We propose a theory of the instability of the space-charge field induced in
a semiconductor by the movement of light fringes against subharmonic gener
ation. A crucial element of this theory is taking into account three-dimens
ional perturbations and higher spatial harmonics of the space-charge field.
It is shown that, irrespective of fringe contrast and velocity, the strong
est instability corresponds to the excitation of subharmonics that are not
split in the transverse direction. The theory is applied to experimental da
ta on subharmonic generation in sillenites. (C) 1999 Optical Society of Ame