A light modulator includes a light guide to guide light, a diffraction grating at a surface of the light guide, and a fluid having a refractive index substantially matched to a refractive index of the diffraction grating. The diffraction grating is to couple out a portion of guided light from the light guide using diffractive coupling. The index-matched fluid is to be selectively moved both into contact with the diffractive grating to defeat the diffractive coupling and out of contact with the diffraction grating to facilitate the diffractive coupling to modulate the coupled out guided light.