OBSERVING DIFFRACTION FROM PHOTOINDUCED TE GRATINGS IN THE PROCESS OFTHEIR FORMATION IN THIN-FILMS

The formation of photo-induced periodic structures (PPS) has been inve stigated under the condition of the excitation of limiting TE-modes by a single laser beam (lambda = 633 nm) in a thin-film structure, consi sting of a thin light-sensitive film with refractive index n, a buffer film (n(1)') and a glass substrate (n(1)), with n(1) > n(1) > n(1)'. It is found that at normal incidence of the laser beam, the period of PPS changes within the range lambda n(1)(-1) to lambda(n(1)')(-1) as t he buffer layer thickness increases. In the latter case, when the PPS are formed by the mode with propagation constant beta = kn(1)', diffra ction from PPS is observed directly on the surface of the sample due t o the mode refraction in the substrate, multiple reflections and light scattering in films. This fact brings us to the method of direct obse rvation of the PPS growth during their formation. Various examples of the diffraction patterns are presented, which were obtained from the P PS induced by normal and off-normal incident laser beams of various po larizations.