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.     

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.     

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.     

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) .     

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.     

Nonlinear optical grating diffraction in dye-doped blue-phase liquid crystals

Low-power cw lasers are employed to study grating formation in blue-phase liquid crystals. We observed that undoped samples exhibit vanishingly small optical nonlinearities whereas methyl-red-dye doped samples produce strong nonlinear self-diffraction effects. The nonlinearities are attributed to director axis reorientation, disorder, and lattice distortion by the laser-excited dye molecules. The magnitude of the observed intensity-dependent index coefficient is in the range of 10^-4-10^-3 cm²/Watt.     

Random lasing from dye-doped chiral nematic liquid crystals in oriented and non-oriented cells

These are the experimental results describing random lasing in dye-doped chiral nematic liquid crystals. A novel random lasing emission is studied in this article based on the helical domains of dye-doped chiral nematic liquid crystals in oriented and non-oriented cells. Under frequency doubled 532 nm Nd:YAG (yttrium aluminum garnet) laser-pumped optical excitation, we carefully observed and analysed random lasing from dye-doped chiral nematic liquid crystals with wavelength ranges from 600 nm to 620 nm. In addition, the line-width of multi-mode peaks is less than 0.2 nm. The difference between the two random lasing behaviours in the oriented and non-oriented cells arises from the fact that random lasing appearing in the oriented cell results from stronger multiple scattering of light generated by the spiral domains of the liquid crystal molecules. Furthermore, chiral nematic liquid crystal micro-domains with different orientations can induce variation of the diffusion constant, thereby resulting in a decrease or increase in the lasing intensity of the random lasers, and an increase or decrease in their energy thresholds. In addition, a detailed comparison of the two experimental results is also presented in the article, showing the dependence of the lasing threshold and the number of lasing modes on the transport mean free path, the excited area, and the sample size. This process allows us to obtain a random laser by changing the structure of the sample, realising tunable random lasers at low cost.     

Specific features of optical diffusion in nematic liquid crystals

Methods of computer simulation are used to study multiple light scattering in the ordered phase of a nematic liquid crystal. The development of the diffusion regime is studied in detail. It is demonstrated that the transient time depends on the external magnetic field and the direction of the incident radiation. The diffusion coefficients along and across the director are calculated. The validity of the diffusion approximation is controlled using the moments of the distribution function. Analysis of the diffusion coefficients versus the external field shows that the diffusion coefficients remain almost unchanged in the experimental range of the magnetic fields. In the presence of relatively strong fields, the diffusion coefficients increase due to a variation in the shape of the scattering indicatrix and the free-path length of photons. The dependence of the diffusion coefficients on the radiation wavelength is also calculated. Comparison of the simulated results and the experimental data shows that the calculated diffusion rate across the director virtually coincides with the experimental rate. The calculated diffusion coefficient along the director is substantially greater than the corresponding experimental rate.     

向列相液晶中光致非线性效应的研究

分析了向列相液晶中的光致非线性效应.在此基础上,基于液晶显示器工作原理设计了光控光器件.提出简化模型并利用琼斯矩阵给出光在该模型中的传输理论;引入斜入射的控制光束代替外加电压来调控液晶盒中分子的排列,实现对出射信号光的位相和光强的光-光调控;利用5CB液晶的温度特性分析环境温度对光-光调控的影响.理论分析表明:出射光位...     

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.     

Paramagnetic nematic liquid crystals

The paper reports new nematic liquid crystals that are paramagnetic.     

Elastic constants of some nematic liquid crystals with banana-shaped molecules

Results are given for the elastic constants in the nematic phase of some molecules with a curved (banana-like) molecular shape. In principle this could lead to a reduced bend elastic constant due to the possibility of adjusting to a bend deformation by a redistribution of the molecules. However, the elastic constants turn out to have quite normal values as compared with similar non-curved molecules.     

First-order polarization-activated optical transitions in smecticC and planar nematic liquid crystals

Summary It is shown that polarization-activated intrinsic optical bistability (OB) may be obtained in smecticC or planar nematic liquid crystals without external feed-back resonator. First-order transitions for twist are proved to occur, in principle, irrespective of the liquid-crystal material constants. The light intensity, needed to obtain OB, ranges from 10 kW/cm² to 1 MW/cm². To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

向列相液晶中强非局域空间光孤子的相互作用

在数值和实验上研究了向列相液晶中空间光孤子的相互作用.向列相液晶中的响应函数为指数衰减函数，这不同于Snyder-Mitchell模型.数值模拟发现向列相液晶中的孤子相互作用仍然满足交叉点位置与输入功率的平方根成反比，强非局域下的Snyder-Mitchell模型交叉点位置与两束光的间距无关，而在向列相液晶中交叉点位置与两束光的间距有关；在实验上发现，向列相液晶中两束光交叉点的位置与输入功率平方根成反比，还和形成孤子的临界功率以及两束光之间的间距有关.     

向列相液晶中强非局域空间光孤子的相互作用

在数值和实验上研究了向列相液晶中空间光孤子的相互作用.向列相液晶中的响应函数为指数衰减函数,这不同于Snyder-Mitchell模型.数值模拟发现向列相液晶中的孤子相互作用仍然满足交叉点位置与输入功率的平方根成反比,强非局域下的Snyder-Mitchell模型交叉点位置与两束光的间距无关,而在向列相液晶中交叉点位置与两束光的间距有关;在实验上发现,向列相液晶中两束光交叉点的位置与输入功率平方根成反比,还和形成孤子的临界功率以及两束光之间的间距有关. 关键词： 强非局域非线性 向列相液晶 孤子相互作用 交叉点     

Optically induced space-charge fields, dc voltage, and extraordinarily large nonlinearity in dye-doped nematic liquid crystals

We have observed extraordinarily large optical nonlinearity in Methyl Red-doped nematic liquid-crystal film. Grating diffraction can be generated with an optical intensity as low as 40 microW/cm(2) , and a refractive-index change coefficient of more than 6 cm(2)/ W is obtained. The effect is attributed to formation of an optically induced dc space-charge field and to the resulting reorientation of the highly birefringent nematic director axis.     

Interaction of light with a dye-doped nematic liquid crystal

The interaction of linearly and circularly polarized beams with a nematic liquid crystal doped with light-absorbing dyes has been studied by light-diffraction and microprojection methods. It has been found that there is a threshold for the emergence of light-induced anisotropic structures, which, depending on the type of dye, can be axisymmetric or extended in a direction determined by the light field. Possible mechanisms leading to the formation of the anisotropic structures are discussed. Zh. éksp. Teor. Fiz. 111, 2059–2073 (June 1997)