Tunable reflective liquid crystal terahertz waveplates

Tunable liquid crystal devices that can change terahertz wave polarization continuously have many potential applications in terahertz optical systems. We present a reflective liquid crystal terahertz waveplate with sub-wavelength metal grating and metal ground plane electrodes. The thickness of the liquid crystal layer can be reduced to similar to 10% of that needed for the same phase shift at a given frequency in a transmissive waveplate. We experimentally demonstrate the same tunability as in the transmissive type just using half the thickness. We discuss the dependence on the angle of incidence for phase shift tunability, which can achieve beam steering and polarization conversion simultaneously. The proposed design can be applied in terahertz imaging, sensing, and communications. (C) 2017 Optical Society of America