Index Of Liquid Crystal Research Articles

Tuning characteristics of fluorescent light source by dye-doped liquid crystal filled hollow fiber

The fluorescent fiber light source has been widely used in many areas, such as optical fiber communication and medical imaging, owing to its low cost and wide optical spectrum. The temperature-sensitive refractive index of liquid crystal makes it a suitable filling material used in the fluorescent light source. The existing work has investigated the filling of liquid crystal into the air holes in cladding of photonic crystal fiber. However, the photonic crystal fiber has the disadvantages of complicated craft and high cost. As is well known, the hollow fiber has the advantages of the easy preparation and low cost, but the filling of liquid crystal into the hollow fiber of fluorescent light source is rarely investigated. In this paper, we investigate that a tunable hollow fiber of fluorescent light source is filled with dye doped liquid crystals. The transmission characteristics of the fluorescent light source are theoretically analyzed. The variation in property of the B4400 fluorescent spectrum is numerically discussed with the dye molecule energy level structure theory. The numerical simulation results show that the relative refractive index is dependent on temperature. It first increases linearly with the increase of temperature and then exponentially increases rapidly till clearing point 61.9 C, finally decreases slowly to a saturated value. In order to find an optimum doping concentration, the doping-concentration-dependent fluorescence output intensity is analyzed by using the super continuum spectrum of YAG pump with a wavelength of 1064 nm. The fluorescence light intensities are amplified at three different selective dye doping concentrations, namely 0.2 wt%, 1 wt% and 2 wt% in the experiment, respectively. The highest output is obtained at the 1 wt% doping concentration, which verifies the selective fluorescence amplification property of the fluorescent source. It is also demonstrated that the transmission characteristics and the tunable range of the liquid crystal filled fluorescent light source can be adjusted by modulating the temperature in experiment. And the temperature-dependence of the fluorescence source is experimentally demonstrated by using the 1 wt% doping concentration dye-doped liquid crystal. Using a pulsed YAG pump with a wavelength of 532 nm, tunable characteristics of the fluorescent light source composed of a dye-doped liquid crystal filled hollow fiber, are studied and show that the central wavelength increases from 590 nm to 605 nm and the spectral width broadens from 228 nm to 236 nm with the increase of the temperature. The proposed fluorescent light source can be controlled by adjusting the temperature within limits. These findings will give a guidance for the practical applications of the dye doped liquid crystal based fluorescent light source, and offer a theoretical foundation for the further study of the liquid crystal filled fluorescent fiber light source.

Refractive index and absorption coefficient of blue phase liquid crystal in terahertz band

ABSTRACTThe refractive index and absorption coefficient of blue phase liquid crystals are investigated by terahertz time-domain spectroscopy system. Four phases, including isotropic phase, BPII, BPI, and cholesteric phase are investigated. The refractive index of liquid crystal sample is dependent on the temperature, which is related to the different phases. In our experiment, in the frequency range of 0.2–1.2 THz, the refractive index of BPII and BPI is about 1.620 and 1.625, respectively. For BPII and BPI, the measured absorption coefficients increase with the increase of frequency, with absorption peak appearing at 1.11 and 1.05 THz, respectively. These properties of BPLC provide useful information for BPLC-based terahertz components such as phase shifter, wave-plates and so on.

The Study of Refractive Indices of Liquid Crystal Mixtures

In the present study the ordinary refractive indices of Nematic liquid crystal (LC), Cholesteric liquid crystal and their mixtures were measured by multiple wavelength refractometer in the visible range with temperature range from 20°C to 80°C. As the wavelength increases, the refractive indices may decrease. As the temperature increases, the refractive index may decrease. The ordinary refractive index decreases or increases depending on the crossover temperature of the Liquid Crystal employed. The wavelength- and the temperature-dependent refractive indices are fundamentally interesting and practically important for optimizing the display performances and other photonic devices employing LCs.We also tried to investigate factors affecting the temperature gradient of ordinary refractive index (dno/dT).Liquid crystal should exhibit a large temperature gradient refractive index to reduce the required laser intensity for triggering the nonlinear optical effects. The two crucial parameters for achieving large dno/dT are high birefringence and low clearing point. However, these two requirements are often conflicting each other. The temperature at which dno/dT = 0 is called the cross-over temperature To. If Tc, the clearing temperature of liquid crystal material is much higher than the room temperature then the cross-over temperature would be relatively high and dno / dT < 0 at room temperature. From all the investigations it is observed that the values of refractive index change with temperature, wavelengths as well as with the mixture. Some of the liquid crystal mixtures show excellent index matching property.

