Design of high-efficiency ultrabroadband dielectric gratings

We present a design concept of dielectric gratings containing resonant high (TiO₂) and resonant low (SiO₂) index dielectric thin-film layers between the grating and the underlying multilayer reflector. We use numerical simulations and the genetic algorithm optimization method to achieve high diffraction efficiency (>97%) in the first diffracted order over a wide wavelength range (∼160  nm) at around 800 nm. The basic concept of the structural optimization contains a high refractive index binary grating with alternating low- and high-index reflector layers, the thicknesses of which are also among the optimization parameters. We introduce two resonant dielectric layers directly below the corrugated TiO₂ grating structure and we choose a small (<10%) filling fraction for the grating. This concept allows us to broaden the spectral range where high DE can be achieved. We investigate here gratings for both Littrow-mount and off-Littrow applications.