We have observed a sharp threshold for the process of optically induced glide at which the velocity changes by more than a factor of 103 when the excitation intensity changes only 20%. This threshold is insensitive to doping and to the presence of a p-n junction. The effect is shown not to be related to recombination-enhanced motion or to local heating. An explanation in terms of the reduction of frictional forces by interaction with unrecombined carriers is offered. © 1978 The American Physical Society.