Azo-dye Doped Liquid Crystals Research Articles

Binary liquid crystal alignments based on photoalignment in azo dye-doped liquid crystals and their application

This work demonstrates the feasibility of binary liquid crystal (LC) alignments, in which two forms of LC alignment are in a single pixel or a specific area using a surface-treated alignment layer and a photoalignment film of the adsorption of azo dyes onto the polymer surface in azo dye-doped liquid crystals. Binary LC alignments that involve two of common LC alignments, which are homogeneous, homeotropic, hybrid, and twisted nematic alignments, are initially demonstrated. Then, a binary LC alignment that comprises hybrid and homeotropic alignments is adopted to fabricate a viewing-angle-dependent liquid crystal display, which displays two images by changing viewing angles.

Variable liquid crystal pretilt angles generated by photoalignment in homeotropically aligned azo dye-doped liquid crystals

This letter demonstrates the feasibility of producing variable liquid crystal (LC) pretilt angles using light-induced ripple structures (LIRSs) in homeotropically aligned azo dye-doped liquid crystals (ADDLCs). Illuminating homeotropically aligned ADDLCs with a linearly polarized light for a suitable period yields the LIRSs which provide LCs an anisotropic homogeneous anchoring force. Experimentally, the effective alignment force produced by the homeotropic alignment layer and the LIRSs determines the LC pretilt angle (24° to 63.5°), defined from the normal to the substrate. A no-bias pi cell for liquid crystal displays is demonstrated using this approach.

Measurement of helical twisting power based on axially symmetrical photo-aligned dye-doped liquid crystal film

This investigation demonstrates a simple but accurate method for measuring the helical twisting power of chiral doped liquid crystals using axially symmetrical photo-alignment in azo dye-doped liquid crystal films. As reported in our previous paper, a reversed twist effect produces a disclination line in photo-aligned axially symmetrical liquid crystal films. The pitch and helical twisting power can be obtained by measuring the rotation angle of the disclination line in chrial doped liquid crystal. This method is independent of cell gap and provide an error below 0.5%.

Photo-induced reorientation inversion in cis-enriched azo-dye doped liquid crystals in the isotropic phase

Nanosecond laser pulse induced molecular reorientation in azobenzene doped liquid crystals (LCs) in the isotropic phase was studied by the optical heterodyned optical Kerr effect method. Reorientation inversion was found by artificially preparing a cis isomer enriched initial state. The interaction strength ratio of trans and cis isomers with the LC molecules was deduced based on a modified mean-field theory model. Our results provide a further understanding of the photo-induced reorientation of photo-isomerizable dye-doped LC systems.

Modeling the Dynamics of Photo-induced Reorientation of Nematic Liquid Crystals doped with Azo-dye

We have used a continuum model of azo-dye doped liquid crystals to describe the dynamics of photo-induced enhancement of director re-orientation. By describing the dye distribution as a superposition of spherical harmonic functions we numerically integrate the dynamic equations and identify various time constants associated with cis to trans isomerization, excited state lifetime, and the various diffusion times of the different species. Our results give an experimental response time which is intensity dependent and characterized by a saturation intensity. This saturation intensity is dependent on the pump beams angle of incidence. Numerical results will be presented.

Holographic image storage and multiple hologram storage in a planar Methyl Red-doped liquid crystal film

Holographic images are recorded and the diffraction characteristic of the hologram is investigated in a planar Methyl Red-doped liquid crystal film without an external electric field. Multiple holograms are stored in this film with no external electric field, and peristrophic multiplexing, angular multiplexing, and spatial multiplexing are demonstrated to be suited for the implementation of azo-dye doped liquid crystal films among the multiplexing techniques. The diffraction efficiency is theoretically studied for multiple storage. Optical reconstruction for peristrophic multiplexed holograms and angular multiplexed holograms is discussed, which involves the peristrophic rotation angles and the recording angles, respectively.

