Dye-doped Nematic Liquid Crystal Cells 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.

Random lasing and mode tunability by extraordinary strong and controllable scattering during the nematic-isotropic phase transition

Random lasing emission from the lateral end face of dye doped nematic liquid crystal (DDLC) cells was investigated during the nematic-isotropic phase transition process and abundant lasing behaviors were observed. There was a temperature range of ~10 °C for the phase transition process of DDLCs, in which anisotropic light scattering was investigated. Light scattering of DDLCs was greatly enhanced during the phase transition process and the enhancement of scattering was strongly dependent on the temperature. And a peak scattering coefficient of more than 1000 cm−1 was achieved at a certain temperature, which we called the peak-scattering temperature. Random lasing induced by resonant feedback was observed near the peak-scattering temperature under the appropriate pump energies. In this situation, broad emissive spectra originating from non-resonant feedback or fluorescence were not evident. When the temperature departed from the peak-scattering value and the pump energy was fixed, the hybrid mode caused by both resonant and non-resonant feedback was achieved. And the proportion of the two feedbacks induced lasing modes could be tuned by temperature and pump energy. All the above random lasing phenomena were observed from 6-µm-thick DDLC cells. The strong scattering in the phase transition process was mainly induced by the inhomogeneity of DDLC due to coexistence of both nematic and isotropic phase. The rich lasing behaviors could be used to control lasing modes by temperature or energy of pump pulse.

Shear-enhanced diffraction from thin permanent gratings in dye-doped nematic liquid crystal cells

Permanent gratings are recorded in planar-aligned dye-doped nematic liquid crystal cells under visible light illumination. By increasing the irradiation intensity and exposure time, several diffraction orders of the recorded gratings are obtained in the Raman-Nath diffraction regime. By applying a dynamic transverse shear on one of the confining plates of the cell, an enhancement of the diffraction efficiency is achieved, which follows the period of the grating. By microscope inspection under static displacement of the upper plate, surface gratings formed by the dye adsorption are revealed in both the front and rear windows of the cell, indicating that the diffraction amplification originates from a coherent superposition of the diffracted orders when the gratings are displaced half a period. The effect provides self-matched amplification of diffraction with a simple cell, a single photo-inscription stage, and elementary displacement steps.

Study on the Polarization of Random Lasers from Dye-Doped Nematic Liquid Crystals

Random lasers from dye-doped nematic liquid crystal (DDNLC) cells with different rubbing methods were observed due to different random ring cavities that were formed. Through constructing cells with different rubbing methods on the forward and backward surfaces of light-emitting sides, we can get two random laser beams with different polarization directions from one DDNLC cell at the same time, and the polarization direction is along the rubbing direction of the light-emitting sides. Additionally, the influence of external electric field on the polarization degree of random lasers was also studied.

The influence of Ag nanoparticles on random laser from dye-doped nematic liquid crystals

The threshold energy and electric field response characteristic of random laser have been investigated in dye-doped nematic liquid crystal (DDNLC) with the addition of different concentrations of Ag nanoparticles (NPs). Due to the localized surface plasmon resonance (LSPR) induced by Ag NPs, random laser from DDNLC with Ag NP doping had a lower threshold energy. From another point of view, nematic liquid crystals (LCs) in a DDNLC cell with the addition of Ag NPs could be more easily influenced by the electric field, which allowed the random laser to be controlled at a lower applied voltage. The turn-off time and turn-on time of random laser also decreased in the DDNLC cells with increasing the concentration of Ag NPs. This is due to the enhancement of the electro-optical characteristic of LC and the restoring force imparted by the locally ordered LCs induced by the Ag NPs, respectively.

The influence of the easy axis on laser-induced ripple structures in dye-doped liquid crystal films

We have investigated the role of the easy axis in the photoalignment effect induced by an Ar+ linearly polarized laser beam on a dye-doped nematic liquid crystal cell in a pump–probe experiment. The induced permanent structure in the irradiated zones was studied using a polarizing optical microscope, and the laser-induced surface morphology was studied by atomic force microscopy. The depth of the ripple structure depends on both the pump power and the irradiated surfaces. The azimuthal anchoring energy provided by the ripple structure was evaluated and it was concluded that it varies with the cell's symmetry.

