An optical device includes a substrate, a first surface-relief grating including grooves and ridges formed on or in the substrate, a first overcoat layer in the grooves of the first surface-relief grating, and a first antireflective layer on the first overcoat layer. The ridges of the first surface-relief grating include high-refractive index, photoactive metal oxide nanoparticles and a material of the first overcoat layer in regions between the metal oxide nanoparticles, or the first overcoat layer includes the metal oxide nanoparticles and a material of the first antireflective layer in regions between the metal oxide nanoparticles. Methods of fabricating the optical device are also described.