Déformation photoinduite dans les films minces contenant des ...

Chapter 1. The azobenzene molecules: photochromic materials and photodeformationeffects 13with the exposure light dose, either under illumination with interfering beams[10, 16–18] and single focused beams [35, 36]. On the other hand, in relation tothe light field polarization configuration various different deformation mechanismsseem to be activated, which result in different SRG growth amplitudes, speed andspatial distribution with respect to the optical intensity.Since the light field polarization has been observed to be a critical parameter influencingthe photodeformation phenomena, we will briefly resume the principal observations sofar reported under various polarization configurations.1.3.1.1 The dependence on the polarizationpastel-00527388, version 1 - 19 Oct 2010Several studies have shown that the SRG formation is very weak under s-polarizedlight, while a higher efficiency is observed using p-polarized light [5, 10, 16, 37, 38].In particular, under projection of an intensity gradient, by illumination either with afocused gaussian beam or with an interference pattern, the resulting deformation is inπ − phase with the illumination: reliefs are formed in the dark areas, while grooves areinduced in the bright zones. Under illumination with a polarization interference pattern(±45 ◦ or orthogonally circularly polarized interfering beams) the highest deformationefficiency is observed [37–39]. This indicates that an intensity gradient is not a necessarycondition, while the effect of the polarization seems preponderant. On the other hand, aquite weak deformation is observed under an s- and a p-polarized interfering beams withthe formation of a double period SRG. Very recently, it has been demonstrated, using thesuperimposition of an interference pattern to an homogeneous, incoherent p-polarizedassisting beam, that the deformation efficiency obtained under s-polarized interferenceis increased [10], with a relief growth in phase with the light field [40]. This supportsthe critical influence of the light field polarization on the photodeformation phenomena.1.3.1.2 Photodeformation mechanisms: interpretationsThe understanding of the mechanisms at the origin of these phenomena is still debated.The observation of a deformation in π − phase with the illumination has suggestedthe existence of a mechanism of matter migration, due to the photoisomerization ofthe azobenzene molecules induced in the enlightened zones [2]. The mechanical workdone during each interconversion would produce an overall transport of matter, whichis accumulated in the darker zones, where the molecules are not photoactivated. Thisinterpretation is the most shared, even if hardly compatible with the high efficiency observedunder irradiation with a polarization interference pattern, and several modeling