Controlling the orientation of microgrooves and the depth of the ripple structure in dye-doped liquid crystal cells

The permanent structure induced by an Ar+ linearly polarised laser beam on an asymmetric dye-doped nematic liquid crystal cell was investigated in a pump-probe experiment. The polarisation direction of the probe beam was parallel to the easy axis and two configurations of pump beam were used: polarisation directions perpendicular and parallel to the easy axis. The transmitted intensity of the probe beam was recorded during irradiation and it was observed that it depended both on the power and polarisation direction of the pump beam. We explained the evolution of the transmitted intensity and evaluated the start time of formation of the ripple structure. The induced permanent structure in the irradiated zones and the laser-induced surface morphology was studied using a polarising optical microscope and an atomic force microscope, respectively. The surface morphology in the irradiated zones was also dependent on both the power and polarisation of the Ar+ laser beam. The orientation of the microgrooves in the ripple structure was parallel to the polarisation direction of the pump laser beam in both configurations. For a given pump power, the depth of the ripple structure was greater in the case of an Ar+ beam polarised parallel to the easy axis. The induced azimuthal anchoring energy provided by the ripple structure was evaluated.

Theory and experimental studies of all-optical transmission switching in a twist-alignment dye-doped nematic liquid crystal

We present a theoretical model and experimental demonstration of an all-optical switching process occurring in twist alignment dye-doped nematic liquid crystal cells. A Landau-deGennes theory is applied together with the thermal buildup and diffusion equation to analyze the laser-induced temperature, the order parameter, and the birefringence changes in the liquid crystal cell. The resulting transmission switching processes are calculated using a modified Jones matrix method. Experimental observations of the switching processes in cells of varying thicknesses and their dependence on the input optical power are in good agreement with theoretical predictions. We have also studied the dynamics of the switching processes and observed microseconds switching times. These studies demonstrate the possibility of low-threshold power and microseconds speed switching applications.

Dynamics in Dye-Doped LC Systems in Presence of Trans-Cis Isomerism

The excitation as well as relaxation dynamics of dye-doped nematic liquid crystal cells has been explored both experimentally and theoretically. Overshoots in the build up of the probe signal diffracted from gratings written onto dye-doped liquid crystal systems have often been observed. The overshoot behaviour makes the accurate measurement of nonlinear optical (NLO) response magnitude difficult and ambiguous. Moreover, it complicates the understanding of the dynamics and the physics of the NLO processes. On the basis of the system with trans-cis isomerisation as a mechanism of the NLO effect the quantitative model has been built to describe the experimental results which we observe. The two unknown material parameters: diffusion coefficient and cis species lifetime are calculated from the relaxation data. A quantitative model of the signal build-up uses these parameters. The calculated dynamic behaviour based on this model correlates very well with the experimental data. The model is used to predict the performance of the system with various dopant diffusion properties.

On the possible correlation between dark conductivity, photoconductivity, and photorefractivity in dye-doped nematic liquid crystals

The dc conductivity of dye-doped nematic liquid crystal cells exhibits a characteristic transition from a cubic current–voltage relationship at applied voltages smaller than about 1 V to a linear one at voltages larger than about 1.5 V. The photoconductivity, and the related photorefractive response of these cells exhibit an apparent threshold at about the same characteristic voltage: An applied voltage larger than about 1.5 V is needed to measure sizeable photocurrents and photorefractive responses. We propose a model that is consistent with all these observations. At low applied voltages, a residual space charge limits the dc current, and prevents the participation of photoexcited charge carriers to photoconductivity and photorefractivity, while at higher applied voltages, the disappearance of the residual space charge allows the manifestation of these photoinduced responses. Experimental results seem to confirm the validity of the model.

Transient and steady state photorefractive responses in dye-doped nematic liquid crystal cells

The photorefractive response of several dye-doped nematic liquid crystal samples is studied experimentally. Particular attention is given to the observation and modeling of the multiexponential dynamics of the photorefractive grating, and of the photocurrent. Both involve at least two different mechanisms with largely different time constants. These phenomena occur on time scales on the order of seconds, for applied voltages about 1 V, and light intensities as low as milliwatts/cm2.