- Phase-only modulation is shown to be possible upon reflection off one-dimensional photonic crystals. Within the photonic bandgap, photons travel to some penetration depth in the periodic structure and experience phase change. Modulation of the effective index or layer thickness within the gap will then produce phase-only modulation. Phase modulation of more than pi is possible upon an equivalent variation of few nm of the layer thickness. This could be the basis for phase-only modulators used in mode locking of photonic crystal lasers and reflective adaptive optics components.