Generating, Separating and Polarizing Terahertz Vortex Beams via Liquid Crystals with Gradient-Rotation Directors

Abstract

Liquid crystal (LC) is a promising candidate for terahertz (THz) devices. Recently, LC has been introduced to generate THz vortex beams. However, the efficiency is intensely dependent on the incident wavelength, and the transformed THz vortex beam is usually mixed with the residual component. Thus, a separating process is indispensable. Here, we introduce a gradient blazed phase, and propose a THz LC forked polarization grating that can simultaneously generate and separate pure THz vortices with opposite circular polarization. The specific LC gradient-rotation directors are implemented by a photoalignment technique. The generated THz vortex beams are characterized with a THz imaging system, verifying features of polarization controllability. This work may pave a practical road towards generating, separating and polarizing THz vortex beams, and may prompt applications in THz communications, sensing and imaging.

A Terahertz Vortex Beam Emitter With Tunable Topological Charge and Harmonic Excitation

“…Metasurfaces composed of the V-shaped unit cells or complementary split rings are designed to produce the THz vortices via precisely controlling the geometric parameters of the elements [17]. Furthermore, silicon binary diffractive optical elements [18], liquid crystals [19], and ZnTe cubic crystal [20] have also been proposed to generate the THz vortices.…”

Generation of high-quality terahertz OAM mode based on soft-aperture difference frequency generation

“…However, their conversion efficiencies from an incident Gaussian beam to an optical vortex mode are typically limited to a few percent owing to their narrow spectral bandwidth and low transmittance. Also, liquid crystal q-plates enable us to generate broadband THz vortices [52,53]. However, they exhibit relatively low damage threshold, and severe transmission loss.…”

Narrowband Terahertz Emission With Tunable Orbital Angular Momentum by Vortex Laser-Beam Interaction

“…The THz vortex can be directly structured from THz Gaussian beams by phase modulation, including spiral phase plates [14], computer-generated holograms [15], metasurfaces [16], liquid crystals [17], etc. Especially, the derivatives of liquid crystal can bring some versatile functions [18], [19]. However, these methods are limited by mode conversion efficiency due to low transmittance.…”