Slab waveguide layer for enhanced near-eye-display surface relief grating lightguide

Embodiments of the present disclosure generally relate to augmented reality waveguide combiners. The waveguides includes a waveguide substrate, having a substrate refractive index (RI) nsub, a slab waveguide layer disposed over the waveguide substrate, the slab waveguide layer having a slab RI nswg and a slab depth dswg, the slab depth dswg from a lower surface to an upper surface of the slab waveguide layer, at least one grating defined by a plurality of grating structures, the grating structures are disposed in, on, or over the slab waveguide layer, and a superstrate between and over the grating structures, the superstrate having a superstrate RI nsuperstrate and an interface with the slab waveguide layer. The slab RI nswg is greater than the substrate RI nsub and the slab RI nswg is greater than the superstrate RI nsuperstrate.