向列相液晶中光学相位共轭技术的研究进展

对向列相液晶中光学相位共轭技术的研究进展进行了介绍,分析总结了向列相液晶中光学相位共轭过程的机理与实验方法,并对向列相液晶中的光学相位共轭技术的应用前景进行了展望。     

Nonlinear TM modes in cylindrical liquid crystal waveguide

We consider a nonlinear cylindrical fiber with a nematic liquid crystal (LC) core having initially the escaped configuration and subject to the action of a normal mode propagating electromagnetic field of arbitrary intensity. We derive a set of coupled equations governing the nonlinear dynamics of the electromagnetic field and the confined LC. We solve numerically these coupled equations and find simultaneously the distorted textures of the nematic inside the cylinder and the Transverse Magnetic modes in the guide. We analyze the dependence of these solutions on the electromagnetic field intensity by assuming consistently soft boundary conditions. We have found a dramatic correlation in the spatial distribution of the nematic's configuration and the Transverse Magnetic modes. Thus, the director adopts configurations completely guided by the specific mode involved. We show that the cut-off frequencies and dispersion relations can be tuned by varying the intensity of the electromagnetic propagating field.     

Optical phase conjugation by degenerate four-wave mixing in basic green 1 dye-doped gelatin film using He–Ne laser

Optical phase conjugation in gelatin film doped with basic green 1 has been measured using CW laser radiation (λ=632.8 nm) generated by He–Ne laser of total power 35 mW. The degenerate four-wave mixing (DFWM) experiment allowed for measurement of phase conjugate reflectivity as a function of dye concentration, backward beam intensity, forward beam intensity, probe beam intensity, mean pumping beam intensity and angle between the forward pumping beam and probe beam. For 1 mM concentration of basic green 1-doped gelatin film, 0.1% phase conjugate reflectivity has been observed.     

Polarization dependent guiding in liquid crystal filled photonic crystal fibers

A theoretical study of nematic liquid crystal filled photonic crystal fibers (LCPCFs) is presented. Detailed investigations including the polarization dependent bandgap formation and the modal properties are given for LCPCFs, in which alignment of the molecules could be controlled by external static electric field. The polarization dependent bandgap splitting caused by the high index difference between the ordinary and the extraordinary dielectric index of nematic liquid crystals provides the possibility of single-mode single-polarization guiding. A polarization operation diagram is proposed to describe the guiding behavior of LCPCFs. The influence of rotation angle ? of the director of liquid crystals on the modal properties is investigated. It is shown that the polarization axis of the guided mode is determined by the rotation angle ?, which could be controlled by external electric field.     

Directional scattering and self-pumped phase conjugation in photorefractive crystal

The directional scattering in photorefractive SBN crystal is reported.The self-pumped phase conjugation has been realized with the feedback of this directional scattering andthe performance parameters are compared with those of cat self-pumped phase conjugator.     

Optical phase conjugation by resonant degenerate four-wave mixing in liquid phase medium

In this paper, frequency-doubled Nd:YAG laser's phase conjugation has been investigated experimentally using resonant degenerate four-wave mixing (DFWM). The iodine solution was used as resonant medium. In this way, the energy of the DFWM phase conjugation beam arrived at 43μJ/pulse while the total pump beam's energy reached 8mJ/pulse. The relation between the pulse energy of the DFWM phase conjugation beam and that of the pump beam was investigated. We also measured the optical field distributions of pump beam and DFWM phase conjugation beam, from which it can be noticed that the DFWM phase conjugation can improve the laser beam quality. At the same time, we observed the influence of the pump beam's disturbance on the DFWM signal.     

用He—Ne激光在KNSBN晶体中实现高反射率自泵浦相位共轭

本文报道了用He-Ne激光在(K_(O.5)Na_(0.5)_2y(Sr_(0.61)B_(0.39)))_(1-y)Nb_2O_6(KNSBN)光折变晶体中实现高反射率自泵浦相位共轭,其共轭光的反射率高达40%.研究了共轭光反射率随入射角度变化的关系以及响应时间与入射光功率密度的关系.     

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.     

Stimulated Brillouin scattering phase conjugating properties of long multimode optical fibers

This paper describes and characterizes a detailed experiment that proves the phase conjugating properties of stimulated Brillouin scattering in multimode fibers (km of length), a problem still in debate in the literature, by studying the random aberrations correction abilities of this nonlinear interaction. Detailed qualitative and quantitative evaluation of aberration correction, by using the M² beam propagation ratio as a criterion to assess beam quality of the beams, proves the phase conjugation via SBS in long MM fibers. A new parameter to characterize the efficiency of the SBS phase conjugation process is proposed and conditions to obtain a complete healing of distortions (complete phase conjugation) are discussed.     

