Saturation effects in All-Optical Poling of dye molecules: 3D studies

Abstract

Photoinduced Anisotropy (PIA) and All-Optical Poling (AOP) are now well known techniques for orienting dye molecules in polymer films, at room temperature. They are efficient when the resonant excitation leads to a reversible photoisomerization (particularly for Trans-Cis isomerization of azodyes). In both cases, molecules are excited selectively (angular hole burning) and rotate, when they relax to the ground state (angular redistribution). After many photoisomerization cycles, molecules accumulate in the directions of smallest excitation probability. In PIA, the linear interaction with a polarized light aligns molecules perpendicularly to the polarization. In AOP, the nonlinear pumping by two coherent waves, ω and 2ω, produces a non-centrosymmetric depletion of the Trans state. Nevertheless, unavoidable one-photon (2ω) and two-photon (ω) absorption terms are centrosymmetric and induce anisotropy, which is detrimental for building χ(2). For that reason, we decided to measure simultaneously anisotropy and second harmonic generation (SHG), in AOP, and we tried to pump with a third beam, for correcting anisotropy.