Design of a subwavelength all-metal grating for generating azimuthally polarized beams based on modified particle swarm optimization.

A subwavelength metal grating for generating azimuthally polarized beams was designed. The grating material was determined by calculating the total thermal conductivity coefficient. A modified particle swarm optimization (PSO), which has a two-step algorithm structure, and a particle position-determined inertia weight was performed to establish the grating's structural parameters. Results show that an Au-Ti-Cu all-metal structure provides good thermal conductivity. A high duty cycle structure gives the grating high polarization selectivity of approximately 82.63% at 10.6 μm and a minimum of 26.39% in the 9.6-11.6 μm band. The modified PSO is more efficient and requires fewer calculations while maintaining accuracy. The geometrical parameter tolerances were also analyzed. The groove depth fabrication tolerance is 200 nm, and the sidewall angle fabrication tolerance is 19°, which means that the optimized structure is insensitive to structural parameter deviations.