Ye Yin

Electric heating effects in nematic liquid crystals

Electric heating effects in the nematic liquid crystal change the liquid crystal physical properties and dynamics. We propose a model to quantitatively describe the heating effects caused by dielectric dispersion and ionic conductivity in the nematic liquid crystals upon the application of an ac electric field. The temperature increase of the liquid crystal cell is related to the properties of the liquid crystal such as the imaginary part of the dielectric permittivity, thermal properties of the bounding plates, and the surrounding medium as well as frequency and amplitude of the electric field. To study the temperature dynamics experimentally, we use a small thermocouple inserted directly into the nematic bulk; we assure that the thermocouple does not alter the thermal behavior of the system by comparing the results to those obtained by a noncontact birefringent probing technique recently proposed by Wen and Wu [Appl. Phys. Lett. 86, 231104 (2005)]. We determine how the temperature dynamics and the stati...

FAST SWITCHING OPTICAL MODULATOR BASED ON DUAL FREQUENCY NEMATIC CELL

We demonstrate a fast optical modulator capable of switching large amount of optical retardation(a few microns)in less than 1 ms. The result is achieved by employing a dual frequency nematic in cells with high pre-tilt alignment and by providing a special addressing scheme that features amplitude and frequency modulated voltage. We explore the effect of surface alignment and dielectric heating on the switching time. We also report the measurements of dielectric permittivities and crossover frequency of dual frequency nematic.

Fast Switching of Nematic Liquid Crystals by an Electric Field: Effects of Dielectric Relaxation on the Director and Thermal Dynamics

Liquid crystals (LCs) represent a special state of soft matter in which the anisotropic molecules or their aggregates demonstrate a long-range orientational order but little or no positional long-range order [1]. In the simplest case of a uniaxial nematic LC (NLC), the long-range positional order is absent altogether; the molecules form a uniaxial centrosymmetric medium, as they are predominantly aligned along a single direction called a director and described by a unit vector n (such that n≡−n . The director can be realigned by electric and magnetic fields that thus change the optical properties of sample. The orientational dynamics of director in the applied electric field is a fundamental physical phenomenon that is at the heart of modern display technologies. Discovered in 1930s by Frederiks (initially for the magnetic case) [2], this phenomenon, rather surprisingly, still poses a fundamental unanswered question regarding the behavior of NLCs with the finite rate of dielectric relaxation. The problem can be understood as follows. The dielectric torque that reorients the director has the densityM t =D t ×E t , where E t is the electric field and D t is the electric displacement at the moment of time t. In the widely accepted standard approach [1, 3, 4], the director response is assumed to be instantaneous, i.e., the displacement D t is determined by the electric field at the very same moment D t = 0 E t , where 0 is the free space 277

Fast-switching dual-frequency liquid crystal optical retarder for beam steering applications

We demonstrate a fast optical modulator capable of switching large amount of optical retardation (a few microns) in less than 1 ms. The result is achieved by employing a dual frequency nematic in cells with high pre-tile alignment and by providing a special addressing scheme that features amplitude and frequency modulated voltage. We explore the effect of time delay and dielectric heating in process of the optical retardation switching.

Effects of Dielectric Relaxation on the Dynamics and Dielectric Heating of Nematic Liquid Crystals

We present recent experimental and theoretical results that link the phenomenon of dielectric relaxation in nematic liquid crystals to their fast electro-optical switching and to the problem of dielectric heating. We develop the theoretical model that allows one to relate director reorientation to the “history” of the electric field. We present an experiment that is well described by the proposed theory but which cannot be described within the classic model of an instantaneous response. We also analyze the effect of dielectric heating that is especially profound at the frequencies corresponding to dielectric relaxation.