Phase grating generation and optical phase conjugation in photorefractive polymer/dissolved liquid crystal composites

The orientational photorefractive properties of photorefractive mesogenic composites have been investigated by means of two-beam coupling and degenerate four-wave mixing. Photorefractive mesogenic composites, consisting of low molar mass liquid crystals, polymer, and photoconductive sensitizer, constitute novel organic materials possessing high performance photoreractivity. The refractive index change was estimated on the basis of the theory of two-beam coupling and four-wave mixing, and large index modulation of over 0.01 was obtained.     

Helical superstructures in the mesophase of compounds derived from 2-cyanoresorcinol

The mesophase exhibited by four members of a new homologous series of compounds composed of banana-shaped molecules derived from 2-cyanoresorcinol is reported. The characteristic optical textures showing helical filamentary growth patterns, coupled with the results from X-ray diffraction studies of these seven-ring compounds containing a biphenyl moiety indicate that the mesophase is indeed B7.     

Helical superstructures in the mesophase of compounds derived from 2-cyanoresorcinol

The mesophase exhibited by four members of a new homologous series of compounds composed of banana-shaped molecules derived from 2-cyanoresorcinol is reported. The characteristic optical textures showing helical filamentary growth patterns, coupled with the results from X-ray diffraction studies of these seven-ring compounds containing a biphenyl moiety indicate that the mesophase is indeed B 7 .     

Liquid crystalline properties of dissymmetric molecules IX. The effect of a terminal nitro group on the thermal properties of the smectic A phase in 4-nitrophenyl 4-(4-alkoxybenzoyloxyphenyl)benzoates

This paper describes the effect of alkoxy chain length on the physico-chemical properties of 4-nitrophenyl 4-(4-alkoxybenzoyloxyphenyl)benzoates. The homologous series shows a smectic A (SmA) phase commencing from the hexyloxy homologue. While the pentyloxy homologue shows only a nematic (N) phase under microscopic observation, X-ray diffraction shows that the N phase has a cybotactic domain with a layer structure of the SmA phase, where the layer spacings are longer than the calculated molecular lengths by c. Å. The layer spacings for the heptyloxy-nonyloxy homologues are longer than the calculated molecular lengths by 2.2-4.8 Å and are almost independent of temperature. The layer spacings are correlated with the calculated molecular lengths, and based on these results, a new molecular arrangement model for the SmA phase is proposed.     

Molecular alignment and field-induced reorientation in a twisted nematic liquid crystal pi-cell

A twisted nematic pi-cell has been studied by optical transmission measurement, polarized Fourier-transform infrared (pFTIR) absorption spectroscopy, and Raman spectroscopy. Our pFTIR results suggest that the LC molecules undergo a restricted rotation about the molecular long axis. The rise and decay times of the optical response were found to be 6 ms and 1.6 ms, respectively. The switching dynamics of the twisted pi-cell was also studied using time-resolved Raman spectroscopy. A normal mode associated with the C-H out-of-plane wag on the LC core was found to be enhanced after the electric field was switched off. Our data show that LC molecules in the twisted pi-cell do not rotate like a rigid molecule during the field-induced reorientation process. The methods employed in this study have yielded valuable information about LC alignment and field-induced reorientation with respect to functional group specificity.     

Formation of lanthanide ion networks by orientation of the mesophases of their complexes in magnetic fields

A complex of a Tm(III) perfluoroalkylsulsphate with salicylaldimine Schiff's base was synthesized and its liquid crystallinity characterized by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. An additional phase transition was detected. 2D or 3D anisotropically-organized media were obtained by cooling of the lanthanide complex from different phases in an applied magnetic field. A huge positive magnetic anisotropy was observed for the resulting material.     

Determination of the second order susceptibility tensor in banana-shaped liquid crystals

Optical second harmonic generation (SHG) measurements have been carried out on the B2 phases of three liquid crystal materials composed of conventional banana-shaped molecules (P-10-O-PIMB, P-8-O-PIMB and P-8-PIMB). Quantitative values of the second order susceptibilities d_ij have been deduced from SHG measurements at oblique incidences on polydomain racemic samples under an electric field. The d_ij coefficients are in the range 1-10 pm V^-1.     

