and Β in Figure 4 should be present in essentially equal number density. Due to the rapid crystallization of the monotropic C* phase of this material, it has proven possible only to obtain preliminary data. This data is, however, completely consistent with the picture presented in Figure 4. That is, an observed polarization of -79 nC/cm (7 times smaller than that observed for compound 3) with a tilt angle 6=39° is what would be expected for the compound with no nitro group at all (P for the nonyloxy homologue with no nitro substituent has been reported by the Chisso group to be -49 nC/cm (24), but no tilt angle data is given). We feel that these data in fact show that the o-nitroalkoxy functional array is indeed oriented along the polar axis in the FLC thin film as evidenced by the observed sign and magnitude of the macroscopic electric dipole moment of the film. This, of course, means that the molecular β associated with this functional array must also be oriented along the polar axis of the film, which should therefore possess a substantial χ< >· 2

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Preliminary Evaluation of the Nonlinear Susceptibility of Compound 3 The measurements of polarization for compounds 3 - 5 were accomplished in a thin (2 μπι) parallel aligned cell with geometry as illustrated in Figure 1. In order to easily measure the second order susceptibility of, e.g. SCE9 by the SHG method, however, a different cell geometry is more preferred. That is, by appropriate surface treatments, an alignment wherein the layer normal is perpendicular to the glass bounding plates, called homeotropic alignment, may be obtained. In this geometry, the "crystal" is oriented with the polar axis parallel to the plates, and the resulting near-normal incidence of the fundamental at the phase-matching angle is convenient.



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Data for compound 4

X —55 — Ε — 64.8 — C* —93.2 — A —96.4 — I

Figure 3. Phase sequence, polarization and tilt angle data as a function of temperature for compounds 3 and 4.


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Figure 4. Important conformational and rotational orientations relative to the tilt plane for compound 5 in the C* phase according to the Boulder Model.



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Unfortunately, homeotropic alignment was not possible with compound 3, even using a magnetic field to attempt to align the sample. We believe this is due to the lack of a nematic phase in the phase sequence in combination with a fairly tight C* helix pitch. In any event, phase matched second harmonic generation using F L C material 3 has not yet been demonstrated. Encouraging preliminary results have, however, been obtained using a 1 0 μπι thick parallel aligned cell and prisms to couple the pump light in at a steep angle relative to the bounding plates for phase-matching. Thus, with reference to the drawing in Figure 1 , Type I eeo angular phase matching was attempted using light polarized in the tilt plane (extraordinary beam), and rotating the cell about the axis normal to the director and lying in the tilt plane. At large incidence angles, the output SHG intensity began increasing dramatically, but it was never possible to hit the top of a phase-matched peak due to total internal reflection at very large incidence angles. Even so, a lower limit of 55 times the SHG efficiency of SCE9 was obtained for nitroalkoxy compound 3, and thus dcff (3)/d ff (SCE9) > 55. This sets the lower limit of dcff for 3 = 0 . 0 7 pm/V — the largest observed to date for an F L C material. It should be stated that an electric field of < 1 0 ν/μιτι was applied to the cell in order to unwind the F L C helix of 3, and the observed NLO behavior is a combination of the electric field induced SHG (EFISH) and that due to the spontaneous polar order in the phase. While other FLCs give much lower SHG efficiency with the same applied fields, and achiral smectic L C phenylbenzoates in our hands give unobservable SHG under identical conditions, we cannot completely rule out at this time the possibility that a significant amount of the response from compound 3 is due to the electrical poling. Control experiments to test for this (e.g. by SHG from compound 5 and/or racemic 3) are in progress, as are further experiments aimed at obtaining the phase-matched SHG efficiency for 3. Nevertheless, at this stage we feel the best interpretation of the results obtained is that the observed response is due to the high degree of spontaneous polar orientation of the nitroalkoxy β along the polar axis in the phase. Given the expected value of β, the density of the material, and the symmetry of the system, one may expect based upon the measured value of Ρ that the large coefficients of the d tensor for 3 (d23 is the largest) should be much greater than any observed to date for F L C materials. Synthesis of second generation targets is proceeding. Problems to address in the future include: 1 ) Increasing the density of NLO active units in the phase; 2) Orientation of functional arrays with larger β; and 3) Developing materials with better processibility. Finally, it should be mentioned that often solids are more desirable than liquids in typical applications of χ< ) films. The prospects for obtaining polymer films with useful thermodynamically stable χ< ) seems high given the recent demonstration that functional group orientation in FLC side chain polymers appears very similar to that observed for the low molecular weight materials (1Ω). The fact that FLC polymers possess thermodynamically stable polar order in a non-crystalline solid film would appear to make this novel type of polymer glass uniquely suited for many second order N L O applications. 2

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Acknowledgments This work was supported in part by the Office of Naval Research.


