Quasi-permanent all-optical encoding of noncentrosymmetry in azo-dye polymers

Abstract

We present an original poling technique that uses a purely optical excitation process. The experiment consists of a seeding-type process. Writing and probing periods are alternated. Writing periods correspond to simultaneous irradiation of the sample by the coherent superposition of the 1064-nm fundamental and the 532-nm second-harmonic light of a picosecond-pulsed Nd:YAG laser. The sample is a spin-coated film of a poly(methyl methacrylate) copolymer onto which the azo-dye molecule Disperse Red 1 is grafted. We demonstrate efficient and quasi-permanent poling of the molecules with a spatial period that satisfies the phase-matching condition for second-harmonic generation. The influence of seeding parameters such as the relative phase and the relative intensities between the writing beams is studied both theoretically and experimentally. Tensorial properties and the spatial profile of the photoinduced χ(2) are analyzed from a microscopic point of view. Dark and photostimulated relaxation processes are investigated from a chemical-physics point of view. The physical origin of the photoinduced molecular orientation process is discussed. A minimal model involving the relevant experimental parameters is developed. Numerical simulations are in agreement with the experiment.