Theoretical study on 2.52 terahertz beam shaping and polarization conversion based on the transmissive all-dielectric metasurface

Abstract

Based on the physical optics vector diffraction integral formula and the commercial electromagnetic software EastWave, a transmissive all-dielectric square metasurface with a side length of 12.4 mm is designed in this paper. It can simultaneously modulate the phase and polarization of the terahertz wave. The 2.52 terahertz x-polarized normal incident Gaussian beam can be shaped into a radially polarized narrow-width annular Bessel–Gaussian beam (radii of the inner and outer circles are 4.125 mm and 5.5 mm, respectively) at a propagation distance of 23.8 mm away from the metasurface. The metasurface is composed of the poly (4-methyl-1-pentene) substrate and the rectangular silicon resonators with different dimensions and rotation angles (counterclockwise rotation angle of the rectangle long axis as the reference of positive x axis). The relative square error between the amplitude distribution of the shaped beam and the target beam is 7.36%, and the fitting coefficient of them is 96.6%. The diffraction efficiency (defined as the ratio of the total light field energy in the target region to that in the rear plane of the metasurface along the incident beam propagation direction) of the shaped annular region is 75.5%. By focusing the narrow-width annular Bessel–Gaussian beam obtained in this paper, we can get smaller focal spot and longer focal depth, which is of great significance for using terahertz wave in particle acceleration and material processing. In addition, the emergent polarization can be flexibly switched between radially and azimuthally polarized beams by changing the polarization direction of the linearly polarized incident beam.