Abstract: An active photonic crystal device for controlling an optical signal is disclosed. The device includes a planar photonic crystal with a defect waveguide bounded on the top and bottom by an upper cladding region and a lower cladding region. An optical signal propagating in the defect waveguide is confined in the plane of the photonic crystal by the photonic bandgap, and in the direction normal to the photonic crystal by the upper clad region and the lower clad region. The propagation of the optical signal in the defect waveguide is controlled by varying the optical properties at least one of the upper clad region or the lower clad region. The variation of the optical properties of the controllable regions may be achieved using a thermo-optic effect, an electro-optic effect, a stress-optic effect, or a mechano-optic effect, or by moving a material into or out of the controllable region.