Terahertz Bessel Beam Scanning Enabled by Dispersion-Engineered Metasurface

Bessel beam steering operating at the terahertz (THz) band holds great potential in improving the depth-of-field (DOF) for imaging systems and the quality of short-range high-speed communications. This article reports a THz reflective metasurface for frequency-controlled Bessel beams steering. A multiresonance structure with a large reflection phase range is designed as the fundamental element of the metasurface by varying its dimensions. At a designated frequency, the desired phase distribution to generate an off-axis Bessel beam is determined by four terms, i.e., phase to emulate an axicon, phase compensation for the incident plane waves, phase to deflect the outgoing beam, and a constant phase for optimization. Then, the metasurface, composed of 79 $\times$ 79 half-wavelength elements with different dimensions, is constructed to fit the desired phase distributions at multiple frequencies. Illuminated by linearly polarized (LP) waves, the metasurface can produce Bessel beams with controllable directions by adjusting frequencies. As a proof-of-concept, a metasurface prototype operating from 0.8 to 1.1 THz was fabricated and experimentally demonstrated for Bessel beams generation and beam scanning from 6.4 $^{\circ}$ to 15.2 $^{\circ}$ off normal.