Biphotonic effect of azo-dye-doped liquid crystals using the sequential Z-scan technique

This study investigates the biphotonic effect of azo-dye-doped liquid crystals (ADDLCs) using the sequential Z-scan technique. A spot on the sample is illuminated by a green light for 6s, and then the same spot is illuminated simultaneously with a red light and a green light for 6s. Measurements are made by scanning the sample near the beam waist of the green laser. The results show that the biphotonic effect is important to the nonlinear coefficient of the sample. The variations of the optical Kerr constant with intensity of red light are measured. Measurement results demonstrate that the molecular reorientation of liquid crystals induced by the photoisomerization of the azo dyes dominates at low red-light intensity, but the thermal effect compensates for the molecular reorientational nonlinearity of the sample at high red-light intensity.

Measurement of nonlinear refraction of dyes doped liquid crystal using moiré deflectometry

Recently the nonlinear effects of the azo-dye doped liquid crystals have attracted much interest. In this paper the nonlinear refractive indices, n 2, of two dyes (Sudan Black B, Sudan III) doped nematic liquid crystal were measured at low laser powers using moiré deflectometry technique. The results show when a nonlinear sample is placed in moiré deflectometry setup, the moiré fringe patterns will rotate around the beam center because of self-focusing effect in the sample. By measuring this rotation, the magnitude and the sign of nonlinear refractive index were calculated. The n 2 values for Sudan black B is larger than Sudan III, because of some differences in molecular structure and molecular polarization.

Enhancement factor formulation of doped nematic liquid crystals in the presence of photoisomerization

The mechanism of the orientational optical nonlinearity enhancement in the azo-dye doped liquid crystals is studied. The expressions of the enhancement factors based on the multiple-level transition model of azo-compound are derived. Our results show that, the simpler the model, the fewer the number of items contained in the expression of the enhancement factor. Using the enhancement factor expression obtained by the four-level model to describe the influences of the photoisomerization of azo-dyes on the enhancement of orientational optical nonlinearity is better than using that acquired by the three-level model. Besides, the expression of the enhancement factors in the case that the cis molecules do not absorb light is also obtained from the enhancement factor expression obtained by the four-level model. Except for a minor difference, the enhancement factor expression obtained in this case is the same as what was obtained for the anthraquinone-dye doped liquid crystals by Marrucci et al in 1997. This result also shows that under a given simplified condition, our theory is consistent with the last theory.

Observation of transient reorientation of dye-doped liquid crystals by pumped attenuated total reflection

A transient reorientation of liquid crystals in azo-dye-doped liquid crystals (ADDLCs) is observed by the improved attenuated total reflection leading a pump light into a coupling prism, namely, pumped ATR experiment. The evanescent wave of the pump light induces the photoexcitation of azo dye at the limited effective depth, and the evanescent wave of the probe light detects the dynamic behavior in real time. The time response of the photoexcitation in ADDLCs in the pumped ATR experiment is less than 2ms and it is believed that the reaction occured in the local region of 100nm.

Enhancement factor sign inversion triggered by the variation of the incident direction of light in the azo-dye doped liquid crystals

The optical reorientation process of the azo-dye doped liquid crystals (LCs) is studied and the dependence of the enhancement factor on the incident light direction is explained. By analysing the relation between the order parameter and the cis isomer concentration in the azo-dye doped LC system, an analytical expression that describes the dependence of the order parameters on the direction of the incident light is obtained. It is found that, since the order parameters of the guest–host LC system depend on the direction of the incident light, the intermolecular orientational interaction potentials are also related with the incident light direction. In order to describe the interaction of the cis isomer with the liquid crystalline molecules, a revised Maier–Saupe potential expression that allows for a higher-order interaction is used. A microscopic formula of the enhancement factor for the azo-dye doped LC system is derived on the basis of a simplified two-level model. From the microscopic formula, the mechanism behind the dependence of the enhancement factor on the incident direction of light is revealed. The comparison of our computational results with the existent experimental data verified our enhancement factor's microscopic form.