English

We report the determination of refractive indices of polymer dispersed liquid crystal composite within visible spectral range 404-706nm for various temperatures. The droplet domain formation effect in polymer liquid crystal composite system with variation of different polymer dispersed in liquid crystal and pure liquid crystal has been studied. The optical parameters due to scattering by sapphire prism have been measured for pure liquid crystal and polymer dispersed liquid crystal. It is found that the optical anisotropy of polymer dispersed liquid crystal composite system is greatly changed by polar order present in the polymer droplet environment. The scattering is due to both reorientation and thermal effects. The wavelength and temperature dependent refractive indices are fundamentally interesting and practically important for optimizing display performances and other photonic devices employing LCs such as high temperature gradient LCs, high thermal tunable LCs, and photonic liquid crystal fibers

Reflective terahertz tunable polarization controller

This paper proposes an optical device which can continuously change the polarization state of terahertz (THz) waves. The device consists of metal gate, anti-reflection coatings, liquid crystal and mirror. By changing the refractive index of liquid crystal in the interface between the metal gate and the mirror, the phase difference between two beams with orthogonal polarization is varied and a continuous phase shift is achieved. The phase shift of the device is calculated by using the finite difference time domain (FDTD) method, and the transmittance and reflectance are calculated by using the rigorous coupled wave analysis (RCWA) method. The results reveal that the structure can realize continuously tunable phase shift for THz wave at 1 THz.

Basic Performance of Refractive Index Measurement Method for LC Materials in Super High Frequency Region by Using Coplanar Wave Guide

Novel measurement method of complex refractive indices of liquid crystal (LC) materials in the ultrahigh frequency region such as microwave and millimeter-wave (MMW) is investigated to reduce the quantity of test sample by using a coplanar waveguide (CPW) substrate, which is known as an excellent planar type waveguide. Although it needs some calibration method for precise measurement, the amplitude and phase properties of the propagating electromagnetic waves are sufficiently sensitive to the extremely small amount of test sample and the basic performance for the loss and refractive index measurements is investigated by using the well known K15 LC material.

Interference Method for Determining Dispersion of Refractive Indices of Liquid Crystals

An efficient, quick and pretty accurate interference method for determining dispersions of ordinary n o and extraordinary n e refractive indices and optical anisotropy of nematic liquid crystals, i.e., ne(λ), no(λ) and Δn(λ) ≡ n e (λ)–no(λ), as functions of wavelength λ (in visible and near infrared region), is presented. The method relies on measurements of positions of interference fringes in spectrogram produced by a flat-parallel nematic liquid crystal cell in only one run.

Infrared sensitive liquid crystal photorefractive hybrid cell with semiconductor substrates

Refractive index grating recording is achieved in the infrared in a nematic liquid crystal placed between semiconductor CdTe substrates. Light-induced space-charge field created in photorefractive semiconductor substrates varies an alignment of liquid crystal molecules inhomogeneously in space forming a spatial modulation of the refractive index in liquid crystal. Two-beam coupling is studied in the hybrid cell, in which a gain factor Γ=16 cm−1 is achieved in the layer of liquid crystal of the sandwich.

Tunable T-Shaped Waveguide in Two Dimensional Photonic Crystals Based on Liquid Crystals

In the last years, in order to achieve active tuning of photonic crystals devices, the possibility to use liquid crystal inside photonic crystals has been explored. On this line of argument, in this paper, we numerically investigate a tunable T-shaped waveguide diplexer, based on a two-dimensional square lattice photonic crystal composed of silicon rods in a liquid crystals. We prove that complete splitting of the entire input wavelengths range in two sub-ranges symmetrical with respect to the middle (switching) wavelength, and propagating in right and left arms respectively, can be achieved. Moreover, changing the refractive index of liquid crystals by electro-optical effect, a tuning of switching wavelength of about 60 nm can be obtained.

