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 ∼700  nm for zero refractive-index contrast Δn=0, whereas a high-contrast device with Δn=2 yields a bandwidth of ∼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.