向列相液晶中空间光孤子的观测

分别以He-Ne激光器、半导体激光器和Ar⁺激光器作为光源照射液晶材料，在低光强入射的情况下均观察到了光束的自聚焦现象以及空间光孤子的形成，观察到了长度l>5mm的孤子波和多模空间光孤子，并对空间孤子的产生条件及特性进行了讨论. 关键词： 自聚焦 空间光孤子 向列相液晶     

Discrete propagation and spatial solitons in nematic liquid crystals

We investigate, for the first time to our knowledge, the discrete propagation of near-infrared light in a voltage-controlled array of channel waveguides in undoped nematic liquid crystals under planar anchoring conditions. This novel geometry enables us to drive the system from one-dimensional bulk diffraction to discrete propagation and, for larger excitations, to discrete spatial solitons, or nematicons. The observed phenomena are adequately described by a scalar model that encompasses the voltage-dependent reorientational response of the material.     

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     

负性介电各向异性向列相液晶中空间光孤子的理论研究

对非局域非线性介质向列相液晶中介电各向异性为负时的情况进行了研究.理论研究表明,负性介电各向异性的向列相液晶具有负的非线性系数.文中给出了其空间非局域响应特征宽度和非线性系数的表达式,并求出了其非局域响应函数;其次,用数值计算的方法给出了其空间孤子的传输结果.最后,研究了光束功率和偏置电压的改变对负性介电各向异性向列相液晶中光束传输的影响,发现偏置电压的改变会导致光束在负性介电各向异性液晶中形成孤子所需的临界功率发生改变.     

非局域程度对向列相液晶中空间光孤子的影响

理论分析了非局域程度对向列相液晶中(1+2)维空间光孤子的影响.得到了非线性系数以及特征长度和预倾角的关系,并且给出了强非局域性的非线性薛定谔方程,最终得到了单孤子和临界功率的解析解.实验上,通过改变偏压从而间接测量了向列相液晶中的临界功率,并与数值模拟结果相结合与理论结果进行了比较,发现基本相符. 关键词： 空间光孤子 非线性系数 非局域程度 临界功率     

Self-healing generation of spatial solitons in liquid crystals

We demonstrate that, in suitably designed cells with undoped nematic liquid crystals, extraordinary-wave spatial solitons can be excited at every applied voltage without adjustments in the input polarization. Their walk-off, hence direction of propagation, is externally controlled over angles as large as 7 degrees. The results pave the way not only to polarization-forgiving generation but also to voltage readdressing of these extraordinary-wave nematicons.     

Nonlocal spatial soliton interactions in nematic liquid crystals

The interaction between spatial optical solitons in undoped nematic liquid crystals is governed by reorientational nonlinearity with a significant nonlocality. We demonstrate several cases of interactions, including crossing, interlacing, and merging of spatial solitons in voltage-biased liquid-crystal cells. A scalar model accounts for the main features that were experimentally observed.     

Incoherent spatial solitary waves in nematic liquid crystals

(2+1) -dimensional spatial solitary waves are generated by launching of milliwatt-power linearly polarized light beams in voltage-biased planar cells with undoped nematic liquid crystals, regardless of the degree of spatial coherence of the input. Coherent and incoherent self-trapping, as well as guidance of a weaker copolarized signal, is demonstrated.     

Electric-field-induced steering in soliton direction of propagation in chiral nematic liquid crystals

In this work we demonstrate the properties of spatial solitary waves in chiral nematic liquid crystals with an external electric field. Such self-trapped beams, called nematicons, were created due to the optical reorientation nonlinearity for a light power of a few tens of milliwatts. We show that the direction of propagation of such nematicons can be changed by applying an external electric field. Additionally, this effect can be modified by changing the input polarization of the light beam. The experimental results were obtained in four independent guiding layers created by a chiral nematic structure.     

