Azo-dye Doped Nematic Liquid Crystals Research Articles

Creation of topological charges by the spontaneous symmetry breaking phase transition in azo dye-doped nematic liquid crystals

Liquid crystals are anisotropic fluids with long-range directional order. They can be easily used to create topological defects, but creating a controlled topological defect is difficult and involves many tasks and complex processes. However, in our study, we were able to easily generate disclinations in the symmetry-breaking boundaries of an azo dye-doped nematic liquid crystal cell owing to photoisomerization and symmetry-breaking isotropic-nematic phase transition. The method proposed here marks the starting point for the easier control of topological defects in liquid crystals.

Fabrication, morphological investigation and spectral characterization of nano-encapsulated azo dye-doped nematic liquid crystals

Polymeric nanocapsules (NCs) containing dye-doped liquid crystal (DDLC) with an average diameter of 390 nm and good stability were successfully fabricated by the emulsification solvent diffusion (ESD) method. The surface morphology and flexibility of polymeric nanoshell were investigated by the field-emission scanning electron microscopy (FE-SEM) and the core-shell structure of the obtained spherical nanocapsules was shown by transmission electron microscopy (TEM) as well. Nanoencapsulation efficiencies (NEE) of the liquid crystal and azo disperse dye were also measured separately by using a comparative all spectroscopic analysis instead of weight-based methods. Thereafter, a polymeric thin-film consist of the fabricated DDLC nanocapsules was produced and then, the polarization rotation feature and spectral response of the obtained DDLC-NCs were investigated by polarized optical microscopy (POM) and UV/Vis spectroscopy, respectively.

Irreversible phase and anchoring transitions of chiral azodye-doped nematic liquid crystal triggered by photostimulation

Reported herein are the simultaneous phase and anchoring transitions of the chiral azodye-doped liquid crystal (LC) mixture. The transitions are influenced by photostimulation and are stable against light and thermal treatments. The photochromic trans state of the azobenzene molecule used in the study is mesogenic with a chiral nature. The photoinstigated cis form of the molecule has a much-enhanced dipole moment with a broken chromophore symmetry. Due to the high dipole–dipole interaction, the cis-state molecules stack together and form nano-sized aggregates. Subsequently, the cis aggregates get phase-separated, resulting in the chiral- to achiral-phase transition of the host LC. The phase-separated cis aggregates diffuse to the interface between indium-tin-oxide and LC and modify the surface, causing planar to homeotropic anchoring transition of the host LC. The stability and irreversibility of the transition, and the fact that there is no need for pre-treatments for LC alignment, provide a fascinating opportunity for LC device applications.

The effect of Sudan dyes concentration in the linear dichroism of the nematic liquid crystals

In this work the absorption spectrum of the azo-dye doped nematic liquid crystals in the visible region of parallel aligned samples were investigated. Dichroic ratio R and order parameter S of the samples are calculated and the effect of concentration on Dichroic ratio R and order parameter S is studied. The results indicate that Dichroic ratio R and order parameter S increase with the increase in dye concentration.

Photo-stimulated phase and anchoring transitions of chiral azo-dye doped nematic liquid crystals

We report concurring phase and anchoring transitions of chiral azo-dye doped nematic liquid crystals. The transitions are induced by photo-stimulation and stable against light and thermal treatments. Photochromic trans- to cis-isomerization of azo-dye induces an augmented dipole moment and strong dipole-dipole interaction of the cis-isomers, resulting in the formation of nano-sized dye-aggregates. Consequent phase separation of the aggregates of a chiral azo-dye induces phase transition from a chiral to nonchiral nematic phase. In addition, the deposition of dye-aggregates at the surfaces brings about anchoring transition of LC molecules. The stability and irreversibility of the transition, together with no need of pretreatments for LC alignment, provide fascinating opportunity for liquid crystal device applications.

