Orientational manipulation of chromophores through photoisomerization

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Abstract
Dye molecules are widely used as optical active dopants in polymer (or sol-gel) films designed for integrated optics. A large effort is made to control the orientation of dye molecules, in order to produce macroscopic anisotropy (permanent and reversible for optical memories or transient for polarization holography) or to pole the film for electro-optic applications or for harmonic generation. In this paper, optical methods for inducing orientational order in photoisomerizable molecules are discussed. Three mechanisms are analyzed, the Angular Hole Burning (AHB), the orientational redistribution and the orientational relaxation and the dynamics of their competition is studied. Experimental illustrations are given using azobenzene derivatives (like DR1) in PMMA or in sol-gels. These experiments provide an efficient method for the characterization of the angular mobility of molecules in different host materials and of the long term stability of anisotropy and of (Chi) (2) properties. Preliminary results of a general mathematical model are given: they look very promising for the understanding of orientation mechanisms in a broad variety of materials.

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