In this letter we present a switchable THz notch filter. The filter contains a liquid crystal layer that acts as a half wave retarder in one state and as an isotropic layer in the other state. The device combines three unique properties: it can be switched electrically, it provides a filter depth of 35 dB at 350 GHz and it can be tuned over wide frequency range from 350 GHz to 700 GHz.
Keywords Liquid crystals . Switchable terahertz devices . BirefringenceIn recent years terahertz technology has experienced a tremendous development [1, 2]. Many applications for THz systems are presently discussed. They range from nondestructive testing [3][4][5] and process monitoring in the industry [5, 6] to free-space communications with THz carrier waves [7][8][9].Besides sources and detectors, a mature THz technology also requires devices to guide and manipulate THz waves. Among them are reflectors [10,11], waveguides [12, 13], splitters [14], lenses [15], filters [16] and modulators [17, 18]. Modulators and switches could be based on liquid crystals (LCs) the dielectric THz properties of which have recently been characterized with high precision [19-25]. First devices have been demonstrated by Pan et al., including THz phase shifters [26, 27] and polarizers [28]. Wilk et al. reported on a LC based switchable THz Bragg structure [29] and Ghattan et al. presented an on-off switch for sub-THz-waves, which was based on a two-dimensional photonic crystal infiltrated by LCs [30]. A THz device, which combines liquid crystals with an artificial medium, was reported by Khoo et al. [31] and Chen et al. presented a magnetically tunable Lyot filter [32].In this paper we report on the fabrication of an electrically tunable liquid crystal filter which uses the THz birefringence of LCs. It provides a high filter depth and is electrically tunable over a broad frequency range. The filter consists of a 2.4 mm thick liquid crystal layer, which is sandwiched between two 710 μm thick fused silica windows (Fig. 1). As a