The dielectric spectrum of the antiferroelectric smectic Cb phase exhibits a low and a high frequency absorption peak (PL, PH) which have been studied as a function of temperature and bias electric field. Measurements from 10Hz to 10MHz were carried out with smectic layers parallel and quasi-perpendicular to the cell plates for the multicomponent mixture CS-4000 (Chisso). In addition to the orthogonal smectic A* phase, this material has four tilted phases, three narrow phases with a dielectric behaviour permitting us to classify two of them as C*(alpha) (82.800 to 81.98 degrees C) and C*(gamma) (80.100 to 79.17 degrees C), and one broad antiferroelectric phase (79.17 degrees to - 10 degrees C). On applying an increasing bias field, Delta epsilon for both processes first increases by about a factor of two, then exhibits a maximum at a threshold field E(C) corresponding to the antiferroelectric-ferroelectric transition at which it decreases by almost one order of magnitude. In fact, at E(C) the P-H peak vanishes and the P-L peak shows up at a frequency slightly lower than that corresponding to zero field. In contrast to the Delta epsilon behaviour, the relaxation frequency of the two absorptions does not show any appreciable bias field dependence for E < E(C). We attribute the P-H process to the collective reorientation of the molecules around the cone in the opposite direction (anti-phase in the phi variable). P-L may be attributed to a similar collective reorientation in the same direction (in-phase) around the cone, where the coupling to the electric field is mediated by the helical superstructure, and a corresponding small shift in the local polarization directions. There is no antiferroelectric soft mode coupling to an electric field, but the anti-phase cone motion acts electro-optically in a way similar to the electroclinic effect.

The inner surfaces of a cell of conventional type were covered with an SiO, aligning layer evaporated at an angle α = 60° and subsequently treated with lecithin in order to achieve symmetrical boundary conditions. The alignment of a nematic liquid crystal layer with negative dielectric anisotropy (Δε < 0) was found to be homeotropic below a critical temperature Tc, (low temperature range). Above that particular temperature the homeotropic alignment abruptly transforms into a planar one, which remains with increasing temperature up to the clearing point TNI (high temperature range). The alignment transition was found to be reversible, and is attributed to a packing change of the lecithin layer with the temperature. A simple model based on the different temperature dependence of the anchoring strengths W H, (T) and W P, (T), characterizing the homeotropic and planar alignment respectively, is proposed to explain the surface induced alignment transition.

In order to better understand which features in dielectric spectra of antiferroelectric liquid crystals (AFLCs) are due to the bulk director geometry, and which are due to surface-induced structures, we have performed dielectric spectroscopy measurements with simultaneous texture monitoring on two SmC*-exhibiting AFLC homologues (11- and 12F1M7), at varying cell gap. Such AFLCs are strongly affected by surface action even in fairly thick cells ( d , 15 w m), with heavy supercooling of the SmC* phase as the most obvious result. We show that the supercooled structure can be removed by AC-field treatment in the SmC a * phase, but some domains may stay in a polar geometry, as reflected in both texture and dielectric signature. On heating from the antiferroelectric SmC g * subphase into SmC*, meta-stable non-helical domains may form at cell gaps much larger than the helical pitch. These domains give rise to a low-frequency dielectric absorption not seen in bulk SmC* samples.

Molecular dynamics has been studied by broadband dielectric relaxation spectroscopy in the Sm-A, Sm-B, and Sm-E phases (Sm denotes smectic) of a homologous series of nonchiral stilbenes. An assignment of modes is presented based on their dependence on temperature and molecular length, and, as far as they obey the Arrhenius law, their activation energy has been determined. In general, reorientations of entire molecules around their short axis are active, whereas reorientations of entire molecules around their long axis are locked out in the Sm-E phase of shorter homologs, yet intramolecular reorientations of polar sites have been established. Strong evidence is presented for an interdependence of reorientations of entire molecules around the short and long axes within the biaxial Sm-E phase of longer homologs.

Broad band dielectric measurements reveal that the reorientation of non-chiral rod-shaped low molecular mass liquid crystals is active around their molecular long axis and a short axis, in the smectic A and hexatic smectic B phase, respectively, as well as in the soft crystalline E phase of two isomeric stilbene compounds possessing an equal molecular length of their all-trans -conformations. One ('generalized') Arrhenius equation describes the temperature dependence of the reorientation around a molecular short axis for each of these phases of both compounds. A change of the activation energy related to the reorientation around a molecular short axis is accompanied by a slowing down of the reorientation around the molecular long axis in the soft crystalline E phase in one of these compounds, compelling evidence for a coupling of both reorientations. This result is discussed with respect to the biaxiality of the soft crystalline E phase.

