Abstract
Terahertz (THz) technology has unique applications in, for example, wireless communication, biochemical characterization, and security inspection. However, high-efficiency, low-cost, and actively tunable THz modulators are still scarce. We propose a broadband tunable THz beam deflector based on liquid crystals (LCs). By a periodic gradual distribution of the orientation of the LC in one direction, a frequency-independent geometric phase modulation is obtained. The LC device with this specific orientation distribution was obtained through ultraviolet polarization exposure. We have verified the broadband beam deflection in both the simulation and experiment. The device can achieve a good spin-coupled beam deflection effect in the 0.8–1.2 Thz band, and the average polarization conversion efficiency exceeds 70%. Moreover, because the electro-optical responsivity of LCs is excellent, graphene transparent electrode layers introduced on the upper and lower substrates enable the deflection modulation to be switched and dynamic tuning to be achieved.
Highlights
Researchers have developed THz beam deflectors based on metasurfaces [11,12], which are composed of sub-wavelength resonator arrays
The beam deflector is realized by the geometric phase of liquid crystals (LCs)
The phase has a double relationship with the orientation angle of the LC
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Dynamic modulation of terahertz beams can be achieved with many materials, such as semiconductors, superconductors, Phase Change Materials (PCM), Micro Electro Mechanical Systems (MEMS) Semiconductor materials, such as silicon and gallium arsenide (GaAs), can change their carrier concentrations by applying electric fields or optical pumping. While the liquid crystal is non-resonant, and the theoretical polarization conversion efficiency can reach more than 95% after optimizing the liquid crystal layer thickness and using low loss liquid crystal materials This method has the advantage of relatively simple processing compared with the metasurface. THz LC devices mainly use magnetic fields or temperature control to change the refractive index to modulate the phase and polarization, and achieve functions such as filtering and phase shifting [27,28]. The required grating phase for beam deflection φ is expressed as:
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