Introduction of liquid crystal device into THz phase imaging

Abstract

We investigate the terahertz (THz) phase imaging using liquid crystal (LC) device. Recently, there have been extensive efforts to measure the refractive indices and transmission losses of some LC materials in THz region. From these results, LC materials show relatively large refractive index anisotropy and they can have a potential application to some control devices similar to the display application. Furthermore, tunable LC THz phase shifter has been reported. In this study, an attempt was made to introduce the LC device into the THz phase imaging which was based on four-step phase-shifting algorithm. Four-step phase-shifting algorithm is one of the most effective methods of phase imaging and moving mirror is needed to introduce phase shift in standard measurements. In addition, we should prepare the two rays with common source, therefor experimental setup becomes complicated. While on the other hand, it is just needed to insert a LC phase shifter into the light path when we adopt LC device. Furthermore, low-voltage application is enough to introduce the phase-shifting, hence it is not necessary to prepare the moving mirror. In this work, we fabricate the electrically tunable LC phase shifter which has sandwich cell structure. The fundamental phase-shifting properties were measured by using an optically pumped gas laser system which can generate continuous wave (CW) THz waves. In addition, we investigated the phase shifting interferometry which was based on four-step phase-shifting algorithm by using LC phase shifter. We also estimate the birefringence of X-cut crystalline quartz at 2.5 THz.

Electrically Tunable Hydrogen-Bonded Liquid Crystal Phase Control Device

“…We previously reported that the birefringence of E44 is 0.2 [33]. The thickness of the LC layer was 800 μm in this case, and the retardation R of the E44 phase shifter was 160 μm.…”

“…Here we also show the reported values of birefringence, absorption coefficients, and dielectric anisotropy for several LCs from References [8,12,22,32,33]. The birefringence of LC1 at 2.5 THz was We successfully observed the transmittance oscillation by increasing the applied voltage.…”

“…This is due to the difference in the absorption coefficient between the extraordinary and ordinary direction. In the case of E44, the extraordinary absorption coefficient was smaller than that of the ordinary directions [33]. The electric field in the x direction felt the extraordinary absorption, and the electric field in the y direction felt the ordinary direction of absorption at 0 V. However, upon increasing the voltage, the LC molecules rose up, and the electric fields in both the x and y directions gradually felt the ordinary direction of absorption.…”

“…In other words, the absorption in the ordinary direction α o was not equal to the extraordinary absorption α e . Previously, we investigated phase-shifting interferometry by using an LC phase shifter and reported that the dichroism of the LC caused unwanted variation in the intensity of the terahertz phase shifter [33]. Therefore, LC materials with no dichroism are significant not only in phase-shifting interferometry, but also in future terahertz applications.…”

Twisted Nematic Liquid-Crystal-Based Terahertz Phase Shifter using Pristine PEDOT: PSS Transparent Conducting Electrodes

“…In contrast, PEDOT: PSS films can be easily fabricated by spin coating [12]. In a previous work, Ito et al [13] investigated THz phase imaging using an 800 µm thick homogeneous LC device using pristine PEDOT: PSS electrode layers and achieved a 360 • phase shift at 2.5 THz. More than 75 V of bias, however, was needed to achieve a 90 • phase shift.…”