Reconfigurable photonic crystal filters for multi-band optical filtering on a monolithic substrate

Many applications require the ability to image a scene in several different narrow spectral bands simultaneously. Absorption filters commonly used to generate RGB color filters do not have the flexibility and narrow band filtering ability. Conventional multi-layer dielectric filters require control of film thickness to change the resonant wavelength. This makes it difficult to fabricate a mosaic of multiple narrow spectral band transmission filters monolithically. This paper extends the previous work in adjusting spectral transmission of a multi-layer dielectric filter by drilling a periodic array of subwavelength holes through the stack. Multi-band photonic crystal filters were modeled and optimized for a specific case of filtering six optical bands on a single substrate. Numerical simulations showed that there exists a particular air hole periodicity which maximizes the minimum hole diameter. Specifically for a stack of SiO₂ and Si₃N₄ with the set of filtered wavelengths (nm): 560, 576, 600, 630, 650, and 660, the optimal hole periodicity was 282 nm. This resulted in a minimum hole diameter of 90 nm and a maximum diameter of 226 nm. Realistic fabrication tolerances were considered such as dielectric layer thickness and refractive index fluctuations, as well as vertical air hole taper. It was found that individual layer fluctuations have a minor impact on filter performance, whereas hole taper produces a large peak shift. The results in this paper provide a reproducible methodology for similar multi-band monolithic filters in either the optical or infrared regimes.