High-performance plasmonic mid-infrared bandpass filters by inverse design

Abstract Plasmonic spectral filters made of periodic nanostructured metal films provide new opportunities to develop multispectral imaging technologies in mid-infrared. However, traditional plasmonic spectral filters are often designed by a forward algorithm in which common structures such as nanoholes or nanorings based on manual design with limited geometry complexity are employed. The resultant filtering spectra from these structures suffer from a relatively low bandpass window and strong crosstalk, and thus can rarely find practical applications. In this work, we employed an inverse design algorithm to design high-performance bandpass filters in mid-infrared range which have an average transmittance over 80% in the bandpass window and less than 10% in the stop band, comparable to commercial multilayer Bragg filters. Nanofabrication processes were employed to transfer the designed pattern into the gold film on ZnS substrate that is transparent in the mid-infrared range. The resultant filters have a spectral performance similar to the inversely designed ones, and thus can be directly integrated onto mid-infrared photodetector arrays for multispectral imaging.