The reason for the optical frequency doubling in the70Ga2S3–30La2S3glass is investigated. The tetragonalGa6La10/3S14phase due to crystallization of the glass is responsible for the frequency doubling. Dense and transparentGa6La10/3S14crystalline thin films have been prepared by means of the controlled surface treatment of the glass due to an ultrasonic surface treatment. The effective second-order nonlinear optical susceptibilities of the crystalline thin films are analyzed. ©1998 American Institute of Physics.