Diffraction measurement for grating formed in optically poled polymeric materials

Recent development of all optical poling technique enables the creation of the photo-induced noncentrosymmetric polarization in the polymeric matrix. In this new method, third-order optical nonlinearity of coherent superposition between fundamental and second harmonic waves is significantly responsible for the creation of polar field which induces the noncentrosymmetric polarization (chi ((2)) polar structure) in the polymeric matrix. In this study, we employed a diffraction technique to investigate the gratings formed in optically poled polymeric materials. Increase of the diffraction intensity follows the increase (,growth) of the second harmonic wave induced by the chi ((2)) polar structure. Diffraction efficiency increases with increasing film thickness up to 4.5 mum. The origin of the diffraction is ascribed to the higher-order of diffraction, i.e. the unsymmetrical grating formed at double spatial frequency between unpoled and poled fringes. These results gave direct evidence of existence of the periodic chi ((2)) polar structure in optically poled polymeric materials.