Optical properties and optimized conditions for polymer dispersed liquid crystal containing UV curable polymer and nematic liquid crystal

Polymer dispersed liquid crystal was synthesized by combining a UV curable polymer and a nematic liquid crystal. Optimized conditions for the optical properties of the PDLC were found to be the concentration ratio of LC and polymer at 7:3, UV curing time of 18 min, and the thickness less than 25 μm. In the case of the high LC concentration (≥70%) sample, the amount of liquid crystal segregated in the polymerization process was enough to form a spherical shape of droplet, and the threshold driving voltage was reduced. The response time for the turn-on process was nearly independent of the concentration, while the turn-off process was almost proportional to the concentration. From microscopic image and UV–visible spectrum analysis, the relation between LC droplet morphology and optical properties were explained.     

Double signal phase conjugation with a Cat conjugator

The phase conjugate reflection of double signals, induced by the self-pumped phase conjugation, is demonstrated in a 16° cut Cu-doped potassium sodium strontium barium niobate (KNSBN) photorefractive crystal. The phase conjugate reflectivities of signals were measured versus the pump-signal beam ratio. A comparison was made between the signal's reflectivity with and without the presence of the other signal beam. The multi-region four-wave mixing model within the same crystal has been employed to explain the geometry performance and the experimental results.     

Electrically switchable finite energy Airy beams generated by a liquid crystal cell with patterned electrode

A pair of electrically switchable finite energy Airy beams is generated by a liquid crystal (LC) cell, where one electrode is patterned by a photomask with a binary-phase pattern. Applying voltage across the LC cell, an index modulation is produced due to the liquid crystal molecules realignment, and the finite energy Airy beams can be generated or erased corresponding to the phase difference between the regions with and without electrode. The diffraction-free and transverse acceleration dynamics of this binary-phase element based finite energy Airy beams were experimentally verified, exhibiting excellent agreement with the theory.     

Electro-optic study of acrylate based holographic polymer dispersed liquid crystals with broad band photosensitivity

We describe the electro-optic (EO) modulation of diffraction in holographic polymer dispersed liquid crystals, recently developed for broad band photosensitivity, ranging from visible to near infrared. More than 90% EO modulation achieved for relatively low electric fields confirms the phase separation and formation of liquid crystal sub-micrometer sized droplets in the polymer matrix. Molecular reorientation induced by the applied voltage and a residual electrical current induced heating are at the origin of the EO modulation.     

Tunable wavelength-selective add-drop in liquid crystals on a silicon microresonator

We propose and numerically investigate a tunable add-drop consisting of an optical disk microresonator embedded in nematic liquid crystals. The electro-optic cladding enables tunability via an external low-frequency voltage. All loss mechanisms as well as the features of the tunable filter are accounted for via a full-dimensional (3D + time) parallel Finite Difference Time Domain code.     

Full prediction of the broadband optical modulation performance of a twisted nematic liquid crystal cell

We present a procedure to obtain the physical parameters responsible of twisted nematic liquid crystal (LC) cells optical modulation. The novelty of our approach is based on the use of spectroscopic measurements of the light transmitted by the system polarizer-LC cell-analyzer, combined with a previously proposed simple physical model of the LC twist and tilt distribution along cell. The procedure involves two steps: the first one yields off-state parameters like the LC director orientation, the twist angle, and the optical path difference (cell gap); the second step yields the effective retardances of the central and edge LC layers. The use of a spectroscopic method provides a full characterization of the LC cell as a function of both the voltage and the wavelength. The complete procedure leads to a very accurate prediction of the transmitted light broadband spectrum, as well as the complex (amplitude and phase) modulation for any wavelength within the calibration range.     

铁电液晶MBOPDOB相变的光散射研究

我们对铁电液晶 MBOPDOB 进行了温度有关的拉曼散射研究。在低频范围记录了三条拉曼谱线(15,49,93cm~(-1))。其中15和49cm~(-1)谱线消失于 C-Sm C相变温度,没有检测到任何频移现象。93cm~(-1)拉曼线产生部分频移,消失于各向同性相,联系着偶极-偶极互作用。高频范围四个振动模(1178和1116cm~(-1),1710和1738cm~(-1))之间能量转移归于由电子转移引起的 C=0键硬化和 C-0健的弱化。同时也讨论了该液晶 Sm C-Sm A 相变的二级连续型特征。     

Non-reciprocal transmission via phase conjugation in multimode optical fibres

We demonstrate phase conjugate correction of modal distortion of a single mode beam on double-passing a passive highly multimode fibre. Phase conjugation is achieved in a self-pumped photorefractive BaTiO₃ crystal which conjugates only the e-polarised input component. We show that only a small fraction of the multimode fibre output is required to faithfully conjugate the single mode input beam. This technique enables the demonstration of a novel non-reciprocal transmission device, which for example has immediate application in gain-grating holographic resonator designs with multimode fibres as the active gain medium.     

Pump induced normal mode splittings in phase conjugation in a Kerr nonlinear waveguide

Phase conjugation in a Kerr nonlinear waveguide is studied with counter-propagating normally incident pumps and a probe beam at an arbitrary angle of incidence. Detailed numerical results for the specular and phase conjugated reflectivities are obtained with full account of pump depletion. For sufficient strengths of the pump a normal mode splitting is demonstrated in both the specular and the phase conjugated reflectivities of the probe wave. The splitting is explained in terms of a simple model under undepleted pump approximation.