Dye-doped Nematic Liquid Crystal Research Articles

Alignment-to-polarization projection in dye-doped nematic liquid crystal microlasers

We report a dye-doped nematic liquid crystal microlaser that allows the two-dimensional alignment of the liquid crystal to be projected directly on the output polarization of the laser beam. The laser cavity is composed of a pair of dielectric multilayers sandwiching a dye-doped nematic liquid crystal with patterned alignment, and exploits the fact that the resonance modes in such systems are split into two orthogonally polarized modes experiencing either the extraordinary or ordinary refractive index of the liquid crystal. Azimuthally polarized lasing is demonstrated using a concentrically aligned liquid crystal layer.

Sub-milliwatt dissipative nematicons and spontaneous light emission in dye-doped nematic liquid crystals

Blue light self-confinement in active dye-doped nematic liquid crystals is reported, demonstrating trapping of the emitted red light in dissipative spatial solitons formed by all-optical reorientation.

Photorefractive Holograms in Dye-doped Nematic Liquid Crystals for Three-dimensional Displays

Photorefractive Holograms in Dye-doped Nematic Liquid Crystals for Three-dimensional Displays

Nonlinear experimental dye-doped nematic liquid crystal optical transmission spectra estimated by neural network empirical physical formulas

In this paper, two complementary objectives related to optical transmission spectra of nematic liquid crystals (NLCs) were achieved. First, at room temperature, for both pure and dye (DR9) doped E7 NLCs, the 10–250 W halogen lamp transmission spectra (wavelength 400–1200 nm) were measured at various bias voltages. Second, because the measured spectra were inherently highly nonlinear, it was difficult to construct explicit empirical physical formulas (EPFs) to employ as transmittance functions. To avoid this difficulty, layered feedforward neural networks (LFNNs) were used to construct explicit EPFs for these theoretically unknown nonlinear NLC transmittance functions. As we theoretically showed in a previous work, a LFNN, as an excellent nonlinear function approximator, is highly relevant to EPF construction. The LFNN-EPFs efficiently and consistently estimated both the measured and yet-to-be-measured nonlinear transmittance response values. The experimentally obtained doping ratio dependencies and applied bias voltage responses of transmittance were also confirmed by LFFN-EPFs. This clearly indicates that physical laws embedded in the physical data can be faithfully extracted by the suitable LFNNs. The extraordinary success achieved with LFNN here suggests two potential applications. First, although not attempted here, these LFNN-EPFs, by such mathematical operations as derivation, integration, minimization etc., can be used to obtain further transmittance related functions of NLCs. Second, for a given NLC response function, whose theoretical nonlinear functional form is yet unknown, a suitable experimental data based LFNN-EPF can be constructed to predict the yet-to-be-measured values.

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.

Nonlinear optical investigation of Gaussian laser beam propagating in a dye-doped nematic liquid crystal

The laser beam shape and variation of the curvature radius of the wavefront have been simulated when the Gaussian laser beam passes through a dye-doped nematics liquid crystal. The effect of different dyes is investigated in the wavefront distortion as well as the beam shape due to its quality factor. We have reported the dependency of the curvature radius of the wavefront to the nonlinearity of the sample. Also we have obtained the beam shape of three dyes doped nematic liquid crystal as a nonlinear optical material compared with the experimental results.

Liquid Crystals Tunable Optical Metamaterials

A critical discussion with detailed calculations for the complex refractive indexes of metamaterials of various nanoconstituent makeups, sizes, shapes and concentrations, and periodic nanostructured patterns/geometry are presented. These metamaterials possess tunable subunity, zero, or negative refractive indexes, on account of the large birefringence of their nematic liquid crystal constituent. We also studied the special case involving laser dye-doped nematic liquid crystals that could provide optical gain. It is shown that the resulting metamaterials' loss can be minimized, while achieving the desired tunable birefringence. Some preliminary experimental studies have demonstrated fast optically induced modulation of the nematic liquid crystals and indicated the possibility of ultimately realizing fast tunable or nonlinear optical metamaterials.

