A diffraction grating includes a substrate and an array of triangular ridges extending from the substrate. The ridges run parallel to one another and have triangular cross-sections such that first sides of the ridges face in a first direction and adjacent second sides of the ridges face in a second, different direction. An array of grating lines is disposed over the first sides of the array of ridges, each grating line of the array of grating lines comprising a slab of transparent material supported by the first side of a corresponding ridge of the array of ridges. A refractive index of the array of grating lines is different from a refractive index of the array of ridges.