We measure the diffraction of a cw probe laser beam from an absorption index grating set up in a thin film semiconductor by illumination with a moving optical interference fringe pattern. Taking advantage of the heterodyne detection principle whereby the weak diffracted laser beam is brought to interfere with the probe beam we are able to work with diffraction efficiencies as low as 10−14. When applied to amorphous hydrogenated silicon the diffraction efficiency depends in a characteristic way on an applied external electric field. We identify two physical mechanisms for the diffraction, namely a periodic temperature profile related to the absorption of the laser grating and the quadratic electro‐optical effect. ©1996 American Institute of Physics.