A method of forming an optical thin film, comprises providing an assembly comprising a layer of semiconductor material deposited on a substrate, the semiconductor material comprising a compound of at least one metal and a group VI element; depositing a masking layer onto the layer of semiconductor material, the masking layer being patterned to expose one or more regions of the layer of semiconductor material; applying to the assembly a plasma of the group VI element in order to cause indiffusion of the group VI element into the semiconductor material in the exposed regions while the masking layer blocks indiffusion in unexposed regions, the indiffusion causing a reduction in carrier density in the semiconductor material; and removing the masking layer; thereby forming, from the layer of semiconductor material, an optical thin film having a variation in carrier density and corresponding variation in optical properties matching the patterning of the masking layer in a plane parallel to the substrate.