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Quadratic optical polarizabilities in polymer films obtained by dc-electric field and pure optical poling

We describe a new method for poling polymers containing nonlinear chromophores. It consists in taking advantage of the polar character of the coherent superposition of a laser beam and second harmonic. Though the resulting electromagnetic field averages to zero, its odd harmonics do not. This offers the possibility of a polar selective excitation of nonlinear chromophores via the interference between one- and two-photon absorption. Experiments were performed on thin films of disperse-red1 dissolved or chemically attached to a PMMA matrix, using a Nd-YAG laser at 1064 nm and its second harmonic obtained after frequency doubling in a KDP crystal. The resulting electro-optic coefficients are comparable to the values obtained by the usual dc-field poling. We have also determined the linear [(chi) (2)(-(omega) ;(omega) ,0)] and quadratic ](chi) (3)(-(omega) ;(omega) ,0,0)] electro-optic coefficients wavelength dispersion in corona-poled thin films, using the modulated ellipsometry technique. One advantage of this technique is the possibility to perform measurements inside the absorption band. From the obtained values, it was possible to determine microscopic parameters of the chromophores such as the dipole moment difference between fundamental and excited states. A resonance of the (chi) (2) dispersion is observed, red-shifted with respect to the one- photon transition.