Refractive Index Of Liquid Crystal Research Articles

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.

Photonic-crystal-waveguide based Mach-Zehnder interferometer for terahertz switch and modulator

We propose a novel tunable photonic crystal (PC) waveguide Mach-Zehnder interferometer (MZI) based on nematic liquid crystals (LCs) 5CB and investigate its interference properties numerically by using the finite-difference time-domain method. We can change the refractive index of LC by rotating the directors of the LC molecules. The line defect modes of the PC waveguide with different liquid crystal refractive indices are analyzed by using the plane wave expansion method. Owing to the slow group velocity region of the line defect mode, when the index of 5CB changes from 1.53 to 1.63, the variation of the effective index of the line defect mode arrives at 0.168. This property helps to significantly control the phase of light propagation in a PC waveguide MZI. The novel interferometer can be used as either an optically controlled on-off switch or an amplitude modulator in optical circuits.

Tunable lightwave propagation in two-dimensional hole-type photonic crystals infiltrated with nematic liquid crystal

Photonic crystals have many potential applications because of their ability to control lightwave propagation. We have investigated the tunability of lightwave propagation in two-dimensional hole-type photonic crystal structures. The linear waveguides can be obtained by the infiltration of liquid crystals into air holes in hole-type photonic crystal with square lattices. The refractive indices of liquid crystals can be changed by rotating the directors of liquid crystals. Therefore, we can control the lightwave propagation in two-dimensional hole-type photonic crystal structures. Such a mechanism of lightwave adjustment should open up a new application for designing components in photonic integrated circuits.

Integrated Bragg reflectors in low-index media: enabling strategies for wavelength tunability in electro-optic liquid crystals

This paper reports two configurations of Bragg reflectors based on liquid crystals confined between two small glass plates. Both approaches employ the efficient electro-optic effect in liquid crystals, which allows tunability of the reflectors by using low voltages. The molecular reorientation induced by an applied electric field implies a refractive index modulation seen by polarized light propagating into the liquid crystal. We show design criteria and profile optimization of the electrodes to induce a liquid crystal refractive index periodic modulation, providing a wavelength selective propagation of confined light in the liquid crystal. The two proposed device configurations differ for the top-bottom electrode configuration in one case and coplanar electrodes in the other case. Modeling of both configurations has been carried by calculating the applied electric field distribution and its interaction with the liquid crystal elastic properties taking into account the boundary conditions due to the alignment layer on the inner faces of the glass substrates. The calculated performance in terms of high wavelength selectivity and ultrawide spectral tuning range indicate that the two designed structures can be proposed for both optical filtering and to produce novel low power integrated distributed feedback resonators in dense wavelength division multiplexed fiber optic systems.

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.

Transient Modulation of Order Parameter and Optical Non-Linearity in a Chiral Nematic Liquid Crystal

Light-stimulated transformation of a molecule in a liquid crystal (LC) greatly affects the ordering of its nearest neighbors. At early stage of excitation this local order parameter modification is characterized with extremely short times, while a minor concentration of photo-modified molecules could contribute a lot into modulation of refractive indices of LC. Dispersion of optical activity at the edge of the selective reflection band of a chiral LC enhances significantly the nonlinear optical response. Combination of these effects has a strong potential in different aspects of optical signal processing, all-optical switching and photonics.

Remote and direct plasma regions for low-temperature growth of carbon nanotubes on glass substrates for display applications

A novel method for growing carbon nanotubes (CNTs) on glass substrates is introduced in this study. A two-stage plasma was used to achieve low-temperature and vertically aligned CNTs. Ni deposited on indium tin oxide/glass substrate was used as the catalyst and hydrogen and acetylene were used as gas feeds. In this investigation a new technique was developed to grow vertically aligned CNTs at temperatures below 400 °C while CNT growth by plasma-enhanced chemical vapour deposition required high temperatures. Low-temperature growth of vertically aligned CNTs was suitable for the fabrication of micro-lens and self-oriented displays on glass substrates. Also, we have reported a new configuration for CNT-based display by means of controlling the refractive index of liquid crystal around the CNT by applying a proper voltage to the top and bottom array.

Reverse mode operation polymer dispersed liquid crystal with a positive dielectric anisotropy liquid crystal

AbstractThe development of electrically activated chromogenic materials is important for their potential applications in smart windows. Several previous works have reported on reverse mode operation polymer dispersed liquid crystals (PDLCs) based on negative dielectric anisotropy liquid crystals. They have a transparent OFF state, which turns opaque after the application of a suitable external electric field. Nevertheless, these devices have some limitations such as the use of large amount of expensive liquid crystals with peculiar physical‐chemical properties. In addition, a good matching between the refractive index of liquid crystal and the polymer matrix one is required. The main result of this work is the achievement of reverse mode operation devices prepared with a positive dielectric anisotropy liquid crystal and characterized by a high OFF state transmittance obtained by the onset of high intensity built‐in DC electric fields in a direct mode operation PDLC, which allows the OFF state homeotropic alignment of liquid crystal directors. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011

Electrical Control of LSPR from Gold Nanoparticles Using Electrochemical Oxidation

AbstractLarge shift of localized surface plasmon resonance (LSPR) spectrum of gold nanoparticles was attained by electrochemical oxidation of the nanoparticle surface. This oxidation occurred in a cell consisting of a pair of indium tin oxide (ITO) electrodes with water medium between the electrodes. On one side of the ITO electrode, the gold nanoparticles were adsorbed. The LSPR spectrum was moved consecutively to the red by increasing the applied positive voltage. By the application of 5 V to the cell, the spectrum shift as large as 55 nm was obtained. Though the spectrum shift has already been observed by changing liquid crystal (LC) orientation surrounding gold nanoparticles, the amount of the shift was not large (11 nm). That was because the variation of the effective refractive index of LC was rather small. Our large shift due to electrochemical oxidation resulted from the large refractive index of Au-O. The upper limit of the LSPR spectrum shift by our method is estimated to be 138 nm.

