Optically induced birefringence in dye-doped blue phase liquid crystals

This paper investigates the photoinduced change of refractive index in dye (methyl red, MR)-doped blue phase (DDBP) cells by illumination of a pump beam. Through excitation of light irradiation with proper photon energy, MR can transform from trans-state to cis-state and successively diffuse and anisotropically adsorb on the inner glass substrate of the DDBP cell along the direction perpendicular to the polarisation of the pump beam. The adsorbed MR molecules can effectively rotate the orientation of the liquid crystal (LC) molecules and thereby modulate the effective refractive index of the DDBP cell. The SEM images of the adsorbed regions of the illuminated DDBP samples were also taken for discussing the relation between the pump intensity and the photoinduced birefringence.     

Temperature- and solvation-dependent dynamics of liquid sulfur dioxide studied through the ultrafast optical Kerr effect

The ultrafast dynamics of liquid sulphur dioxide have been studied over a wide temperature range and in solution. The optically heterodyne-detected and spatially masked optical Kerr effect (OKE) has been used to record the anisotropic and isotropic third-order responses, respectively. Analysis of the anisotropic response reveals two components, an ultrafast nonexponential relaxation and a slower exponential relaxation. The slower component is well described by the Stokes-Einstein-Debye equation for diffusive orientational relaxation. The simple form of the temperature dependence and the agreement between collective (OKE) and single molecule (e.g., NMR) measurements of the orientational relaxation time suggests that orientational pair correlation is not significant in this liquid. The relative contributions of intermolecular interaction-induced and single-molecule orientational dynamics to the ultrafast part of the spectral density are discussed. Single-molecule librational-orientational dynamics appear to dominate the ultrafast OKE response of liquid SO2. The temperature-dependent OKE data are transformed to the frequency domain to yield the Raman spectral density for the low-frequency intermolecular modes. These are bimodal with the lowest-frequency component arising from diffusive orientational relaxation and a higher-frequency component connected with the ultrafast time-domain response. This component is characterized by a shift to higher frequency at lower temperature. This result is analyzed in terms of a harmonic librational oscillator model, which describes the data accurately. The observed spectral shifts with temperature are ascribed to increasing intermolecular interactions with increasing liquid density. Overall, the dynamics of liquid SO2 are found to be well described in terms of molecular orientational relaxation which is controlled over every relevant time range by intermolecular interactions.     

Structural relaxation in the hydrogen-bonding liquids N-methylacetamide and water studied by optical Kerr effect spectroscopy

Structural relaxation in the peptide model N-methylacetamide (NMA) is studied experimentally by ultrafast optical Kerr effect spectroscopy over the normal-liquid temperature range and compared to the relaxation measured in water at room temperature. It is seen that in both hydrogen-bonding liquids, beta relaxation is present, and in each case, it is found that this can be described by the Cole-Cole function. For NMA in this temperature range, the alpha and beta relaxations are each found to have an Arrhenius temperature dependence with indistinguishable activation energies. It is known that the variations on the Debye function, including the Cole-Cole function, are unphysical, and we introduce two general modifications: One allows for the initial rise of the function, determined by the librational frequencies, and the second allows the function to be terminated in the alpha relaxation.     

Dynamic behaviors of molecular assemblies at liquid/liquid interfaces studied by time-resolved quasi-elastic laser scattering spectroscopy.

The dynamic behaviors of molecular assemblies at two immiscible liquid interfaces are intriguing topics in many fields of science and technology. However, it is generally difficult to investigate the dynamic behaviors of such molecular assemblies because of the buried nature of liquid/liquid interfaces. In the present paper, our recent investigations on dynamic behaviors of various molecular self-assemblies at liquid/liquid interfaces are reviewed. We monitored dynamic behaviors of the molecular assemblies by time-resolved quasi-elastic laser scattering (TR-QELS) and fluorescent spectroscopy. The former method allows us to monitor the change in interfacial tension with millisecond time-resolution. As molecular assemblies, bis(2-ethylhexyl)sulfosuccinate (AOT) microemulsion, phospholipid biomembrane models, and liposome-DNA complexes have all been studied, since they are relevant in material sciences and biological technologies. At liquid/liquid interfaces, these molecular assemblies showed characteristic behaviors. We review the finding of rebound response of the interfacial tension at the liquid/liquid interface induced by the adsorption of the AOT microemulsion. We monitored the hydrolysis reaction of phospholipid biomembrane models formed at oil/water interfaces, observing the different types of behavior of liposome-DNA complexes at biomembrane models with different kinds of phospholipids.     

