  We demonstrate a method to locally change the refractive index in planar
optical devices by photodarkening of a thin chalcogenide glass layer deposited
on top of the device. The method is used to tune the resonance of GaAs-based
photonic crystal cavities by up to 3 nm at 940 nm, with only 5% deterioration
in cavity quality factor. The method has broad applications for postproduction
tuning of photonic devices.
