Wideband reflectors with zero-contrast gratings

We present wideband resonant reflectors designed with gratings in which the grating ridges are matched to an identical material, thereby eliminating local reflections and phase changes. This critical interface thus possesses zero refractive-index contrast; hence zero-contrast gratings. We design reflectors with zero-contrast gratings and high-contrast gratings and compare the results. For simple gratings with two-part periods, we show that zero-contrast grating reflectors outperform comparable high-contrast grating reflectors. An example silicon-on-glass reflector exhibits a 99% reflectance bandwidth of similar to 700 nm for zero refractive-index contrast Delta n = 0, whereas a high-contrast device with Delta n = 2 yields a bandwidth of similar to 600 nm. It follows that local Fabry-Perot modes residing in the grating ridges and reflecting off a high-contrast interface are not the root cause of wideband reflection. (C) 2014 Optical Society of America