Abstract. Liquid crystals are nowadays widely used in all types of display applications. However their unique electro-optic properties also make them a suitable material for nondisplay applications. We will focus on the use of liquid crystals in different photonic components: optical filters and switches, beam-steering devices, spatial light modulators, integrated devices based on optical waveguiding, lasers, and optical nonlinear components. Both the basic operating principles as well as the recent state-of-the art are discussed. C 2011 Society of Photo-Optical Instrumentation Engineers (SPIE).[ DOI: 10.1117/1.3565046] Subject terms: liquid crystals; photonic applications; review; liquid-crystal lasers; spatial light modulators; tunable lenses; nematicons; optical nonlinearity.Paper 100913SSR received Nov. 5, 2010; revised manuscript received Jan. 11, 2011; accepted for publication Jan. 13, 2011; published online Jun. 14, 2011. IntroductionLiquid crystals (LCs) are organic materials that are liquid but that show a certain degree of ordering (positional and/or orientational). With this definition, many materials can be classified as liquid crystals, but the majority of liquid crystals that are used in photonic applications are of the thermotropic type. Thermotropic means that the liquid-crystal phase exists within a certain temperature interval (in contrast to lyotropic materials for which the material is liquid-crystal within a certain concentration range). Various types of thermotropic liquid-crystal materials exist, and many different mesophases have been discovered in the last decades: nematic, smectic A, smectic C, columnar, blue phases, and many many more. The diversity of liquid-crystal materials is huge, but this diversity is even overshadowed by the number of applications in which liquid crystals are used nowadays. The majority of applications are related to informationdisplay applications. Liquid crystals have conquered the major market share in different display application areas: television screens, laptop screens, screens in mobile phones, etc. Only in projection displays does a tough competitor exist, namely microelectromechanical systems or licenced by Texas Instruments: Digital light processing. Organic light emitting diodes (OLEDs) are the obvious next generation technology that could overtake the LC domination, but today OLED displays have not penetrated the market and only the future will tell if they will. In this review article, we will focus on nondisplay applications, but because we cannot go too broad, we restrict ourselves to photonic applications in which the light is actively manipulated by the LC. In this review article, a certain external influence (surface anchoring, electric fields, optical fields) is used to (re)orient the LC in a certain way. In turn, the LC then changes the light that is propagating through it. Of course, as in every review article, it is impossible to list all the fascinating new scientific results or breakthroughs. Therefore, the authors apologize for not i...

Abstract-The accurate analysis of optical waveguides is an important issue when designing devices for optical communication. Waveguides combined with liquid crystals have great potential because they allow waveguide tuning over a wide range using low voltages. In this paper, we present calculations that combine an advanced algorithm for calculating liquid crystal behavior and a finite-element mode solver that is able to incorporate the full anisotropy of the materials. Calculation examples demonstrate the validity of our program.Index Terms-Finite-element mode solver, nematic liquid crystal, optical waveguides.

An efficient full-vectorial finite element beam propagation method is presented that uses higher order vector elements to calculate the wide angle propagation of an optical field through inhomogeneous, anisotropic optical materials such as liquid crystals. The full dielectric permittivity tensor is considered in solving Maxwell's equations. The wide applicability of the method is illustrated with different examples: the propagation of a laser beam in a uniaxial medium, the tunability of a directional coupler based on liquid crystals and the near-field diffraction of a plane wave in a structure containing micrometer scale variations in the transverse refractive index, similar to the pixels of a spatial light modulator.

The strong influence of the complex reverse flow phenomenon on the dynamic temperature behavior of vertically aligned liquid crystal displays (VA-LCDs) has been demonstrated. Good agreement was obtained between theoretical and experimental switching profiles over a wide temperature range (25–75°C). This was achieved using the Leslie–Ericksen theory in a one-dimensional model with material viscosity coefficients obtained from an improved estimation procedure. Such accurate numerical simulations can have a large impact on further improvements of VA-LCDs (e.g., the development of temperature-compensating driving schemes).

