Déformation photoinduite dans les films minces contenant des ...

Chapter 1. The azobenzene molecules: photochromic materials and photodeformationeffects 11In general, the absorption bands of the trans isomer are located at higher wavelengthswith respect to the cis form. In the case of the pseudo-stilbene, the absorption peaks ofthe trans and the cis forms are very close, so the photoisomerization of the two speciescan be excited by the same wavelength, generally in the blue-green range. In condition ofconstant temperature and illumination, it is possible to generate continuous trans → cis→ trans photoisomerization cycling, leading to a photostationary equilibrium betweenthe trans and the cis populations. Turning the light off, the cis isomers undergo thermalrelaxation towards the trans state, with a characteristic time of a few seconds or evenshorter.1.2.4 Molecular reorientationpastel-00527388, version 1 - 19 Oct 2010The strong anisotropy of the absorption probability characterizing the trans isomer contributes,together with the statistical reorientation of the molecules, to the depletion ofthe molecule orientational distribution along the light electric field direction. After atrans-cis isomerization cycle, each molecule can return to the trans state with an orientationslightly different from the initial one. Assuming that the initial orientation wasalong the polarization axis, the probability of absorption and further isomerization isreduced. This leads to a statistical re-orientation of the molecules away from the directionparallel to the light polarization, a phenomenon known as angular hole-burning,which, under linearly polarized light, causes birefringence and linear dichroism [30], thatcan be observed in viscous liquids and polymers. The photoinduced optical anisotropycan relax, in the absence of irradiation, due to Brownian motion. It can also be partiallyerased by irradiation using circularly polarized light, which favors a more isotropicorientational distribution of the molecules [31].1.3 Photoinduced deformation in azo-containing materials1.3.1 Photodeformation phenomena in polymeric matrices containingazo-derivativesThe incorporation of azobenzene derivative molecules into a polymeric matrix leads topeculiar photodeformation phenomena, which have drawn great attention in the past15 years. As it was discovered in 1995 by two research teams simultaneously [17, 18],the projection of an interference pattern on an azo-containing thin film induces theformation of Surface Relief Gratings (SRG) having the same spatial period of the opticalinterference grating and an amplitude up to hundreds of nm. An example of SRG formedusing this method on a sol-gel film containing DR1 molecules is shown in Figure 1.5.