Particular attention of this paper is paid to the mechanism of fast transformation from the glassy state to crystalline state of chalcogenide alloy thin films: Sb2Se3, In2Se3, Sb2Te3 and In2Te3. Previous theory of these transformations is based on thermal effects which are formed through the nucleation and growth mechanism. An optical process, caused by laser pulse, has been carried out. The coupling of laser beam to film depends strongly upon its electronic structures. The band structures and densities of states of these films are calculated