The stationary solution is obtained for the photorefractive subharmonic gratings excited in crystals of the sillenite family by a standing light interference pattern and an applied ac electric field. We show that the main subharmonic with doubled spatial period may become unstable against excitation of the subharmonic with quadrupled spatial period. The threshold condition for this bifurcation is found. [S1050-2947(98)05808-9].
Physical Review a (atomic, Molecular and Optical Physics), 1998, Vol 58, Issue 2, p. 1601-1604