Magnetic field effects on the optical properties of an azo-dye doped liquid crystal

Homeotropically aligned nematic liquid crystals doped with azo-dye were subjected both to a linear polarized light of a He–Ne laser and to a magnetic field perpendicular to the incident light beam. We found that the emerging light was elliptically polarized when using magnetic field strengths higher than the threshold value for the magnetic Freedericksz transition. The light transmission, the rotatory power (induced by azo-dye) and the ellipticity varied quasiperiodically when increasing magnetic field strength. The number and positions of maxima and minima depend on the cell thicknesses. Changes in the phase difference between the emergent ordinary and extraordinary rays were computed from the experimental data and the magnetic field dependence of the birefringence was determined.

P-206: Circular Polarization Separation Elements for Display Applications

We report on a circular polarization discrimination element for potential use in display applications. A highly stable and efficient polarization grating based on an azo-dye doped liquid crystal (azoLC) is fabricated using a polarization holographic method. In this paper we present the design, fabrication, electro-optical characterization, efficiency and stability of these polarization gratings and their potential use in projection displays.

Nonlinear optical property of azo-dye doped liquid crystals determined by biphotonic Z-scan technique

This work investigates the optical Kerr property of azo-dye doped nematic liquid crystal films using the biphotonic Z-scan technique. The results indicate that the nonlinear effect measured using the Z-scan technique with a red light can be modulated or switched with the simultaneous application of a green light, because of photoisomerization and thermal effects, as determined by dynamic measurements. The former dominates in the early stage when the green light is applied, while the latter dominates in the later stage.

Light driven motion of the nematic director in azo-dye doped liquid crystals

We show that under laser and white light action, reorientation of the nematic director for azo-dye doped liquid crystals follows a three-dimensional trajectory, composed by in and out-of-plane motion with respect to the incidence plane of the light. We relate the two components to the surface and bulk effects, respectively, and we show that the bulk contribution is mainly dominated by the photoisomerization of the azo-dye molecules.

Light Induced Reorientation in AZO-DYE Doped Liquid Crystals: The Relative Role of Surface and Bulk Effects

We report an experimental investigation on the origin of the large optical non-linearity in azo-dye doped nematic liquid crystals. We show that a very large response is obtained for extremely low input light intensity, of the order of a few tens of W/cm 2 , and that it is mediated by the action of the photo-induced voltage on the specific surface coupling agent, which is an ionic surfactant. By performing circular polarization experiments we show that, during the reorientation, the nematic director follows a three-dimensional trajectory composed by in and out-of-plane motion with respect to the incidence plane of the light. We suggest that the bulk contribution is dominated by the photoisomerization of the azo-dye molecules and we report a new kind of self-sustained gratings.

Analysis of gratings induced in azo-dye doped liquid crystals

We investigate the formation of photoinduced gratings in a homeotropic film of nematic liquid crystal doped with methyl red. Using a grating translation technique, we determine the time evolution of the amplitudes and phase shifts of both index and absorption modulations. With low writing intensities, a fast grating caused by photoisomerization of the dye competes with a slower grating, originating from a reorientation of the director. This slower grating only appears if the light polarization has a nonzero projection on the director, i.e. at oblique incidence. The grating decays in the dark. We suggest that its formation may be explained by a light-induced dielectric torque, rather than a photorefractive effect. With high intensities, an even slower grating is formed at normal incidence. We stipulate that it originates from a change in the anchoring parameters caused by the adsorption of photoisomerized dyes on the surfaces. The resulting reorientation of the director is permanent.

Two-wavelength reversible holograms in azo-dye doped nematic liquid crystals

We describe reversible holographic effects in azo-dye doped liquid crystals. Surface-stabilized holograms are recorded at 514 nm using beams polarized parallel to the molecular director of the liquid crystals. While these holograms persist for several hours in the dark, complete erasure is achieved in a few seconds by exposure to 337 nm light.