Reorientations of molecules in orthogonal smectic phases (SmA, SmB, and SmE) of a homologous series of stilbenes have been studied by broad band dielectric spectroscopy and their activation energies have been determined. The effect of the molecular length on the reorientation around a short and the long molecular axis is described in detail. The reorientation around the long molecular axis is locked out in the SmE phase of homologues with a short alkoxy chain length whereas the reorientation around the short molecular axis is always active. A change of the activation energy describing the reorientation around a short molecular axis is accompanied by a change of the relaxation frequency of the reorientation around the long molecular axis in the SmE phase. This coincidence compels evidence for a relationship between both reorientations in the higher-ordered biaxial smectic E phase.

The smectic layer spacing of a nonfluorinated ferroelectric liquid crystal (FLC) compound with almost no shrinkage and only minor tendency to form zigzag defects was characterized by small angle x-ray diffraction. The material lacks a nematic phase. The smectic-A*-smectic-C* phase transition was studied by measuring the thermal and electric field response of the optical tilt and the electric polarization. These properties are described very well by a Landau expansion even without introduction of a higher-order Theta(6) term. This result suggests a pure second-order phase transition far from tricriticality and differs considerably from the typical behavior of the A*-C* transition in most FLC materials.

Pyroelectric Liquid-crystal polymers were prepared by photopolymerization of binary mixtures of two monomers that exhibit a smectic C* phase: A2c, 4"-{(R)-(-)-2-[(10-acryloyloxy)decyl]oxy}-3-nitrophenyl 4-{4'-[(11-acryloyloxy)undecyloxy]phenyl}benzoate and A1b, 4"-((R)-(+)-2-octyloxy)-3"-nitro phenyl 4-(4'-[(11-acryloyloxy)undecyloxy]phenyl) benzoate. Both liquid-crystal monomers have a NO2 substituent to enhance the nonlinear optical properties, and one of the monomers, A2c, permits polymerization to a cross-linked polymer. During the polymerization an electric field of approximately 25-50 V/mu m was applied over the ferroelectric liquid-crystal cells. All cases of polymers formed from the chiral smectic C* phase showed a second-harmonic-generated signal with no external field present, indicating that polar order became fixed. The orientation dependence of the second-harmonic-generated intensity was similar to that of the ferroelectric Liquid-crystal monomer; however, some changes were observed that might be due to changes in the dielectric axes of the system. The highest d(16) and d(23) coefficients were found to be in the range 0.65-0.8 pm/V and differed depending on the detailed preparation of the sample. Experimental results of several polarization combinations of the pump and frequency-doubled light are presented and discussed. (C) 1998 Optical Society of America.

M-line measurements have been performed on thin films of pyroelectric liquid crystalline polymers (PLCP), in order to determine the magnitude and anisotropy of the refractive index. The films, polymerized under various conditions, are based on materials exhibiting a chiral smectic C (S-C*) phase. The results of the measurements were analyzed and compared with a new numerical method to model the optical properties of anisotropic multilayered thin films. In the model, which is an extension of the analytical approach of Berreman, the eigenmodes and boundary conditions of refracted light waves are handled and solved by numerical matrix manipulations. Model calculations of total reflection measurements for various multi-layered systems were used to investigate the effect of ITO and polyimide aligning layers to the apparent effective index of m-lines.

Pyroelectric Liquid Crystal Polymers (PLCP), a novel class of material with intrinsic polar order developed in our laboratory, are discussed. Thin films (2-4µm) of PLCP were prepared by photopolymerization of ferroelectric liquid crystalline monomers in the chiral smectic C (SmC*) phase. The poly(acrylate) materials have been structurally modified in order to improve their nonlinear optical and thermal properties. Further are their longterm nonlinear optical properties discussed based on results from second harmonic generation (SHG), spontaneous polarization (Ps), and dielectric spectroscopy.