The effect of voltage controlled orientation order of liquid crystals in non-acrylic polymer dispersed liquid crystals films

The nematic liquid crystals (LCs) are randomly dispersed material with random orientation order in polymer dispersed liquid crystal (PDLC) films. The LCs change their orientation from random to vertical as electric field is applied. This transformation of orientation order of nematic liquid crystals in the PDLC films is controlled by many factors operating simultaneously. For instance, some factors like the internal forces of attractions among the neighboring LC molecules, anchoring with polymeric matrix, ITO glass boundaries, and chemical structures of the materials are less studied. The learning of extent of vertical orientation of liquid crystal droplets in an electric field is essential to attain optimum electro optical properties of PDLCs. In this finding, bipolar and radial LCs droplets with random orientation have been observed in non-acrylic polymeric media. It is learned that with small increase of contents of external material, the extent of vertical orientation has been varied intensely. The extent of vertical orientation of LCs molecules increases as the contents of external non-acrylic polymeric material decreased. For this study, the orientations of LCs with respect to material type/contents, external applied force, and restoration of electric filed as hysteresis have been studied in details.     

Low-frequency spectrum of homeotropically aligned liquid crystals: optical heterodyne-detected Raman-induced Kerr effect spectroscopy of 4-octyl-4-cyanobiphenyl

The low-frequency spectrum of homeotropically aligned 4-octyl-4^′-cyanobiphenyl in the smectic-A (Sm-A) phase was obtained by using optical heterodyne-detected Raman-induced Kerr effect spectroscopy. The absence of a narrow band at 9 cm⁻¹, corresponding to a pseudo-lattice vibration propagating in a direction perpendicular to the smectic layers, suggests that the spectrum of homeotropically aligned Sm-A is due to orientational modes associated with rotation about the long axis of the molecule and translational modes associated with motion in the smectic layers. The spectrum is described by a bimodal lineshape function in which the lower and higher frequency components are attributed respectively to the translational and orientational modes.     

Kerr effect of low melting nematic liquid crystals with negative dielectric anisotropy

The two low melting nematic liquid crystals, 2-chloro-4-heptylphenyl 4-pentylbicyclo[2,2,2]octane-1-carboxylate (7CP5BOC) and 2-chloro-4-heptylphenyl 4-heptylbicyclo[2,2,2]octane-1-carboxylate (7CP7BOC) have been investigated to determine their electro-optical behaviour and third order non-linearity by the static Kerr effect method. Both liquid crystals are laterally substituted by a single chlorine atom located close to the ester linking group. The temperature dependence of the electric Kerr constant in the isotropic phase and the pretransitional behaviour have been investigated for these low birefringence nematic liquid crystals in the isotropic phase. Both the compounds, with negative dielectric anisotropy, have a positive Kerr constant. The Landau-de Gennes model was obeyed for these compounds.     

Lamellar-non-lamellar phase transitions in synthetic glycoglycerolipids studied by time-resolved X-ray diffraction

The phase sequences of eight fully hydrated synthetic, stereochemically pure glycoglycerolipids with saturated alkyl chains 12-18 carbon atoms long and a glucose, galactose or mannose head group are followed in real time during heating and cooling scans using synchrotron X-ray diffraction. One of them, 1,2-di-O-hexadecyl-3-O-β-D-glucosyl-sn-glycerol, has been characterized by X-ray diffraction for the first time. A summary of the lamellar-non-lamellar transition sequences and reversibility for all eight glycoglycerolipids studied is provided. It includes also observations of intermediate phases, previously not detected. Lattice parameters of the various phases have been determined as functions of chain length in monoglucosides. While the repeat periods of the lamellar phases increase linearly with chain length, an anomalously high lattice spacing of the inverted hexagonal phase is observed at a chain length of 14 carbon atoms. This maximum coincides with the disappearance of the cubic phases from the phase sequence upon chain elongation from 12 to 14 carbon atoms. It thus appears that the expanded H_II phase in 14-Glc retains structural characteristics of the anticipated cubic phases. Upon heating to high temperatures, its high lattice spacing gradually approaches that of the 'normal' hexagonal phase. A direct transition from lamellar subgel to inverted hexagonal phase has been observed to proceed without intermediate structures, but with an extended phase coexistence region, in 1,2-di-O-tetradecyl-3-O-β-D-galactosyl-sn-glycerol and 1,2-di-O-octadecyl-3-O-β-D-galactosyl-sn-glycerol. This transition is not reversible on cooling when lamellar phases skipped in the heating scan intervene. By contrast, the direct lamellar gel-inverted hexagonal phase transitions are fully reversible with minor or absent temperature hysteresis.     

