Janusz Parka

Terahertz characterization of tunable metamaterial based on electrically controlled nematic liquid crystal

The terahertz time domain spectroscopy (THz-TDS) system is used to determine the effects of an AC bias voltage on the tunable response of a metamaterial transducer. The tunability of the metamaterial structure, which is based on the rod-split-square resonator, is demonstrated at terahertz frequencies through electrical control of the nematic liquid crystal orientation. Experimental results show that the metamaterial device can be tuned effectively (with transmittance change of up to 19%) by changing the magnitude of the AC bias voltage from 0 to 300 V. This type of tunable metamaterial could find application in the development of devices operating in the THz frequency region for filtering, modulating, and switching of the electromagnetic signals.

Terahertz properties of fluorinated liquid crystals

In this article, optical properties of four fluoro-substituted 4-propyl-4′-[(4-ethylphenyl)ethynyl] biphenyls and liquid crystal mixture A are presented in the terahertz (THz) range. Birefringence, refractive indices and absorption coefficients for ordinary and extraordinary ray of liquid crystals are described in the range of 0.3 to 3.0 THz. It shows that the measured parameters are dependent on the number and placement of fluorine atoms in the molecules. Measurements have been performed using time-pulsed spectroscopy.

Dielectric properties of highly anisotropic nematic liquid crystals for tunable microwave components

In this Letter, we report on measurements of the complex permittivity of highly anisotropic nematic liquid crystals at microwave frequencies as a function of the AC bias voltage. Permittivity measurements have been performed by the split post dielectric resonator technique. The experiments have shown that when the AC bias voltage increases from 0 to 8 V, the real part of the permittivity of these liquid crystals changes by up to 28%. The tunability and the relatively low dielectric losses observed in these liquid crystal mixtures mean that they are ideal materials for the design of tunable microwave components.

Holographic grating recording in large area photoconducting liquid crystal panels

Abstract Large area (50 cm 2 ) liquid crystal panels for optical information processing are evaluated. Optically-controlled diffraction gratings are reported in a dye-doped nematic confined between two transparent electrodes and SiO 2 insulating layers. The electrodes are coated with unidirectionally treated alignment layers in order to form a 90° twisted nematic structure. The parameters of the phase gratings obtained in a two-wave mixing experiment and light diffraction are controlled by applied voltage. The role of insulating layers is studied in respect of device performance and response time.

Spectral Properties of Nematic Liquid Crystal Mixtures Composed with Long and Short Molecules in THz Frequency Range

Properties of liquid crystal materials in THz frequency range can be interesting from application point of view. In this paper the influence of short and long molecules added to liquid crystal 6CHBT were investigated. Change of refractive indices and absorption coefficients for 6CHBT mixtures were compared in the 0.3 - 3 THz frequency range. Influence of temperature on the mentioned parameters of liquid crystal were investigated. Our study show that even a slight change in shape and the composition of the molecules dopants affects the macroscopic properties of liquid crystal. These properties depend on the length of chains, the number of benzene or cyclohexane rings or the spatial distribution of molecules and the interactions between them in the liquid crystal mixture. Spectra measurements on the terahertz time-domain spectrometer were performed.

Numerical analysis of THz metamaterial with high birefringence liquid crystal

We analysed the response of a tunable liquid crystal metamaterial (MTM) transducer in the terahertz frequency range. The MTM structure is based on the rod double-split square resonators (SSR) with back-to-back orientation. A full-wave analysis technique based on the finite-difference time-domain method (FDTD) was performed using QuickWave 3D electromagnetic solver. Terahertz transmission properties of the MTM structure can be controlled by the director of the liquid crystal layer. The main aim of this work is to determine the impact of the size of the SSR of the MTM transducer on scattering parameters of the analysed MTM transducer at terahertz frequency range. Numerical simulations were performed in the range 0.32–0.44 THz.

Experimental study on terahertz metamaterial embedded in nematic liquid crystal

We present an electrically tunable metamaterial device consisting of a periodic array of crossed elements embedded in nematic liquid crystal. Our experimental results show that the transmittance and absorption coefficient of the metamaterial device can be substantially tuned (with an absorption coefficient change of up to 22% for 0.82 THz) by switching the liquid crystal alignment, induced by applying an external voltages applied to the wire electrodes. Structured tunable devices may find applications in modulation and switching elements operating in the visible–infrared–terahertz and microwave regimes.

Tunable negative index metamaterial employing in-plane switching mode at terahertz frequencies

We analysed the response of a tunable liquid crystal metamaterial transducer in the terahertz frequency range. Tunability of scattering parameters is achieving by an in-plane switching (IPS) effect. The metamaterial structure is based on Ω-shape resonators. A full-wave analysis technique based on the finite-difference time-domain (FDTD) method was performed using the QuickWave 3D electromagnetic solver. Terahertz transmission properties of the metamaterial structure can be controlled by the director of the liquid crystal layer. The effective refractive index for operation frequency varies from negative to positive values. A novel approach to switching of metamaterial transducer by IPS mode is presented.

Spectral and photorefractive properties of nematic liquid crystals from the CHBT family in the terahertz range

In this paper, the measurement results of 10 nematic liquid crystals from the homologous series of 4-(trans-4′-n-alkylcyclohexyl) isothiocyanatobenzenes (CHBT) in the range of 0.3–3 THz are presented. Birefringence, refractive indices and absorption coefficients for ordinary and extraordinary rays versus frequency are described. In addition, temperature dependence of these parameters is shown. The purpose of this study was to investigate the effect of alkyl chain length on the properties of liquid crystals in the THz range.

Simulation of a tunable metamaterial with nematic liquid crystal layers

Authors analyze response of tunable liquid crystal metamaterial transducer in microwave frequency range. Tunability of scattering parameters is achieving by reorientation of liquid crystal molecules. Metamaterial (MTM) structure is based on Ω-shape resonators. A full-wave analysis technique based on the finite-difference time-domain method (FDTD) was performed using QuickWave 3D electromagnetic solver. Microwave transmission properties of the metamaterial structure can be controlled by director of liquid crystal layer. Effective refractive index for operation frequency varies from negative to positive values.