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

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.