Fabrication and characterization of high-quality waveguide-mode resonant optical filters

Optical filters containing resonant waveguide gratings are designed and fabricated using low-loss, robust materials. The double-layer filters contain a silicon dioxide diffractive element on a hafnium dioxide waveguide deposited on a fused silica substrate. Noise-pattern formation is minimized by use of an antireflective absorption layer during holographic grating recording in photoresist. Subsequent fabrication steps include metallization, lift-off, and oxygen plasma etch to create a metal etch mask for final CF4 plasma etching of a surface-relief grating. Spectral characterization with a tunable laser shows that the resulting filter exhibits 90% efficiency, 1.2 nm linewidth, and low sidebands.