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 memories. Besides their reversible color change which is used in these applications, they have also raised a new interest because other optical or physicochemical properties can be changed under light control. We review here the optical properties induced by irradiation with polarized light. First, it has been known for more than two decades that birefringence and UV-visible dichroism can be induced in azo polymers. More recently, such phenomena have also been found with spiropyrans. Secondly, when thin polymer films are irradiated with an interference pattern, it is possible to create both refractive index and thickness gratings. Other optical properties arise from the coupling between photochromism and nonlinear optical (NLO) properties. First, when the polarized excitation beam is superimposed with a DC electric field directed perpendicular to it, a non centrosymmetric arrangement of dipoles can be created in a polymeric medium, and second order NLO susceptibilities can be measured. Secondly, it is possible to photoswitch the NLO response of previously oriented photochromic and NLO active polymers (photoswitching) by irradiating alternatively each form of the photochrome. Thirdly, when two coherent beams at frequencies omega and 2 omega are superimposed on the same NLO sample, it is also possible to induce non centrosymmetry. This method, called "all-optical poling" is very promising to create NLO devices such as electro-optical modulators or frequency converters.