Fast optical recording in dye-doped liquid crystals

Light-induced director reorientation in dye-doped nematic liquid crystals was recently reported to be an efficient method of writing permanent holographic gratings with high sensitivity [Phys. Rev. Lett. 82, 1855 (1999)]. We report the achievement of stable director reorientation in the same materials by means of a single 4-ns pulse of the second harmonic of a Nd:YAG laser. Fast recording of high-resolution holographic gratings (more than 500 lines/mm) can be obtained with an energy density as low as 7x10(-3)J /cm(2) .     

Enhanced optical nonlinearities in some dye-doped nematic liquid crystals

The nonlinear refractive index of some pure and dye-doped nematic liquid crystals was measured and compared using extraordinary polarized light. The optical torque of the nematic liquid crystals was strongly enhanced by a small amount of dye impurities (Sudan Black B). This observation is found to be in good agreement with the Janossy model. The dependence of the enhancement factor on the nematic host structure was investigated. The sign of all nonlinear refractive indices were determined by the Z-scan technique.     

Nonlinear optical interactions in dye-doped solids

We present a brief review of non-linear optical investigations on dye-doped solids using low-power CW lasers. After a brief introduction to the photophysics of the dye molecules, we discuss specific nonlinear processes such as self-diffraction, optical phase conjugation, two-beam coupling and polarization gratings in these systems. The application potential of dye-doped solid devices is discussed.     

Optical addressing in dye-doped cholesteric liquid crystals

This study investigates a method of optical addressing in dye-doped cholesteric liquid crystals (DDCLCs). Photo-induced randomly adsorbed dyes can change the CLC textures from planar to focal conic. Such patterning can be adopted to develop a display that is initially invisible, but becomes visible upon heating above the clearing temperature, followed by cooling to room temperature. The display can also become visible upon the application of a suitable voltage, and its rapid release. Additionally, the display is thermally erasable, optically rewritable and electrically switchable. It can be applied for use as a smart card.     

Polarization-independent photochromic diffraction in a dye-doped liquid crystal

We report the observation of polarization-independent photochromic diffraction in an azo-dye-doped liquid crystal. The generation of the phase grating is more than 90% independent of the polarization of the writing beams, and the diffraction by the phase grating is more than 90% independent of the polarization of the probe beam. Unpolarized lamp light was also used to generate real-time phase gratings and self-diffraction. For the first time to our knowledge, photochromic phase modulation and light diffraction that exhibit more than 90% polarization independence for both writing and probe beams were produced in an anisotropic liquid-crystalline material.     

Two-wave mixing in chiral dye-doped nematic liquid crystals

Two-wave mixing in nematic liquid crystals doped with dyes and chiral agents is studied. The photo-induced spatial modulation of the chiral structure, together with the diffusion anisotropy of the mixture, determine a relatively fast response time, a spatial resolution of 1 μm, and a maximum gain for circularly polarized interacting beams. The gain is insensitive to changes in linear polarization, while it varies with the grating period and with the pump-to-signal intensity ratio.     

Nonlinear optical ferroelectric liquid crystals and device configurations

Ferroelectric Liquid Crystals (FLCs) are presented which comprise charge-transfer functional groups, such that highly efficient NonLinear Optical (NLO) properties and pronounced ferroelectricity result. By operating the NLO-FLCs in one of the two Bistable states of the Short-pitch Ferroelectric (SPF) conformation, stable, planar waveguides result. The NLO-FLCs exhibit exceptionally large second-order NLO-coefficientsd ₂₂ = 5 pm/V. Moreover, their linear electrooptical effect is shown to be essentially electronic in nature. This renders ultrafast electrooptical modulators with NLO-FLCs feasible.     

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

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 (n₂) 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 n₂ modulated is from −5.26 × 10⁻³ to 1.62 × 10⁻³ cm²/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 n₂ 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.     

Transient grating optical nonlinearities in nematic,cholesteric and smectic liquid crystals

The transients of Kerr-like optical nonlinearities in various liquid crystals observed with picosecond light-induced dynamic gratings are discussed. It is shown that relatively high intensities and strong optical fields of short laser pulses lead to rapid molecular reorientation, ultrasound generation, multiphoton absorption, cholesteric helix deformation and other new phenomena which are not only of interest for basic liquid-crystal research but also determine photonic switching times in liquid crystalline all-optical devices.     

Random lasing in freely suspended dye-doped nematic liquid crystals

Random lasing in fully disordered systems having organic and inorganic nature has been the subject of extensive studies since the beginning of the past decade. The interest mainly emerges from the unexpected role played by disorder in the laser action. The disorder was considered detrimental for the optical feedback in cavity laser, until it was demonstrated that multiple-scattering materials including a gain medium act as random laser. Here, a completely new approach is reported, where freely suspended complex fluid films doped with fluorescent molecules under optical excitation generate narrowband lasing peaks. The constellation of localized modes is selected by properly choosing the gain profile. The idea to have laser action in absence of mirrors and boundaries realizes an unparalleled tunable and moldable laser source.     

