Photoinduced manipulations of photochromes in polymers: Anisotropy, modulation of the NLO properties and creation of surface gratings

Photochromes in solid matrices such as polymers or sol-gels are interesting because of their potential applications as optical filters or optical memo ries. Besides their reversible color change which is used in these applicat ions, they have also raised a new interest because other optical or physico chemical properties can be changed under light control. We review here the optical properties induced by irradiation with polarized light. First, it h as been known for more than two decades that birefringence and UV-visible d ichroism can be induced in azo polymers. More recently, such phenomena have also been found with spiropyrans. Secondly, when thin polymer films are ir radiated with an interference pattern, it is possible to create both refrac tive index and thickness gratings. Other optical properties arise from the coupling between photochromism and nonlinear optical (NLO) properties. First, when the polarized excitation be am is superimposed with a DC electric field directed perpendicular to it, a non centrosymmetric arrangement of dipoles can be created in a polymeric m edium, and second order NLO susceptibilities can be measured. Secondly, it is possible to photoswitch the NLO response of previously oriented photochr omic and NLO active polymers (photoswitching) by irradiating alternatively each form of the photochrome. Thirdly, when two coherent beams at frequenci es omega and 2 omega are superimposed on the same NLO sample, it is also po ssible to induce non centrosymmetry. This method, called "all-optical polin g" is very promising to create NLO devices such as electro-optical modulato rs or frequency converters.