Switchable Terahertz Beam Steering with Nearly 100% Ordinary Transmission

We present a reconfigurable metasurface demonstrating transmission manipulation over a broadband terahertz (THz) wavelength range. This metasurface realizes controllable THz beam deflection utilizing only one layer of patterned vanadium dioxide (VO₂) film, known for its phase-transition properties. When the metasurface is at the OFF state, it has little distortion to the wavefront of incident light, exhibiting nearly 100% ordinary transmission. The simulation and experimental results show highly efficient THz beam steering when the metasurface is turned ON.<br/><br/>To realize the terahertz beam steering, we utilize two gap-split ring resonators as the unit cells of the metasurface and implement VO₂ on a sapphire substrate as the activation material. The metasurface is geometrically optimized to within the frequency range of 0.2 – 1 THz, where anomalous diffraction occurs in the transmitted beam, with polarization orthogonal to that of the incident beam. Eight resonators exhibit nearly identical transmission amplitudes and a π/4 phase increment, effectively covering the entire 2π phase range.<br/><br/>Fabrication is carried out in the University of Delaware Nanofabrication Facilities. About 120 nm thick of VO₂ thin film on the sapphire substrate is deposited using pulsed laser deposition and patterned by standard photolithography and inductively coupled plasma reactive ion etching. The as-deposited VO₂ thin film exhibits higher than 4 orders of magnitude resistance change across its phase transition.<br/><br/>THz time-domain spectroscopy, combined with a rotation stage, was used to characterize the fabricated metasurface. The state of the metasurface was controlled thermally. When the metasurface is in an ‘OFF’ state, the transmission intensity closely matches that of a bare sapphire substrate. When the metasurface is in an ‘ON’ state, it shows a large deflection consistent with the simulation results. For example, at three distinct frequencies: 0.4, 0.6, and 0.8 THz, the corresponding deflection angles are 69°, 39°, and 27°, respectively, aligning closely with the simulated angles of 69.52°, 38.65°, and 27.93°. The efficiency of beam steering is comparable with that of an identical metasurface sample fabricated of gold.