Title: A broadband vortex beam generation by reflective meta-surface based on metal double-slit resonant ring

Abstract: Recently, meta-surface(MS) has emerged as a promising alternative method for generating vortex waves. At the same time, MS also face the problem of narrow bandwidth, in order to obtain a board bandwidth, the MS unit cells structure become more and more complex, which will deduce many inconveniences to the preparation process of MS device. 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 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 2pi phase coverage was satisfied in the frequency range of 0.52THz-1.1THz, 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 waves (RCP) and left-handed circularly polarized waves (LCP) are separately incident on MS with topological charges of l=1,2,3 under multiple resonant frequencies. The generated RCP vortex wave with topological charges of l=-1,-2,-3 and the generated LCP vortex wave with topological charges of l=1,2,3. The numerical simulation results exhibit our designed MS with multiple resonance outcomes can achieve a multi-broadband operation and generate 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.