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

Shi-Jun Ge, Zhi-Xiong Shen, Peng Chen, Xiao Liang,  Xin-Ke Wang,  Wei Hu, OrcID, Yan Zhang, Yan-Qing Lu


http://www.mdpi.com/2073-4352/7/10/314

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.

Abstract-Broadband tunable liquid crystal terahertz waveplates driven with porous graphene electrodes

Lei Wang^1,*, Xiao-Wen Lin^1,*, Wei Hu¹, Guang-Hao Shao¹, Peng Chen¹, Lan-Ju Liang², Biao-Bing Jin², Pei-Heng Wu², Hao Qian³, Yi-Nong Lu³, Xiao Liang⁴, Zhi-Gang
 Zheng¹ and Yan-Qing Lu¹

http://www.nature.com/lsa/journal/v4/n2/full/lsa201526a.html

Versatile devices, especially tunable ones, for terahertz imaging, sensing and high-speed communication, are in high demand. Liquid crystal based components are perfect candidates in the optical range; however, they encounter significant challenges in the terahertz band, particularly the lack of highly transparent electrodes and the drawbacks induced by a thick cell. Here, a strategy to overcome all these challenges is proposed: Few-layer porous graphene is employed as an electrode with a transmittance of more than 98%. A subwavelength metal wire grid is utilized as an integrated high-efficiency electrode and polarizer. The homogeneous alignment of a high-birefringence liquid crystal is implemented on both frail electrodes via a non-contact photo-alignment technique. A tunable terahertz waveplate is thus obtained. Its polarization evolution is directly demonstrated. Furthermore, quarter-wave plates that are electrically controllable over the entire testing range are achieved by stacking two cells. The proposed solution may pave a simple and bright road toward the development of various liquid crystal terahertz apparatuses.