Optical devices including waveguide grating structures are described. In accordance with one embodiment, an optical device is provided comprising a horizontal waveguide grating structure having at least one waveguiding layer and at least one subwavelength periodic grating layer. The optical device further comprises upper and lower cladding layers immediately adjoining respective upper and lower surfaces of the waveguide grating structure and having refractive indices lower than a lowest-index one of the waveguiding layers, incident radiation propagating through one of the upper and lower cladding layers toward the waveguide grating structure. The waveguide grating structure is configured for peak reflection of the incident radiation at a peak reflection frequency. A cumulative thickness of the waveguiding layers is less than one tenth of a free space wavelength of the incident radiation at the peak reflection frequency divided by an average refractive index of the waveguiding layers.