Terahertz broadband achromatic focusing vortices

In this paper, we propose to combine an achromatic lens phase and a spiral phase in the metadevice phase profile to achieve terahertz (THz) broadband achromatic focusing vortices in the frequency range from 0.8 THz to 1.3 THz. The designed metadevices are composed of elliptical high-resistivity silicon pillars with different sizes to cover the propagation phase from -π to π. The particle swarm optimization (PSO) algorithm is implemented to search for the most suitable metaatoms in the structure library to achieve the THz broadband achromatic focusing vortices. The lens phase and spiral phase are simultaneously considered in the PSO algorithm to determine the reference phase values, which can optimize the achromatic characteristics of focusing vortices. The broadband achromatic characteristics are confirmed at design and non-design frequencies, demonstrating the implementation of broadband achromatic focusing vortices across the entire design THz frequency band. Our design enables broadband THz focusing vortices to be detected in the same area, which has great application prospects in THz communication systems and THz information processing.