OBSERVATION OF REVERSIBLE PHOTOCHEMICAL BLOW-OUT OF THE 3RD-ORDER MOLECULAR HYPERPOLARIZABILITY OF PUSH-PULL AZO-DYE IN HIGH GLASS-TRANSITION TEMPERATURE POLYIMIDES

We present evidence of reversible optical control of the third-order m olecular hyperpolarizability (gamma) of nonlinear optical (NLO) azo ch romophore. We show that optically-induced molecular shape change of th e NLO dye from the trans to the cis form, by means of photoisomerizati on which occurs within the picosecond time scale, breaks gamma down ra pidly. The anharmonic movement of the electronic cloud of the NLO dye in strong optical field is ''blown out'' upon optical exitation of the azo chromophores. We show that gamma recovers its initial value upon thermal back-isomerization of the dye to the trans form. This change i n gamma can be optically-cycled many times, leading to a novel all-opt ical light modulation phenomenon. The light polarization and molecular reorientation do not influence this all-optical switching of gamma. W e develop a theoretical model that considers a molecular density resul ting from an intensity-dependent balance between two molecular species with different molecular third-order hyperpolarizabilities imbedded i n a transparent medium. We derive analytical solutions, and we study t he effect of the parameters involved in this all-optical process, incl uding the irradiating light intensity and the change in magnitude and sign of gamma. The theory explains the experimental findings and allow s a physical insight into this optical control of third-order molecula r hyperpolarizability of Light sensitive nonlinear optical isomers.