Shape of the nematic-isotropic interface in conditions of partial wetting

A nematic liquid crystal in contact with a solid substrate is studied in the partial wetting regime. Both a mesoscopic Landau-de Gennes theory and a macroscopic effective interface Hamiltonian approach are considered. A generalized Young equation for the balance of forces at the three-phase contact line is derived, which takes into account corrections due to distortions of the nematic director field. It is also shown that the asymptotic form of the separation of the nematic-isotropic interface from the substrate has a logarithmic correction to the usual linear behaviour. The characteristic length scale of this correction is given by the ratio K/(2σ_NI), where K and σ_NI are the average elastic constant and the nematic-isotropic surface tension, respectively, and is of the order of a few hundred angstroms. Then, a simple form of an effective interface Hamiltonian is proposed, and results consistent with the predictions of the Landau-de Gennes theory are obtained. It is shown, in the framework of this macroscopic approach, that the line tension associated with the contact line remains finite, when the thermodynamic limit is taken, if the anchoring at both the nematic-substrate and the nematic-isotropic interfaces is homeotropic. However, in the case of different anchoring directions, the line tension diverges logarithmically with the system size.     

Molecular alignment and field-induced reorientation in a twisted nematic liquid crystal pi-cell

A twisted nematic pi-cell has been studied by optical transmission measurement, polarized Fourier-transform infrared (pFTIR) absorption spectroscopy, and Raman spectroscopy. Our pFTIR results suggest that the LC molecules undergo a restricted rotation about the molecular long axis. The rise and decay times of the optical response were found to be 6 ms and 1.6 ms, respectively. The switching dynamics of the twisted pi-cell was also studied using time-resolved Raman spectroscopy. A normal mode associated with the C-H out-of-plane wag on the LC core was found to be enhanced after the electric field was switched off. Our data show that LC molecules in the twisted pi-cell do not rotate like a rigid molecule during the field-induced reorientation process. The methods employed in this study have yielded valuable information about LC alignment and field-induced reorientation with respect to functional group specificity.     

Formation of lanthanide ion networks by orientation of the mesophases of their complexes in magnetic fields

A complex of a Tm(III) perfluoroalkylsulsphate with salicylaldimine Schiff's base was synthesized and its liquid crystallinity characterized by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. An additional phase transition was detected. 2D or 3D anisotropically-organized media were obtained by cooling of the lanthanide complex from different phases in an applied magnetic field. A huge positive magnetic anisotropy was observed for the resulting material.     

Surfactant-Water Systems with Small Layer Rigidity Phase Stability,Defects Models and Defects Mobility in Polyoxyethylene Surfactant with Water

Abstract

Starting from recent experiments in non-ionic surfactants, we discuss various topics characteristic of low rigidity K ₁ materials. (1) The stability of the lamellar and cubic phases is studied in a simple model involving K ₂ the saddle-splay constant K ₂, and entropy terms due to the chains. We expect the lamellar phase to have a large range of existence, and the cubic phase to be of small extent. (2) We compare to cubic phases stability in large K ₁ materials. We discuss the mobility of edge dislocations on the basis of a new model of the core which involves stretching of the core layers in one dimension, and easy nucleation of pores in this region. The core extension is a characteristic length of low K ₁ materials which is much larger than de Gennes' length for microemulsions with a similar low K ₁.     

Optical Bragg reflection from the TGBA phase

We have studied theoretically the reflection band properties of the twist grain boundary A phase (TGBA). A 4 x 4 matrix approach for the electromagnetic wave propagation is used. At normal incidence, simple analytical expressions are found, while at oblique incidence numerical methods have to be used since no analytical solution is available. The chart of stability of the propagating waves and the reflection spectra are given and discussed. No evidence of the incommensurability properties of the TGBA phases can be found optically. However, the analysis of the output polarization within the Bragg reflection band, on thick enough samples, allows us to determine the physical parameters characterizing the TGBA phase. In particular conditions, we can get the rotation Delta Theta between two homogeneous slabs and the total optical phase phi of one slab. When Delta Theta and phi go to zero, the optical properties of the TGBA phase become identical to those of the cholesteric phase. The essential difference appears when Delta Theta and phi are increased; an infinite number of reflection bands forms each Bragg reflection order, due to the periodicity of the TGBA parameters.     

