Design of waveguide-grating filters with symmetrical line shapes and low sidebands.

We show that ideal reflection filters can be designed by combining guided-mode resonance effects in waveguide gratings with antireflection effects of thin-film structures. Since the guided-mode resonance effect overrides the antireflection effect this filter provides a symmetrical line shape with near-zero reflectivity over appreciable wavelength bands adjacent to the resonance wavelength. In the single-layer filter the same layer functions as the waveguide grating supporting the resonance and as the antireflection layer suppressing reflection around the resonance. A multilayer design allows the filter resonance peak to have a wide surrounding region of low reflectance. The central resonance wavelength, the filter linewidth, the range of the low sidebands, and the resonance line shape are all under the control of the designer.