Optical control of polarized terahertz waves using dye-doped nematic liquid crystals

Tomoyuki Sasaki,Kohei Noda,Hiroshi Ono,Hiroki Okuyama,Moritsugu Sakamoto,Nobuhiro Kawatsuki

doi:10.1063/1.5041294

Tomoyuki Sasaki, Kohei Noda + Show 4 more

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https://doi.org/10.1063/1.5041294

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Abstract

We propose an optically tunable terahertz (THz) polarization converter based on use of a dye-doped nematic liquid crystal (LC). The dye-doped LC exhibits a photothermally-induced nematic-isotropic phase transition under irradiation by a laser beam with a wavelength that lies within the absorption band of the dye. We investigate the transmission spectra and the polarization-conversion properties of the dye-doped LC cell with twisted alignment using THz time-domain spectroscopy. The polarization state of a THz wave transmitted through the proposed dye-doped LC cell can be controlled using the laser beam irradiation based on the photothermal effect. The experimental results also demonstrate that optical control of this dye-doped LC cell offers an advantage in terms of response speed when compared with voltage-based control of LC cells via their electrodes. The dye-doped LC proposed in this study presents the possibility that a variety of LC-based THz elements with optical tunability may be realized.

Highlights

INTRODUCTIONA variety of useful applications of electromagnetic waves within the terahertz (THz) spectral range have been proposed for a wide range of fields, including wireless communications, biological imaging, nondestructive inspection, and physical chemistry.[1,2] To realize these applications, numerous THz elements including polarization converters, frequency filters, attenuators, demultiplexers, phase shifters, and modulators are in high demand, in addition to THz light sources and THz detectors.[3,4] Liquid crystals (LCs) are promising materials for fabrication of functional THz elements because they are capable of active control and manipulation of THz waves.[5,6,7] liquid crystal (LC) are transparent anisotropic media in the THz spectral range and the LC molecular alignment responds to a variety of external fields
We examined the transmission spectra of a dye-doped nematic liquid crystal (LC) with homogeneous alignment using THz time-domain spectroscopy
The results demonstrated that the complex refractive indices of the ordinary and extraordinary waves in the THz spectral range are not affected by the dye-doping process

Summary

INTRODUCTION

A variety of useful applications of electromagnetic waves within the terahertz (THz) spectral range have been proposed for a wide range of fields, including wireless communications, biological imaging, nondestructive inspection, and physical chemistry.[1,2] To realize these applications, numerous THz elements including polarization converters, frequency filters, attenuators, demultiplexers, phase shifters, and modulators are in high demand, in addition to THz light sources and THz detectors.[3,4] Liquid crystals (LCs) are promising materials for fabrication of functional THz elements because they are capable of active control and manipulation of THz waves.[5,6,7] LCs are transparent anisotropic media in the THz spectral range and the LC molecular alignment responds to a variety of external fields. Resonant response of the order of 7–8% in terms of bandwidth and a change in the signal absorption of two orders of magnitude by infiltrating tiny quantities of an LC into the meta-atoms These reports demonstrate the potential of LCs and LC-based metastructures for realization of active THz elements. Based on the results of these studies, we discuss the optical tunability of a THz polarization converter constructed using this dye-doped LC

EXPERIMENTAL

RESULTS AND DISCUSSION

CONCLUSIONS