Optical path control of solitary waves in dye-doped nematic liquid crystals

The light-induced steering of spatial optical solitary waves in dye-doped nematic liquid crystals is analyzed. Shining a control beam through the cell results in a boundary mediated change in the orientation of the molecular director, thereby a perturbation in the refractive index of the medium. This re-orientational effect can optically steer spatial solitary waves propagating in an orthogonal plane within the liquid crystal layer. The equations governing this steering are solved using a Lagrangian formulation, resulting in modulation equations for the self-trapped beam path. Very good agreement is obtained between solutions of the modulation equations and numerical solutions of the full governing equations. Full Text: PDF

Identification of optical nonlinearities of dye-doped nematic and polymer-dispersed liquid crystals using Z-scan technique

Abstract This study investigates the nonlinear optical properties of azo-dye-doped nematic and polymer-dispersed liquid crystal (ADDPDLC) films with nano-sized LC droplets using the Z-scan technique, which is a simple but powerful technique for measuring the optical Kerr constants of materials. The results indicate that the optical Kerr constant ( n 2 ) of the azo-dye-doped nematic LC (ADDLC) film is large because of the photoisomerization effect and the thermal effect. Therefore, the optical Kerr constant of this material can be modulated by varying the temperature of the sample and the direction of polarization of incident laser. The range of n 2 modulated is from −5.26 × 10 −3 to 1.62 × 10 −3 cm 2 /W. The optical Kerr constants of ADDPDLC films at various temperatures are also measured. The experimental results reveal that liquid crystals in the ADDPDLC film strengthen the nonlinearity. The n 2 of the ADDPDLC film is maximal at ∼35 °C, because of the decrease in the clearing temperature of the ADDPDLC films. The clearing temperatures of the liquid crystals (E7), and the ADDPDLC film used in this work were found to be 61 °C and 43 °C, respectively.

Bistable device using anchoring transition of nematic liquid crystals

The authors have demonstrated a novel bistable memory device writable by a laser beam. The device utilizes the hysteresis of a temperature-driven discontinuous anchoring transition (orientation change) in a dye-doped nematic liquid crystal. The laser light irradiation switches the stable orientation from homeotropic to planar in a liquid crystal on perfluoropolymer surface. The thickness of recorded lines comes down to as low as ∼20 μm. The recorded images were kept at least for a day, i.e., memory effect. We also showed that the temperature range of the hysteresis was lowered down to room temperature using a binary mixture system.

Self-phase modulation and ring pattern observation by applying electrical field on dye-doped nematic liquid crystal

In this work, we report the experimental results of two typical liquid crystals (6CHBT and w1680) in the nematic phase while they have been doped with Sudan dyes compared with pure state. We investigate the AC-applied voltage effect on the nonlinear behavior of dye doped liquid crystal (DDLC). The novel effects on the far-field diffraction patterns of a Gaussian beam depend on the external applied field. The measurements were performed using CW He:Ne laser and Nd:Yag laser at 632.8 nm and 532 nm, respectively. The z-scan technique is used to measure the amplitude and the sign of the nonlinear refractive indices DDLC. The nonlinear refractive indices were in order of 10 − 5 cm 2/W. Also the nonlinear absorption coefficient β was measured using open aperture z-scan technique.

Polarization-controlled contrasted images using dye-doped nematic liquid crystals

We explore the polarization dependence of the nonlinear response of a planar nematic liquid crystal cell doped with 1% wt of methyl red dye. The results obtained show that the refractive index change can be switched from a positive value to a negative one as the polarization of the beam changes from parallel to perpendicular with respect to the rubbing direction. This property is exploited in a phase contrast system, where a dynamic phase filter is photoinduced in a liquid crystal cell placed in the system's Fourier plane. Real-time contrast inversion in the resulting images is demonstrated.

Enhanced linearly polarized lasing emission from nanoimprinted surface-emitting distributed feedback laser based on polymeric liquid crystals

The authors have demonstrated the enhancement of linearly polarized lasing emission intensity using a structure made by a simple fabrication process. The enhanced lasing is achieved using a nanoimprinted distributed feedback structure together with spin-coated polymeric liquid crystals. The backward linearly TE-polarized lasing emission is transformed to left-handed circularly polarized light (L-CPL) by employing a dye-doped polymeric nematic liquid crystal (PNLC) film as a (−1/4)λ[=(3/4)λ] plate. The L-CPL is effectively reflected by a L-polymeric cholesteric liquid crystal film as a reflector and transformed back to TE-polarized light by the PNLC film; as a result one-directional emission intensity is enhanced.

