Experimental and theoretical studies of an amorphous As2S5 semiconductor show the appearance of a nonlinear variation of the refractive index Δne produced by a weak He-Ne laser beam. The refractive-index variation needs long-time exposure (hours) and survives for a long time (a few days) after the laser is switched off. The main feature of the theoretical model proposed here is the existence of electron and hole trapped states resulting in slow relaxation and recombination, and the dependence of generation and recombination rates in a given trap on the space distribution of electron-hole ordering. This dependence results in spatial electron-hole ordering: the ordered state is long living and gives a contribution to the refractive-index variation. Some experimental results supporting the presented model are also reported.