The inertia mode of the mechanically generated solitons in nematic liquid crystals

The existence of a localized relaxation mode of the recently observed solitarylike director waves in nematic liquid crystals is pointed out. This mode describes a nearly instantaneous regression of the perturbed director wave to its initial profile.Work supported by the Swiss National Science Foundation     

向列相液晶中强非局域空间光孤子传输的理论研究

对向列相液晶中非局域空间孤子的传输进行了理论研究.基于非线性液晶孤子传输方程，采用Gauss形式的试探解，不仅得到了空间孤子的解析解,而且还在临界功率附近得到了呼吸子的解析解.通过数值模拟证明我们的结果比Conti和Assanto等人的结果更合理.同时，对液晶中的非局域孤子模型和Snyder等提出的强非局域孤子模型进行了全面的比较. 关键词： 向列相液晶 空间孤子 非局域非线性 呼吸子     

Paramagnetic nematic liquid crystals

The paper reports new nematic liquid crystals that are paramagnetic.     

Controlled generation and steering of spatial gap solitons

We demonstrate the first fully controlled generation of immobile and slow spatial gap solitons in nonlinear periodic systems with band-gap spectra, and observe the key features of gap solitons that distinguish them from discrete solitons, including a dynamical transformation of gap solitons due to nonlinear interband coupling. We also describe theoretically and confirm experimentally the effect of the anomalous steering of gap solitons in optically induced photonic lattices.     

Orientational dynamics in nematic liquid crystals

We examine the behaviour of single-particle orientational time correlation functions in nematic liquid crystals. As well as the expected dynamics involving oscillation in a mean-field potential, and occasional jumps between orientations parallel and antiparallel to the director, we provide the first simulation evidence of long-time tails characteristic of coupling to director fluctuations.     

Thermomechanical reorientation in nematic liquid crystals

Thermomechanical mechanism of the director reorientation in twisted nematic liquid crystals caused by the one-dimensional longitudinal temperature gradient is predicted and studied theoretically. The calculated director reorientations are in the range that can be measured experimentally very easily.     

End-chain ordering in nematic liquid crystals

The ordering of hydrocarbon end-chains in nematic liquid crystals has been calculated by numerical summation over all chain configurations in the molecular field. The results, used in a self-consistent calculation of the orientational ordering for the full system, explain the variation of the isotropic-nematic transition temperatures (including the even-odd effect) and transition entropies with end-chain length.     

Thermotropic biaxial nematic liquid crystals

We have synthesized liquid crystal (LC) mesogens based on a nonlinear oxadiazole unit that exhibit nematic phases near 200 degrees C. Polarized microscopy and conoscopy indicate that these LCs are biaxial nematics. Unambiguous and quantitative evidence for biaxiality is achieved using 2H NMR spectroscopy. "2D powder" spectra, obtained by rotating 2H-labeled samples about an axis perpendicular to the magnetic field at approximately 200 Hz, yield phase biaxiality parameters of approximately 0.1 when coupled with rigorous and proven simulations.     

Bistability in dual-frequency nematic liquid crystals

Different modes of bistable switching in liquid crystals with frequency inversion of the dielectric anisotropy sign are discussed. The study is performed by numerical simulation and experimentally. It is shown that dual frequency driving can be effectively used to control switching between topologically equivalent and non-equivalent director field distributions. The experimental results on temperature performance of the dual-frequency switching and possible driving methods for energy consumption and expanding the temperature range are presented.     

Photoinduced effects in nematic liquid crystals

Temperature, concentration of the solvent and pressure are the parameters that are well known to bring about phase transitions in liquid-crystalline systems. In recent years a new parameter has been added to this list: light. The principle behind these photoinduced transitions is the light-driven shape transformation of certain photoactive materials like, e.g., azobenzene. In this article, we present results of various aspects of our recent investigations on such photoinduced transitions in the nematic phase and highlight the feature that light is a new tool to study phase transitions and the associated critical phenomena.