Light-induced orientational effects in periodic photonic structures with pure and dye-doped nematic liquid crystal defects

We theoretically study and compare the light-induced Freedericksz transition and above-threshold reorientation in pure and dye-doped nematic liquid crystals embedded into periodic dielectric structures. The presence of optical defect modes is found to significantly alter the effective influence of dyes as compared to the standard case of liquid crystal films without periodic structure, which is due to the overall resonant absorption of the dye-doped liquid crystal layer. Both statics and dynamics are discussed. In particular, it is found that optical response time can be up to several orders of magnitude faster with pure liquid crystals than with dye-doped ones. Thermal effects caused by absorption of dyes are also investigated to gauge whether it may induce nematic-to-isotropic phase transition.

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.

Polychromatic liquid crystal laser arrays towards display applications

Band-edge liquid crystal lasers are of interest for a number of applications including laser projection displays. Herein, we demonstrate simultaneous red-green-blue lasing from a single liquid crystal sample by creating a two-dimensional laser array fabricated from dye-doped chiral nematic liquid crystals. By forming a pitch gradient across the cell, and optically pumping the sample using a lenslet array, a polychromatic laser array can be observed consisting simultaneously of red-green-blue colors. Specifically, the two-dimensional polychromatic array could be used to produce a laser-based display, with low speckle and wide color gamut, whereby no complex fabrication procedure is required to generate the individual 'pixels'.

All-optical steering of soliton waveguides in dye-doped liquid crystals

Soliton induced waveguides can be all-optically readdressed in planar cells by modifying the molecular anchoring with an external light source. Using an elliptically shaped control beam we demonstrate refraction and total internal reflection of spatial solitons in dye-doped nematic liquid crystals.

One- and biphotonic reorientational nonlinearities in dye-doped nematic and cholesteric liquid crystals

This investigation elucidates one- and biphotonic reorientational nonlinearities in dye-doped nematic liquid crystals (DDNLCs) and dye-doped cholesteric liquid crystals (DDCLCs) using the Z-scan technique. Three cases—DDNLCs stimulated by one green beam polarized linearly parallel and perpendicular to the LC director and DDCLCs stimulated by one circularly polarized green beam—are considered without and with simultaneous stimulation by one linearly polarized red beam. Green-beam-induced dye reorientation through trans-cis isomerization and red-beam-induced suppression of dye reorientation by cis-trans back isomerization are responsible for the one- and biphotonic nonlinearities, respectively. The nonlinear refractive indices obtained herein are two orders of magnitude larger than those obtained elsewhere [Fuh et al., Opt. Exp. 13, 10634 (2005)]. The induced biphotonic nonlinearities can be modulated by controlling the intensity of the red beam in different ways due to the difference among the capabilities of the red beam to suppress the reorientation of the dyes and LCs in the three cases.

Model for Light Scattering and Lasing in Dye-Doped Nematic Liquid Crystals

Phenomena associated to lasing in random or quasi-random media are of growing interest both from theoretical and experimental points of view. Recent studies show that nematic liquid crystals are good candidates as scattering host for these lasing systems. We present theoretical analysis and Monte Carlo simulations for different modellizations of scattering in nematics coupled with a two-level system which describes the dye action. We also show that a simple description of light diffusion in nematics is inadequate to describe the localization process that is at the basis of the laser action; therefore, different physical phenomena have to be taken into account.

Light-induced gliding of the easy orientation axis of a dye-doped nematic liquid crystal

We studied the effect of light-induced gliding of the easy axis of dye-doped nematic liquid crystal on an aligning polymer surface. The observed drift of the easy axis is over tens of degrees and is caused by light-induced anisotropic adsorption and/or desorption of dye molecules on or from the aligning layer in the presence of light-induced bulk torque. We present a theoretical model that explains the experimental data in terms of the light-induced changes of the adsorbed dye molecules angular distribution due to their exchange with the dye molecules from the liquid crystal bulk.