Thermomechanical effects in uniformly aligned dye-doped nematic liquid crystals

We show theoretically that thermomechanical effects in dye-doped nematic liquid crystals when illuminated by laser beams, can become important and lead to molecular reorientation at intensities substantially lower than that needed for optical Fréedericksz transition. We propose a 1D model that assumes homogenous intensity distribution in the plane of the layer and is capable to describe such a thermally induced threshold lowering. We consider a particular geometry, with a linearly polarized light incident perpendicularly on a layer of homeotropically aligned dye-doped nematics.     

Photorefractive effect induced by polarization gratings in dye-doped liquid crystals

We report on the photorefractive effect induced by a polarization grating in the presence of dc voltage in a dye-doped liquid-crystal (DDLC) film. The writing beams are two orthogonally (left- and right-circularly) polarized laser beams that create a spatially polarization-modulated interference field with constant intensity. The photorefractivity is ascribed to the absorption anisotropy of the azo dye. The unique dichroism of a DDLC cell causes a spatial variation in the absorption of light in response to a polarization-modulated interference field. Such a variation establishes a space-charge field in the presence of dc voltage, generating photorefractivity. Two-beam couplings were also verified and measured dynamically during the formation of the photorefractive grating in this study.     

Tailoring of random lasing characteristics in dye-doped nematic liquid crystals

Amplified spontaneous emission and random lasing are investigated in random systems with dye-doped nematic liquid crystals. And that temporal stability of random lasing is analyzed. The influence of pumping polarization as well as the multiple scattering and reflection between boundaries on the emission behavior and the formation of coherent feedback is investigated in detail. For freely suspended samples, certain emission wavelength can be obtained by changing the pump wavelength. This feature is useful in making wavelength-tunable lasers. Moreover, as the pumping thickness of wedge sample increases, the emission spectrum is red shifted and the average spacing of adjacent spikes decreases. This property can be applied in laser mode selection, i.e., the number of modes within certain wavelength range can be chosen.     

基于透射式液晶/聚合物光栅的分布反馈式激光器的研究

本文研究了染料掺杂透射式液晶/聚合物光栅的制备以及基于透射式液晶/聚合物光栅的分布反馈式激光器的激光特性.实验选取DCM作为激光染料,制备了周期为586 nm的掺杂DCM的透射式液晶/聚合物光栅;使用532 nm输出的Nd ∶YAG倍频脉冲激光器作为抽运光源对染料掺杂液晶/聚合物光栅进行侧面抽运,得到了中心波长为603 nm的窄线宽、低阈值激光输出.激光线宽为1.4 nm、阈值能量约为17.3 μJ,与之前国外的报道相比,阈值能量有了很大幅度的降低. 关键词： 液晶/聚合物光栅 分布反馈式激光器 阈值 线宽     

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.     

Nonlinear optical properties of composites based on conductive metal-alkanoate liquid crystals

The dynamic holography in new composite materials based on a novel class of metal-alkanoate ionic liquid crystals (ILCs) is studied experimentally and theoretically. The composites are formed as a dielectric dye film covered by lyotropic metal-alkanoate ILC and ionic smectic glasses with doped dye molecules. The dynamic gratings are created by nanosecond pulses of double frequency Nd:YAP laser, the recording demonstrates fast erasure time of residual thermal gratings. The nonlinear optical properties are determined by the resonance nonlinearity in photosensitive centres of ILC. Note, that permanent relief gratings will be formed on a dielectric dye film only as well as in composite cells either with nematic LC or with polymers under action of pulsed laser radiation. Lyotropic ILC layer applied over the dye film provides the dynamic regime of grating recording in composite cells. We found a secondary thermal grating is much smaller, the conductive ILC matrix provides effective heat dissipation and erasure of this thermal grating. A theory of Raman-Nath self-diffraction holography on thin films followed from the wave equation and the nonlinear mechanism of absorption saturation is developed to explain experimental results.     

Biphotonic effect-induced phase transition in dye-doped cholesteric liquid crystals and their applications

This work studies the biphotonic effect in samples that are cholesteric liquid crystals (CLCs) doped with azo-C5. The experimental results show that the photo-isomerization of azo-C5 not only changes the clearing point of the sample, but also shifts the reflection band that is associated with the planar texture. Additionally, azo dye-doped CLCs (DDCLCs) have bi-stable or tri-stable states, as determined by the ambient temperature. Photo-switching between these bi-stable/tri-stable states is systematically studied. The result indicates that photo-addressing one of these states (planar, focal conic, and isotropic states) using a low- or high-intensity Ar⁺ laser beam is feasible. The results thus obtained are used to fabricate a photo-rewritable DDCLC display.