Temperature gradient-cooled smectic A phase orientation and its effects on Cl orientation in ferroelectric liquid crystals

The temperature gradient cooling of ferroelectric liquid crystals (FLCs) confined to a closed long narrow rectilinear space causes the molecules constituting the smectic layers to acquire significant movement. The ordinary layer structure of the SmA phase consequently undergoes deformation which in turn determines the mode of the SmA SmC* phase transition. SmA stripe-shaped texture was found to result from molecular movement only when the direction of the temperature gradient is the same as the rubbing direction. For FLCs whose SmA temperature range exceeds 20 C, the SmA phase undergoes virtually defect-free C1 orientation without change to C2 orientation, when the direction of the temperature gradient is opposite that of rubbing. Defect-free C2 orientation is possible irrespective of the SmA temperature range in temperature gradient cooling. C1 and C2 orientations may combine with no zigzag defects through the use of such FLCs.     

Fast polarisation-insensitive optical shutter supported by backflow in dichroic dye-doped dual-frequency liquid crystal

This paper describes a novel implementation of a dual-frequency liquid crystal optical shutter of the guest–host type. The transmissive state of the filter is obtained by applying a low-frequency electric field that brings the dichroic dye in a homeotropic orientation. The light-absorbing state is realised by a twisted planar configuration for which the absorption is quasi-independent of the polarisation. Switching between the two states occurs in about 1 ms and the devices show no scattering for wavelengths inside or outside the absorption band of the dichroic dye. Simulations and experiments reveal how a twisted state is obtained through the backflow phenomenon.     

Characterization by mirage effect technique of dye diffusion in columnar discotic liquid crystals

The anisotropic behaviour of the matter diffusion in columnar discotic liquid crystals is studied by the 'mirage effect' technique. The D_h and D₀ mesophases of C₈HET and C₁₁HET, respectively, are considered. The impurity (a dye, the 1-[4-(xylylazo)xylylazo]-2-naphthol) diffusing in these mesophases is detected by the photothermal deflection technique. Measurements of the diffusion coefficients are performed in two perpendicular directions, along and perpendicularly to the molecular columns. Effects of impurity size, length and type of the branched chains on the discs of triphenylene, and molecule stackings in columns, are presented.     

Arrangement of trace metal contaminations in thin films of liquid crystals studied by X-ray standing wave technique

X-ray standing wave method has been applied to investigate the molecular organization in Langmuir–Blodgett films of liquid-crystalline lanthanide complex deposited on silicon substrates. The X-ray standing waves measurements were carried out at BESSY II on the beamline KMC-2. Energy spectra of characteristic fluorescence emitted from the samples have been recorded at each point in the X-ray reflectivity curve as the incident angle was scanned through the total external reflection region. The integrated intensity under selected peaks was analyzed as a function of the incident angle. Incorporation of metal ions (Fe, Zn, Cu, Ca) to the Langmuir monolayer from the triply distilled water subphase has been established. Owing to the element selectivity of X-ray standing wave technique the arrangement of several types of metal ions inside the Langmuir–Blodgett films was studied individually. The experimental data revealed that the shape of the fluorescence curves for the contamination metal ions is distinctly different indicating considerable differences in the lateral distribution of these ions in the film. These results may be attributed to the phase separation in Langmuir monolayer of liquid-crystalline lanthanide complex due to the incorporation of metal ions from the water subphase.     

Preliminary communication First observation of an optically controlled electro-optic effect in nematic-discotic liquid crystals

An optically controlled electro-optic effect in nematic-discotic liquid crystals was discovered for the first time. The peculiarities of this effect are: (a) high photosensitivity observed in three different radial multiyne liquid crystal materials, allowing optically controlled transmission, i.e. giving the possibility of writing and reproducing optical images; (b) for one of these compounds ( 1a ) the effect occurs at temperatures far below its melting point and even at room temperature.     

