High-efficiency reflective metasurfaces for terahertz vortex wave generation based on completely independent geometric phase modulations at three frequencies

Abstract

One of the considerable disadvantages of a metasurface (MS) is frequency-dependent behavior, which largely restricts its practical applications. To overcome this challenge, a MS is designed to generate vortex beams at three distinct terahertz (THz) frequencies. Specifically, a unit-cell structure comprising three resonators is proposed to operate at three distinct THz frequencies. The cross-polarized reflection coefficients of the unit-cell structure are greater than 70% under circularly polarized (CP) incidence. Based on the geometric phase principle, the full 2 π phase shift can be obtained at each frequency by rotating the corresponding orientation angles of the three resonators. By carefully arranging the unit-cell structure, orbital angular momenta (OAMs) with topological charges of l = ± 1 , − 2 , and, − 3 at 0.706 THz, 1.143 THz, and 1.82 THz, respectively, can be generated for a normally incident right CP wave, and OAMs with topological charges of l = − 3 , − 2 , and − 1 can be generated for a normally incident left CP wave. The generated reflective vortex beams through the designed MS have good mode purity up to 85% at 0.706 THz, 84% at 1.143 THz, and 74.7% at 1.82 THz, respectively. Moreover, the designed reflective MS reveals a convenient and low-cost way to generate vortex beams with different/same OAM modes at three different resonance frequencies and is beneficial for potential applications in THz communication.