Abstract-Highly Efficient Ultra‐Broadband Terahertz Modulation Using Bidirectional Switching of Liquid Crystals

Xuequan Chen, Kaidi Li,  Rui Zhang, Swadesh Kumar Gupta, Abhishek Kumar Srivastava, Emma Pickwell‐MacPherson,

https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.201901321

Accurately manipulating field strength and polarization state are essential in various terahertz applications. Such manipulations are based on the efficient modulation of the amplitude and phase of electromagnetic waves. However, there is a lack of such terahertz modulators with sufficient efficiency and bandwidth. Herein, the Brewster–critical angle is exploited for modulation by using a nematic liquid crystal. Unlike liquid crystal phase shifters that only give a narrowband phase delay via a one‐directional switch, the presented device modulates both the amplitude and phase across an ultra‐broadbandwidth via a bidirectional active switch. An average intensity modulation depth over 99.6% is achieved for 0.2–1.6 THz. Furthermore, highly accurate polarization conversion between linear and circular states is also realized for 0.4–1.8 THz, with the average degree of linear and circular polarizations as high as 0.994 and 0.998, respectively. The superior accuracy, bandwidth, and active control achieved provide great potential for multifunctional terahertz modulation.

Abstract-Towards a Rapid Terahertz Liquid Crystal Phase Shifter: Terahertz In-Plane and Terahertz Out-Plane (TIP-TOP) Switching

Benjamin S.-Y. Ung,  Xudong Liu,  Edward P. J. Parrott,  Abhishek Kumar Srivastava,  Hongkyu Park, Vladimir G. Chigrinov, Emma Pickwell-MacPherson

http://ieeexplore.ieee.org/document/8283624/


Terahertz (THz) phase shifters are an essential component needed to realize many potential applications. Liquid crystals (LC) are commonly used at optical frequencies, yet to achieve an equivalent phase shift at THz frequencies the LC layer needs to be orders of magnitude thicker. Consequently, the time for the LC to relax back to its initial state is prohibitively slow. In this paper, we show for the first time how a thick, nematic phase LC cell can be switched actively in both directions, thereby achieving fast phase shifting of THz light. We call this THz in-plane and THz out-plane (TIP-TOP) switching. To achieve this, we have designed and fabricated a novel electrode structure, able to switch to and from both in- and out-plane orientations (TIP-TOP). The performance of the fabricated device provides an actively controllable phase delay with an ON–OFF cycle switching time of approximately 0.5 s: almost 100 times faster than the usual cycle time which exceeds 40 s. Furthermore, the analysis of the director distributions allows us to understand the causes of the asymmetric switching times. The TIP-TOP cell presents the capability to work as a low insertion loss, fast THz phase shifter and could be scaled up to realize a phased array device.