Calculation of Optical Parameters of Liquid Crystals

efiect on liquid crystal refractive indices is being reported in the present article. This model is based upon the Vuks equation. Experimental data of ordinary and extraordinary refractive indices for two liquid crystal samples MLC-9200-000 and MLC-6608 are used to validate the above-mentioned theoretical model. Using these experimental data, birefringence, order parameter, normalized polarizabilities, and the temperature gradient of refractive indices are determined. Two methods: directly using birefringence measurements and using Haller’s extrapolation procedure are adopted for the determination of order parameter. Both approches of order parameter calculation are compared. The temperature dependences of all these parameters are discussed. A close agreement between theory and experiment is obtained.

Voltage-tuned resonant reflectance optical filter for visible wavelengths fabricated by nanoreplica molding

A guided-mode resonant filter incorporating an electro-optically tunable liquid crystal refractive index is demonstrated at a wavelength of 655nm and a tuning range of 4nm. Rigorous coupled wave analysis and finite difference time domain analysis are used to simulate the characteristics of the filter during liquid crystal reorientation. Tuning performance is demonstrated that is consistent with the device simulations. Tunable filters in the visible wavelength range that are inexpensively fabricated over large surface areas are expected to find applications in optical limiting and video display.

Polarization rotator using a hybrid aligned nematic liquid crystal cell

Using both direct mathematical analysis and numerical modeling based on the predictions by Jones [1] it is shown that if the director in a liquid crystal cell is in a plane which lies at 45 degrees to the incident polarization, then, for normally incident light, the transmission signal which conserves polarization will always have a phase difference of pi/2 from the transmission signal of the orthogonal polarization. This is independent of the director profile in the plane, the cell thickness, the anisotropy of the liquid crystal refractive index and the optical parameters of other isotropic layers in the cell. Based on this realization a hybrid aligned nematic liquid crystal cell has been tested as a thresholdless voltage-controlled polarization rotator. By using a quarter-wave plate to compensate for the phase difference between the two orthogonal output polarizations a simple liquid crystal spatial light modulator has been realized.

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.

Investigation of a tunable T-shaped waveguide based on a silicon 2D photonic crystal

We investigate the tunability of a T-shaped waveguiding diplexer, based on a two-dimensional square lattice photonic crystal (PhC) composed of silicon rods in air and in liquid crystals (LCs), respectively. For both devices we prove that a complete splitting of the entire input wavelength range into right and left arms respectively can be achieved. Moreover, a tuning of the switching wavelength can be obtained by changing the refractive index of silicon by the thermo-optic effect for PhCs in air, and the refractive index of liquid crystals by the electro-optic effect for PhCs in LCs.

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.

Tunable Channel Drop Filter in a Two‐Dimensional Photonic Crystal Modulated by a Nematic Liquid Crystal

Photonic crystals (PCs) have many potential applications because of their ability to control light‐wave propagation and because PC‐based waveguides may be integrated into optical circuits. We propose a novel tunable PC channel drop filter based on nematic liquid crystals and investigate its properties numerically by using the finite‐difference time‐domain (FDTD) method. The refractive indices of liquid crystals can be actively modulated after infiltrating nematic liquid crystals into the microcavity in PC waveguides with square lattices. Then we can control light propagation in a PC waveguide. We analyze the Q‐factors and resonance frequencies of a tunable PC channel drop filter by considering various indices modulation of liquid crystals. The novel component can be used as wavelength division multiplexing in photonic integrated circuits.

Polarization-dependent optical properties of planar photonic crystals infiltrated with liquid crystals

Photonic crystals with infiltrated liquid crystals are investigated in the 1.5μm wavelength regime. Due to the strong temperature dependence of the refractive index of liquid crystals, especially near the clearing point, i.e., at the transition between nematic and isotropic phase, the optical properties of photonic crystals can be tuned by small temperature changes. The refractive index of liquid crystals is birefringent and, therefore, sensitive for the polarization of the incident light. By using microresonators embedded into planar photonic crystal waveguides, the polarization dependence of the optical properties with and without infiltrated liquid crystal is investigated. The experimental results quantitatively agree well with the calculated photonic band structure assuming a temperature depending reorientation of the liquid crystal director field inside the holes.

Refractive Indices of Liquid Crystals for Display Applications

This paper reviews the extended Cauchy model and the four-parameter model for describing the wavelength and temperature effects of liquid crystal (LC) refractive indices. The refractive indices of nine commercial LCs, MLC-9200-000, MLC-9200-100, MLC-6608, MLC-6241-000, 5PCH, 5CB, TL-216, E7, and E44 are measured by the Multi-wavelength Abbe Refractometer. These experimental data are used to validate the theoretical models. Excellent agreement between experiment and theory is obtained.