Broadband generation of abruptly autofocusing terahertz ring-Airy beams with all-silicon metasurface

Abruptly autofocusing terahertz (THz) ring-Airy beam has been generated by transmissive plasmonic metasurface at a single frequency in the earlier work. However, the transmission efficiency of it is low. Based on the waveguide effect, a design strategy for broadband generation of THz ring-Airy beams with transmissive allsilicon metasurface is proposed in this paper, which has an improved transmission efficiency and is more favorable in fabrication. Under the linearly polarized plane wave incidence, the amplitude and phase of the crosspolarized transmitted beam can be simultaneously modulated by controlling the rotation angles of the identical rectangular silicon pillars arranged on the silicon substrate. A polarization-independent antireflection square silicon pillar array is also proposed to be patterned at the other side of the substrate. The metasurface promises a working bandwidth from 2.12 to 2.72 THz with the overall transmission efficiency being over 51.18% (the highest transmission efficiency is up to 77.57% at 2.52 THz). The peak intensity positions of the generated beam during the propagation follow a parabolic trajectory. For the generated ring-Airy beam, the focal length is positively proportional to the frequency, while the abruptly autofocusing property and full width at half maximum (FWHM) of the focus are positively and negatively related with the frequency, respectively. This work can be expected to facilitate the applications of terahertz wave in biomedicine and manipulation of particles.