Periodic photonic filters: theory and experiment

Photonic multiline filters exhibiting periodic resonance lines on a dense spectral grid in a broad wavelength range are demonstrated. We design the filters using rigorous numerical methods and then proceed with experimental verification by patterning, etching, and collecting spectral data. We use standard double-side-polished 300-μm-thick silicon wafers for the experiments. On one side of the wafer, we place a shallow grating whereas the other side has a quarter-wave antireflection layer. An example filter with a ∼200-nm-deep TiO2 grating on the wafer yields 12 narrow resonance peaks within a 10-nm wavelength range centered at 1550 nm. The spectral width of each filter peak is ∼0.1 nm with a free spectral range of ∼0.8 nm. Peak efficiency approaches 80% with low sidebands between the resonant filter lines. We discuss briefly design of polarization-independent periodic filters and the concept of Brewster-angle filters. Possible applications include spectral sampling and wavelength discretization as used, e.g., in sensing gas species and quantifying toxic gas concentrations.