Broadband vortex beam generation by a reflective meta-surface based on a metal double-slit resonant ring

Recently, the meta-surface (MS) has emerged as a promising alternative method for generating vortex waves. At the same time, MS also faces the problem of a narrow bandwidth; in order to obtain a broad bandwidth, the MS unit cell structure becomes more and more complex, which will bring many inconveniences to the preparation process of MS devices. Therefore, we want to design a simple MS unit cell with a multi-frequency selection. In this paper, based on the principle of geometric phase, we design a simple reflective MS unit cell based on a metal double-slit resonant ring. We elaborate on the resonance mechanism of the MS unit cell. Under the normal incidence of circularly polarized (CP) waves, the reflection coefficient of the same polarization was greater than 85%. By rotating the orientation angle of the resonator on the MS unit cell, the continuous 2π phase coverage was satisfied in the frequency range of 0.52–1.1 THz, and the relative bandwidth becomes 71.6%. Based on this, we construct a vortex generator by using a 15×15 MS unit array. The right-handed circularly polarized (RCP) waves and left-handed circularly polarized (LCP) wave are separately incident on MS with topological charges of l=+1,+2,+3 under multiple resonant frequencies. The generated RCP vortex wave has topological charges of l=−1,−2,−3 and the generated LCP vortex wave has topological charges of l=+1,+2,+3. The numerical simulation results demonstrate that our designed MS, capable of achieving multiple resonance outcomes, can effectively operate in a multi-broadband mode and produce a wide-band vortex beam. In addition, we also calculate the pattern purity. Through theoretical analysis and numerical simulation, we prove that our designed MS can generate a broadband vortex wave.