Wideband flat lens of sub-THz beams by geometric phase
Abstract
<p indent="0mm">Geometric phase, also known as Pancharatnam-Berry phase, is widely employed in optical metasurfaces for wavefront manipulation, benefiting from the simple design procedure and 2π dispersion-free phase coverage. The efficiency and bandwidth of geometric-phase-based metasurfaces are usually constrained by the capability of the constituent inclusions in orthogonal polarization conversion. When operating at a frequency deviated from the designed one, the inclusions cannot fully alter the handedness of the circular polarization, leading the wavefront to be partially reshaped by the geometric phase. In this study, we propose a geometric phase metasurface for a wideband beam focusing that benefits from the wideband polarization-rotation capability of the constituent inclusions. The inclusions are one-dimensional (1D) low-index dielectric gratings featuring orthogonal polarization conversion and high transmission efficiency from 130 to 165 GHz. Below <sc>130 GHz,</sc> the polarization conversion fails due to the undesired interactions of two waveguide modes inside the grating layer. Then, the gratings are rotated, cut off, and reconstructed to focus the circular polarized sub-THz waves. Numerical simulation results show that we obtain a focusing efficiency of >40% in the frequency range of <sc>60–160 GHz.</sc> The focusing bandwidth is much wider than that of the constituent 1D gratings as a polarization convertor. This is explained by the different cutoff lengths of the gratings in various zones of the metasurface. The proposed design can be manufactured using cost-effective three-dimensional printing techniques, thus paving the way for compact and wideband sub-THz functional devices.</p>