Optical Bragg reflection from the TGBA phase

We have studied theoretically the reflection band properties of the twist grain boundary A phase (TGBA). A 4 x 4 matrix approach for the electromagnetic wave propagation is used. At normal incidence, simple analytical expressions are found, while at oblique incidence numerical methods have to be used since no analytical solution is available. The chart of stability of the propagating waves and the reflection spectra are given and discussed. No evidence of the incommensurability properties of the TGBA phases can be found optically. However, the analysis of the output polarization within the Bragg reflection band, on thick enough samples, allows us to determine the physical parameters characterizing the TGBA phase. In particular conditions, we can get the rotation Delta Theta between two homogeneous slabs and the total optical phase phi of one slab. When Delta Theta and phi go to zero, the optical properties of the TGBA phase become identical to those of the cholesteric phase. The essential difference appears when Delta Theta and phi are increased; an infinite number of reflection bands forms each Bragg reflection order, due to the periodicity of the TGBA parameters.     

Spatial frequency selective reconstruction using Fourier transform holograms generated in functionalized mesogenic composites

We have demonstrated spatial frequency selective reconstruction (SFSR) of two-dimensional optical images by using functionalized mesogenic composites possessing real-time holographic capability. The two-dimensional optical image was Fourier transformed by a lens and part of the spatial frequency was brought into interference with the reference beam in the Raman-Nath regime. The SFSR images were observed in the self-diffraction patterns and we calculated the expected reconstructed images which were in good agreement with the observed images.     

Electrooptical properties of polarization holographic gratings formed in liquid-crystal composites

The results of experimental study of the electrooptical properties of polarization holographic gratings formed in a liquid-crystal composite by polarization holography are presented. The influence of the formation conditions, the parameters of the controlling ac electric field, and the composite geometry on the diffraction efficiency of such gratings is considered.     

Spatial frequency selective reconstruction using Fourier transform holograms generated in functionalized mesogenic composites

We have demonstrated spatial frequency selective reconstruction (SFSR) of two-dimensional optical images by using functionalized mesogenic composites possessing real-time holographic capability. The two-dimensional optical image was Fourier transformed by a lens and part of the spatial frequency was brought into interference with the reference beam in the Raman-Nath regime. The SFSR images were observed in the self-diffraction patterns and we calculated the expected reconstructed images which were in good agreement with the observed images.     

EPR studies of the dynamics and phase behaviour of dithiadiazolyl radicals derived from mesogenic precursors

A series of liquid-crystalline materials based on 4-substituted cyanobiphenyls, RC(6)H(4)C(6)H(4)CN (R = C(5)H(11), C(6)H(13), C(7)H(15), C(8)H(17) and C(12)H(25), commonly referred to as 5CB, 6CB, 7CB, 8CB and 12CB, respectively) were functionalised to give the corresponding dithiadiazolyl organic radicals RC(6)H(4)C(6)H(4)CNSSN(*) (compounds 1(*), 2(*), 3(*), 4(*) and 5(*), respectively). EPR spectra of n-C(12)H(25)C(6)H(4)C(6)H(4)CNSSN(*) (5(*)) reveal that it adopts a dimeric diamagnetic structure in the solid state with a small number of paramagnetic defect sites. Variable temperature electron paramagnetic resonance (EPR) studies reveal a thermally activated rotation about the molecular long axis at temperatures above 314 K. The energy barrier to rotation about the phenylene-dithiadiazolyl bond was estimated as 25 kJ mol(-1) using density functional theory (DFT). At elevated temperatures thermal annealing was also observed, quenching the sample's paramagnetism.     

Thermal gratings and phase in high-order, transient-grating spectroscopy

Thermal gratings are a well known feature in one-dimensional (i.e., single excitation) transient-grating spectroscopy. This paper presents theory and experiments for thermal gratings in multiple dimensions (i.e., with many excitations). The theory of thermal gratings is extended to an arbitrary number of dimensions using an incoherent Hilbert-space formalism. Interference between Hilbert-space pathways makes it impossible for a thermal grating to propagate across multiple time intervals. The only surviving signal is a hybrid--a population grating between excitations and a thermal grating between the final excitation and the probe. This theory is tested on auramine O in methanol (1D) and in an ionic liquid (3-butyl-1-methylimidazolium hexafluorophosphate) (1D and 2D). In methanol, the ground-state recovery and thermal-grating signals are well separated in time; in the ionic liquid, they are not. Using the results of the theory, accurate subtraction of the thermal-grating signal is possible, extending the useful time range of the experiments. Both the comparison to the theory and the subtraction of the thermal-grating signal are dependent on accurate measurements of the time-dependent phase in these systems. Models are proposed to account for the time-dependent phase. Beer's law is generalized to multidimensional grating spectroscopy. This law provides conventions for consistently comparing the absolute phases and magnitudes between grating and nongrating experiments and between experiments of differing dimensionality.