A light modulator includes a light guide to guide light by total internal reflection, a diffraction grating at a surface of the light guide, and a liquid crystal in contact with the diffraction grating. The liquid crystal has a first state with a first refractive index that substantially matches a refractive index of a material of the diffraction grating to defeat the diffractive coupling. The liquid crystal has a second state with a second refractive index that differs from the first refractive index to facilitate the diffractive coupling.