Abstract:Finding exact analytical soliton profile solutions is only possible for certain types of non-linear media. In most cases one must resort to numerical techniques to find the soliton profile. In this work we present numerical calculations of spatial soliton profiles in nematic liquid crystals. The nonlinearity is governed by the optical-field-induced liquid crystal director reorientation, which is described by a system of coupled nonlinear partial differential equations. The soliton profile is found using an iterative scheme whereby the induced waveguide and mode profiles are calculated alternatively until convergence is achieved. In this way it is also possible to find higher order solitons. The results in this work can be used to accurately design all-optical interconnections with soliton beams.

Reverse flow is undesirable in liquid crystal devices with vertical alignment. The influence of the material properties on the onset of backflow is investigated for commercially available negative dielectric liquid crystals. It is shown that the threshold voltage V BF for the occurrence of backflow is an important material characteristic. This threshold is relevant for applications and a large value is desired in devices to avoid backflow while keeping a wide applicable voltage range. Accurate finite element simulation of the liquid crystal hydrodynamics allows extraction of V BF and the unknown Miesowicz coefficients ij . The resulting values are tabulated at 20.0°C. © 2009 American Institute of Physics. ͓doi:10.1063/1.3242018͔The occurrence of reverse nematic flow 1 in liquid crystal devices with homeotropic alignment 2,3 has important consequences for practical applications. For instance, vertically aligned liquid crystal displays ͑VA-LCDs͒ have become very popular for mobile and television applications because of their excellent contrast ratio and wide viewing angle. However, their dynamic behavior 4 is affected by the possible occurrence of reverse flow, also referred to as backflow. When a voltage step with amplitude exceeding a certain threshold V BF is applied, the initial rapid variation causes reverse flow of the negative dielectric liquid crystal which leads to a complex reorientation mechanism ͑including a twist deformation͒ and a bounced transient transmission profile, 5 increasing the switching times. Therefore, the limitations due to backflow must be taken into account as a special case to design improved driving schemes 6 to achieve fast and reliable switching in VA-LCDs. In this paper, the influence of the material properties on the onset of backflow is investigated for a number of commercially available liquid crystal materials with negative dielectric anisotropy. The viscous flow properties of liquid crystals are described by the four Miesowicz 7 coefficients ij . The three principal coefficients express the viscosity of the liquid crystal for a liquid crystal director parallel to the direction of flow ͑ 11 ͒, parallel to the direction of the velocity gradient ͑ 22 ͒, and perpendicular to both previous directions ͑ 33 ͒, respectively. The fourth coefficient 12 is related to the shear viscosity. The ij coefficients are, in general, not known because of the notorious difficulty in measuring their values accurately. 8 It is shown that the threshold voltage V BF for the occurrence of backflow is a material characteristic comparable to the Freedericksz threshold voltage 9 for elastic deformation. The backflow threshold is related to the viscous flow properties of liquid crystals and is relevant for practical devices because backflow is undesired and should be avoided. Furthermore, it is possible to estimate the unknown Miesowicz viscosity coefficients ij based on measurement and simulation of V BF . The ij coefficients are extracted using this approach and tabulated together with the V BF values at...

We study theoretically and experimentally the propagation of light beams in chiral nematic liquid crystals. Despite the rather complex refractive index distribution of these crystals, their reorientational nonlinearity can compensate for diffraction, leading to robust solitonlike beams propagating along helical trajectories. We demonstrate that, due to a symmetry-breaking instability of the liquid crystal structure, these beams undergo abrupt switching and bistability, features that are of potential interest for applications to all-optical signal processing.

Reducing the pixel dimensions of liquid crystal microdisplays in search of high resolution has a fundamental impact on their electro-optic behavior. The liquid crystal director orientation becomes distorted due to fringing fields and diffraction effects influence the optical characteristics of the device once the structure features approach the wavelength of the incident light. Three-dimensional finite element simulation of the liquid crystal dynamics with a variable order approach is combined with a full-vector beam propagation analysis to investigate how elasticity and diffraction limit the resolution as a function of the pixel size for transmissive and reflective architectures with vertical liquid crystal alignment. The key liquid crystal properties are considered and the importance of materials with high birefringence is confirmed for small pixel devices as these improve the contrast for a fixed pixel size.