Terahertz metasurface with polarization conversion and Gaussian beam shaping

The majority of current beam conversion techniques rely on bulky lens systems for operation. In this paper, we propose a terahertz metasurface that can effectively convert a Gaussian beam into a flat-top beam and convert linearly polarized terahertz waves into their orthogonal counterparts. An improved Gerchberg–Saxton (GS) algorithm was employed in this study to precisely calculate the required phase distribution for the beam shaping device. By adjusting the opening angle of the metal split ring in the middle layer of the metasurface, a full 2 π phase coverage was achieved, ensuring precise control over the phase distribution. Simulation results indicate that within the range of 0.4–1.1 THz, the cross-polarization conversion efficiency is greater than 70%, with a relative bandwidth of 93.3%. The root-mean-square error (RMS) can reach 0.143 at 0.75 THz, the non-uniformity of the rectangular spot in the target region is 0.126, and the energy ratio of the rectangular spot to the whole focal plane is 78.03%, demonstrating the achievement of a high-quality flat-top beam. The metasurface beam shaping device we proposed, with its wide operating bandwidth, low transmission loss, high shaping efficiency, and compact structural design, offers an innovative solution for beam shaping techniques in the terahertz frequency band.