Dispersion of the ordinary refractive indices of smectic liquid crystals in free standing films

The ordinary refractive indices of some smectic-A liquid crystals have been measured by means of interference spectra of free standing films as a function of wavelength and temperature. Numerical data are presented for further use. In the Sm-C phase one of the principal indices can be measured. If the three principal indices in the Sm-C phase would be known the tilt angle could be measured by means of the interference technique with high precision. Predictions of the refractive index of liquid crystals based on quantum chemical calculations of polarizabilities are presented. Using an expression derived by de Jeu and Bordewijk [J. Chem. Phys. 68, 109 (1978)] values for the principal refractive indices have been calculated that can be compared with our experimental results.

Active Photonic Crystals Based multiplexer

ABSTRACTIn the last years, in order to achieve active tuning of photonic crystals devices, the possibility to use liquid crystal inside photonic crystals has been explored.On this line of argument, in this paper, we numerically investigate a tunable T-shaped waveguide diplexer, based on a two-dimensional square lattice photonic crystal composed of silicon rods in a liquid crystals. We prove that complete splitting of the entire input wavelengths range in two sub-ranges symmetrical with respect to the middle (switching) wavelength, and propagating in right and left arms respectively, can be achieved. Moreover, changing the refractive index of liquid crystals by electro-optical effect, a tuning of switching wavelength of about 60 nm can be obtained.

P‐96: Experimental Measurement of Cauchy Values for Δn of Nematic Liquid Crystals

AbstractRefractive indices of liquid crystal materials change dramatically with the wavelength of light due to dispersion. The Cauchy formula provides an empirical way of describing the relationship of various refractive indices of liquid crystal with wavelength of light. It is often difficult to measure individual refractive indices (ne and no) at several wavelengths and hence the birefringence Δn. In most liquid crystal device simulations, it is important to know Δn as a function of wavelength for accurate performance prediction. Here we propose a simple method of measuring Δn of liquid crystalline materials at various wavelengths and obtaining the Cauchy values for the LC material.

Extension of Phase Modulation Ellipsometry to Measure Refractive Indices of Liquid Crystals

We present an extension of phase modulation ellipsometry to measure refractive indices in the visible and near infrared spectrum of multilayer and anisotropic structures, in particular liquid crystal cells, with arbitrary orientation of optical axis. An original software code was derived to process ellipsometer measurements, based on Jones formalism to express sinusoidal components of detected optical intensity reflected by a sample for different angles of the polariser and analyser respect to the sample orientation. In this way it is possible to obtain independent equations whose unknowns are the reflection matrix coefficients. Since it is possible only to obtain values of detected intensity and reflection coefficients given the refractive indices of the sample by using Berreman formalism, an algorithm has been developed to search best refractive index values which fit intensity measurements at each wavelength. Dispersion curves of refractive indices of 5CB have been obtained with accuracy of about 1%.

Copolymer composition-dependent light transmission of polymer/liquid crystals composite films

Electrooptic responses (voltage and angular-dependent transmittance) of polymer/liquid crystal composite films with H, V, and unpolarized lights have been studied based on a nematic liquid crystal (Ro-5921) and four types of homopolymers and copolymers from ethyl methacrylate and styrene with different compositions. In this way, the index ratio of the polymer (np) to the ordinary refractive index of liquid crystal (no)(np/no) has been varied systematically, and the effect of the index ratio on viewing angle, applied voltage, response times, and transient response have been investigated. With increasing styrene content in the copolymer, droplet size increased, threshold (Vth) and saturation (Vsat) voltage, and rise time decreased. With np ' no, maximum transmittance occurred at normal incidence, regardless of the type of polar- ization. On the contrary with npu no, V-polarization gave a peak in the transmittance- voltage curve, and transmittance overshot upon removal of the field, and these were interpreted in terms of effective refractive index and two-step relaxations. q 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 55-64, 1998

Birefringence Measurement of Liquid Crystals

The cylindrical cell method for determining the refractive indices of liquid crystals is described. This method is based on the use of a cylindrical cell composed of a pair of semi-cylindrical prisms and the measurement of the incident angle dependence of transmitted light intensities. It is applied to the measurement of refractive indices of nematic and cholesteric liquid crystals.