Orientational and interaction induced dynamics in the isotropic phase of a liquid crystal: polarization resolved ultrafast optical Kerr effect spectroscopy

The ultrafast dynamics of the isotropic phase of a liquid crystal 4'-pentyl-4-p-biphenylcarbonitrile (5CB) have been investigated using polarization resolved optical Kerr effect spectroscopy. Measurements were made as a function of both temperature and dilution in nonpolar solvents. To separate single molecule and interaction induced components to the relaxation of the induced birefringence, measurements of both the anisotropic and isotropic response were made. The isotropic response was found to be dominated by a damped low-frequency mode of intramolecular origin. There is a minor additional component assigned to an interaction induced contribution. There is at most an extremely weak isotropic signal beyond 1 ps, showing that the picosecond time scale dynamics of 5CB are dominated by orientational relaxation. The isotropic response is independent of temperature in the range studied (0.2-50 K above the nematic to isotropic phase-transition temperature). The anisotropic response exhibits relaxation dynamics on time scales spanning subpicosecond to several hundred picoseconds and beyond. The fastest components are dominated by a librational response, but there are smaller contributions from three low-frequency intramolecular modes, and a contribution from interaction induced effects. The low-frequency spectral density extracted from these data are independent of temperature in the range studied, 0.2-30 K above the phase-transition temperature, but shift to lower frequency on dilution in alkane solvents. In neat 5CB the picosecond time scale orientational dynamics are dominated by temperature-independent reorientation within the pseudonematic domains, while in solution these are disrupted, and the orientational response becomes faster and temperature dependent.     

Observation of spatial distribution of laser-induced refractive index change in dye-doped liquid crystals

Spatial distributions of photothermal refractive index changes in dye-doped liquid crystals were determined by an optical interferometric method. The refractive index change of the order of 10^-1, including spatial distribution, was estimated by the described experimental technique. The absolute value of the refractive index change was proportional to the pump beam power, and the diameter of the index distribution was slightly larger than that of the pump laser beam due to heat conduction.     

Detection of phase transitions in liquid crystals using the mirage effect

Abstract

We describe here a novel technique which makes use of the photothermal mirage effect to study phase transition in liquid crystals. Results of the measurements done with two nematic liquid crystals are given to illustrate this technique. We observe an enhancement in the signal amplitude during phase change which is due to the large magnitude of the refractive index gradient developing at the transition temperature.     

Orientational correlations in two-dimensional liquid crystals studied by molecular dynamics simulation

Orientational correlations in Langmuir monolayers of nematic and smectic-C liquid crystal (LC) phases are investigated by molecular dynamics simulation. In both phases, the orientational correlation functions decay algebraically yet with the different exponents of 1.9 and 0.2 for the nematic and the smectic-C monolayers, respectively. The power law decay, i.e., the absence of long-range orientational order, means the both monolayers should be the ideal 2D system with a continuous symmetry, whereas the large difference in the exponents of power law gives rise to the crucial difference in their optical properties; the nematic monolayer is optically isotropic while the smectic-C monolayer exhibits an anisotropy on the length scale of visible light. Since the exponent is inversely proportional to the molecular exchange energy, the averaged molecular interaction in the nematic monolayer should be an order of magnitude smaller than that in the smectic-C monolayer, which is ascribed to the low molecular density and the weak molecular dipole due to the water molecule. The relation between the molecular interaction and the orientational correlation calculated for the 2D LC system offers much information not only about the 2D LCs but also on the bulk system.     

Fast switching of vertically aligned negative liquid crystals by optically hidden relaxation

We propose a method for fast switching of vertically aligned (VA) negative liquid crystals (LCs) by hiding the relaxation process of LCs. During the turn-off process, a strong in-plane electric field is applied for a short duration of time instead of relying solely on the slow relaxation of LCs. The LC molecules are rotated to the transmission axis of one of the polarisers by the applied in-plane electric field, resulting in turn-off switching that is 5.8 times faster than that of a conventional VA cell. By applying an overdriving scheme, we experimentally obtained a total response time of 3.3 ms.     

Properties of optically addressed liquid crystal spatial light modulators studied by mach-zehnder interferometry

In this work optically addressed liquid crystal spatial light modulators are experimentally investigated. Local reorientation of molecules in the modulator and local changes of effective refractive index n_eff (x,y) are induced by modulated light intensity I (x,y). We present preliminary results of measurements of phase shifts in optically addressed liquid crystal panels using a Mach-Zehnder interferometer. Experiments were performed for the panels filled with nematic and dye-doped nematic liquid crystals. Optical addressing was realized by Ar+ laser beam (λ = 514.5 nm) while the reading beam was supplied by He-Ne laser (632.8 nm). The operation voltage was in the range 4 - 20 V. The total phase shift under the influence of addressing light for the studied systems was 2 - 6 pagr;, sensitivity to the addressing light ∼ μW/cm² per 2π phase change and speed of response to the light was 20 ms - 30 s with total recovery time 0.5 - 120 s.