A photonic crystal interferometric switch has a photonic crystal; a waveguide in the photonic crystal, the waveguide having at least one input portion, at least two output portions and an interference channel connecting the at least two output portions, the waveguide capable of transmitting light within a bandgap of said photonic crystal; and a resonant member connected to at least one of the at least two output portions to control a property of light in the at least one output portion, to control the interference of light in the waveguide. A 1×2 optical switch may be constructed by tuning the parameters of the resonant member, either optically or electronically, resulting in a switching of a light signal from one output portion to another output portion. Furthermore, by isolating one output portion of the interferometer, the apparatus may be utilized as an optical modulator.