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Literature Cited 1. 2. 3. 4.


5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

Clark, Ν. Α.; Lagerwall, S. T. Appl. Phys. Lett. 1980, 36, 899-901. Handschy, Μ. Α.; Johnson, K. M.; Moddel, G.; Pagano-Stauffer, L. A. Ferroelectrics 1988, 85, 279-289. Walba, D. M.; Slater, S. C.; Thurmes, W. N.; Clark, Ν. Α.; Handschy, Μ. Α.; Supon, F. J. Am. Chem. Soc. 1986, 108, 5210-5221. Walba, D. M.; Vohra, R. T.; Clark, Ν. Α.; Handschy, Μ. Α.; Xue, J.; Parmar, D. S.; Lagerwall, S. T.; Skarp, K. J. Am. Chem. Soc. 1986, 108, 7424-7425. Walba, D. M.; Clark, N. A. In Spatial Light Modulators and Applications II, Efron, U., Editor, Proc. SPIE 825, 81-87 (1988). Walba, D. M.; Clark, N. A. Ferroelectrics 1988, 84, 65-72. Walba, D. M.; Razavi, Η. Α.; Clark, Ν. Α.; Parmar, D. S. J. Am. Chem. Soc. 1988, 110, 8686-8691. Walba, D. M.; Eidman, K. F.; Haltiwanger, R. C. J. Org. Chem. 1989, 54, 4939-4943. Walba, D. M.; Clark, Ν. Α.; Razavi, Η. Α.; Eidman, K. F.; Haltiwanger, R. C.; Parmar, D. S. In Liquid Crystal Chemistry, Physics, and Applications, Doane, J. W. Yaniv, Z., Editor, Proc. SPIE 1080, 115-122 (1989). Walba, D. M.; Keller, P.; Parmar, D. S.; Clark, Ν. Α.; Wand, M. D. J. Am. Chem. Soc. 1989, 111, 8273-8274. Walba, D. M.; Razavi, Η. Α.; Horiuchi, Α.; Eidman, K. F.; Otterholm, B.; Haltiwanger, R. C.; Clark, Ν. Α.; Shao, R.; Parmar, D. S.; Wand, M. D.; Vohra, R. T. Ferroelectrics, in press. Walba, D. M.; Clark, Ν. Α.; Razavi, Η. Α.; Parmar, D. S. In Proceedings of the 5th International Symposium on Inclusion Phenomena and Molecular Recognition, Atwood, J. L. (Ed.); Plenum Publishing Corp, in press. Walba, D. M. In Advances in the Synthesis and Reactivity of Solids, Mallouk, T. E. (Ed.); JAI Press Inc., Greenwich, Connecticut, in press. Vtyurin, A. N.; Ermakov, V. P.; Ostrovskii, B. I.; Shabanov, V. F. Phys. Status Solidi Β 1981, 107, 397-402. Shtykov, N. M.; Barnik, M. I.; Beresnev, L. Α.; Blinov, L. M. Mol. Cryst. Liq. Cryst. 1985, 124, 379-390. Taguchi, Α.; Kajikawa, K.; Ouchi, Y.; Takezoe, H.; Fukuda, A. In Nonlinear Optics of Organic and Semiconductors, Kobayashi, T., Editor, Springer Proceedings in Physics, Vol 36, 250-253 (1989). Taguchi, Α.; Ouchi, Y.; Takezoe, H.; Fukuda, A. Jpn. J. Appl. Phys. 1989, 28, L 997-L 999. Liu, J. Y.; Robinson, M. G.; Johnson, K. M.; Doroski, D. Optics Letters 1990, 15, 267-269. Clark, Ν. Α.; Rieker, T. P.; Maclennan, J. E. Ferroelectrics 1988, 85, 79-97. Clark, Ν. Α.; Rieker, T. P. Phys. Rev. A 1988, 37, 1053. Meyer, R. B.; Liebert, L.; Strzelecki, L.; Keller, P. J. Phys., Lett. (Orsay, Fr.) 1975, 36, L-69-L71. Furukawa, K.; Terashima, K.; Mitsuyoshi, I.; Saitoh, S.; Miyazawa, K.; Inukai, T. Ferroelectrics 1988, 85, 451-459. Goodby, J. W.; Gray, G. W.; McDonnel, D. G. Mol. Cryst. Liq. Cryst. 1977, 34, 183-188. Terashima, K.; Ichihashi, M.; Kikuchi, M.; Furukawa, K.; Inukai, T. Mol. Cryst. Liq. Cryst. 1986, 141, 237.

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