Second harmonic generation in novel pyroelectric liquid crystal polymers (PLCP) made from a series binary mixtures, was studied using 1100 nm as the fundamental wavelength. The PLCPs were prepared by photo-polymerization of binary mixtures of two monomers which exhibit a smectic C∗ phase, A2c (4″-(R)-(−)-2-[(10-acrylo-yloxy)decyl]oxy-3-nitrophenyl 4-{4′-[(11-acryloyloxy)-undecyloxy]phenyl}benzoate) and Alb (4″-((R)-(+)-2-octyloxy)-3″-nitrophenyl 4-(4′-(11-acryloyloxy)undecyloxy)-phenyl)-benzoate). The highest d16 and d23 coefficients were found to be in the range 0.65–0.8 pm/V, and differed depending on the detailed preparation of the sample. All cases of polymers formed from the chiral smectic C∗ phase showed an SHG-signal with no external field present, indicating that polar order became fixed. The SHG-signal was found to increase with the tilt angle of the FLC molecules.

We report second-harmonic generation in samples with a pyroelectric liquid crystal polymer (PLCP) prepared from the ferroelectric liquid crystalline acrylate monomer 4-[(R)-(-)-2-(10-acryloyloxydecyl)oxy]-3-nitrophenyl 4-[4-(11-acryloyloxyundecyloxy)phenyl]-benzoate (A2c) by in situ photopolymerization. The relation between SHG efficiency, spontaneous polarization and amount of chiral substance in this and previously synthesized PLCP materials is discussed. Electro-optical properties of the monomeric and crosslinked A2c are investigated. In the crosslinked state, the molecular mobility is found to be very low, supporting the fact that the degree of polar order in the polymer is high. In contrast to the case of a poled electret NLO material, the polarization of this material is not 'frozen-in', but an intrinsic thermodynamic property.

Three selected compounds with a rich liquid crystalline polymorphism of chiral tilted smectic phases were investigated with respect to their pyroelectric behaviour. Measurements for well aligned samples are presented with high temperature resolution and are compared to the calorimetric behaviour. Data of the temperature dependence of the spontaneous polarization is derived for all smectic phases under investigation and compared to the behaviour obtained for the smectic C* phase by the triangular wave method.

The linear flexoelectrooptic effect in short-pitch cholesterics was investigated for four configurations of a compound with two chiral centers. One of the four compounds, the (S,S) configuration, is a single component twist inversion cholesteric and we were able to study the electroclinic effect of the compound in the unwound state around the twist inversion point. This made it possible to separate the electroclinic tom the flexoelectric electro-optic response and our results show that the electroclinic deflection of the optical axis is about 100 times smaller and 100 limes faster than the flexoelectric one Moreover, this is the first investigation of the electroclinic effect, far from the N*-SmC* transition, in a single component N* liquid crystal.

The dynamics of different molecular modes in four antiferroelectric liquid crystal substances have been studied by a combination of spectroscopic methods. The fastest motion is the reorientation around the molecular long axis, here found in the low GHz range by time domain spectroscopy. The reorientation around the short axis has a characteristic frequency of about 10kHz and is detected by frequency domain spectroscopy in the homeotropic configuration. As for the collective excitations, the Goldstone and soft modes, characteristic of the ferroelectric phase, have counterparts in the antiferroelectric phase which appear very different. There are two characteristic peaks in the spectrum, one at high frequency, about 100kHz, the other at low frequency, about 10kHz. The latter has often been mistaken for short axis reorientation and both have been attributed to soft modes. By combining different experimental techniques and different geometries it can be shown that neither is a soft mode, but both are collective modes of different character: the high frequency mode corresponds to fluctuations where molecules in neighbouring layers are moving in opposite phase, the low frequency mode to phase fluctuations in the helicoidal superstructure. In materials exhibiting a C* phase in addition to the C-a* or C-gamma* phases, an additional strong peak appears in at least one lower-lying phase adjacent to the C* phase. We show that this peak, which we call a hereditary peak, has nothing to do with the antiferroelectric or ferrielectric order, but is just the Goldstone peak from a coexisting C* phase. In the same way, a Goldstone mode peak from the C-gamma* phase may appear in the underlying C-a* phase. In a general way, narrow phases like C-gamma*, being bounded by first order transitions on both sides (C-a*-C-gamma*-C*) are likely to show non-characteristic (hereditary) peaks from both adjacent phases.