Reverse design of pixel-type terahertz band-pass filters

Reverse design is a frontier direction in the optical research field. In this work, reverse design is applied to the design of terahertz devices. We have employed direct binary search (DBS) and binary particle swarm optimization (BPSO) algorithms to design pixel-type terahertz band-pass filters, respectively. Through a comparative analysis of the designed devices, we found that BPSO algorithm converged faster than DBS algorithm, and the device performance is better on out-of-band suppression. We have fabricated a sample utilizing femtosecond laser micromachining and characterized it by terahertz time-domain spectroscopy. The experimental results were consistent with the finite difference time domain (FDTD) simulation. Our method can simultaneously optimize multiple characteristics of the band-pass filters, including the peak transmittance, out-of-band transmittance, bandwidth, and polarization stability, which can not be achieved by traditional optical design methods.