A diffraction grating comprises a substrate with a set of protruding ridges and intervening trenches characterized by a ridge spacing Λ, width d, and height h. The substrate comprises a dielectric or semiconductor material with a refractive index n1; the first substrate surface faces an optical medium with a refractive index n2 that is less than n1. Each ridge has a metal layer on its top surface of thickness t; at least a portion of the bottom surface of each trench is substantially free of metal. Over an operational wavelength range, λ/2n1