A grating structure having a fused silica base includes alternating ridges and grooves. The ridges and grooves form a fused silica to air interface. The ridges and grooves are configured such that the grating has a ratio between the effective refractive index difference between s-polarization and p-polarization of about 1/3. As such, for non-polarized light with an incident angle θin of between 40° and 90° and a wavelength λ=350-1600 nm, the grating directs both s-polarization and p-polarization components of the incident light to the −1st order diffraction mode.