Process for working a preform made of an oxide single crystal, and a process for producing functional devices

A process for producing functional devices provided on cut pieces each made of an oxide single crystal and having a given shape, said process consisting essentially of the steps of forming the functional devices on a wafer at positions corresponding to locations of the cut pieces, removing molecules of the oxide single crystal through dissociation and evaporation with an optochemical reaction under irradiation of a single-wavelength laser beam upon the wafer, thereby forming such grooves in the wafer as to surround the respective functional devices, said laser beam having a wavelength near that of an absorption edge of the oxide single crystal and then cleaving, the wafer along the grooves.