Behaviour of photosensitive soft materials: Thermo-optical, dielectric and elastic constant studies on azo-dye doped nematic liquid crystals

Optical absorption, thermo-optical and dielectric studies have been performed on three guest–host nematic mixtures featuring photo-sensitized flexoelectric polarization. The liquid crystal 4-butyl-cyclohexane carboxylic acid 4-pentyloxy-phenyl ester, a room temperature nematic with a negative dielectric anisotropy served as the host material. Three different azo-dye compounds, also exhibiting liquid crystallinity were employed as the low-concentration guest component in the mixtures. In each case illumination of the sample with actinic (UV) light leads to strong photoisomerization driven effects. Correlations among the shift of the isotropic–nematic phase transition temperature, population of the cis isomers, and the change in the length of the azo-dyes upon UV illumination, are discussed. The UV-induced changes in the dielectric constant values have been shown to be dependent on the nature of the photoactive dopant. The article also describes the first observation of the lowering of the bend elastic constant upon photoisomerization, a feature ascribed to the formation of the bent-shaped cis isomers.

Optical Kerr Constant of Azo-Dye-Doped Nematic and Polymer-Dispersed Liquid Crystals Determined by Biphotonic Z-Scan Technique

We have recently investigated the optical Kerr constant (n2) of azo-dye-doped nematic liquid crystal (ADDLC) and polymer-dispersed liquid crystal (ADDPDLC) films using the Z-scan technique. The measurements of ADDLC samples are firstly performed with the simultaneous application of two pump beams, which are referred as the biphotonic Z-scan technique. The results show that n2 for ADDLC films is contributed primarily from the reorientation of liquid crystals induced by the photoisomerization of azo dyes and thermal effect, while that of ADDPDLC films with nano-sized LC droplets results from the thermal effect, and changes markedly with temperature.

Light-scattering experiments in dye-doped liquid crystals both to determine crystal parameters and to construct consistent neural network empirical physical formulas for scattering amplitudes

The aim of this paper is two-fold. Firstly, static laser light-scattering amplitude measurements in azo-dye doped nematic liquid crystals (NLCs) were made versus scattering angle, temperature and applied bias voltage. Three NLC parameters were determined: the elastic constant ratios K11/K22 by regression, phase transition temperatures, and Freedericksz voltages from the graphs. They were all doping ratio dependent. Secondly, as a novel approach, by a nonlinear universal function approximator layered feedforward neural network (LFNN) we constructed an explicit form of empirical physical formulas (EPFs) for theoretically unknown nonlinear azo-dye doped NLC scattering amplitude functions. Excellent LFNN test set (i.e. yet-to-be measured experimental data) predictions prove that the constructed LFNN-EPPs estimate unknown amplitude functions consistently. The LFFN-EPFs, too, confirmed the doping-ratio dependency. Also, comparing LFNN and regression amplitude fits, the LFNN fits were significantly better. In conclusion, physical laws embedded in the physical data can be consistently extracted by LFNN. One major potential application in the nonlinear optics domain is that these LFNN-EPFs, by differentiation, integration, minimization, etc., can be used to obtain further NLC scattering amplitude related molecular structural physical quantities. This could in turn help us to develop new nonlinear optical materials.

Electro-Optical Properties of Dye-Doped Nematic Liquid Crystals

The electro-optical properties of dye-doped nematic liquid crystals have been investigated in this work. The absorption and transmittance spectrum of the azo-dye doped nematic liquid crystal are measured. The band gaps of the samples are calculated and the effect of concentration on band gap is studied. The results indicate that energy band gap decreases with the increase in dye concentration.

Surfactant Controlled Light Induced Reorientation in Dye-Doped Nematic Liquid Crystals

ABSTRACT We report new advancements in the experimental investigation of the giant optical nonlinearity observed in azo-dye doped nematic liquid crystals. We show that the extreme sensitivity of the azo-dye doped nematics results from a combined action of the trans-cis photoisomerization process of the azo-dye molecules and the light-induced changes of the anchoring energy. The molecular reorientation at the surface can be controlled by changing the length n of the aliphatic chain of the ionic surfactant, CnTAB, used as anchoring agent. Polarization holography experiments show that, during the reorientation, the nematic director mainly follows the direction imposed by the linear polarization along the interference pattern. The diffraction gratings show a large response, proving that all the mechanisms related to light intensity gradients, such as charge migration or thermal heating, may be excluded from the effects that are at the origin of the giant nonlinearity.

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 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.

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.