Déformation photoinduite dans les films minces contenant des ...

Chapter 3. Microscopic mechanisms at the origin of the photo-induced deformation inazobenzene-containing thin films 57intensity distribution is represented 1 with a negative sign.Matrix photoexpansion is observed under both s-polarized (Figure 3.1(a-d)) and p-polarized (e-h) interference patterns. As the illumination is turned on (at t = 0), itproduces the rapid growth of a SRG of weak amplitude, spatially in-phase with theillumination. This SRG growth kinetics corresponds to the initial positive rise of thedeformation amplitude A(t). In the case of p-polarized light, subsequently to the photoexpansion,we observe the rapid decrease of A(t) which becomes negative and reacheslarge absolute values. This corresponds to the growth of a new SRG half-a-period spatiallyshifted with respect to the interference pattern. This phenomenon corresponds toa photo-induced lateral matter motion which takes place between the bright and darkfringes. Note that, instead, matter migration is not observed under s-polarized light, ina sol-gel Si-DR1 sample.pastel-00527388, version 1 - 19 Oct 2010Shutting down the illumination, the deformation induced by the matrix photoexpansionhas been observed to be irreversible while a part relaxes [10, 11]. In order to discriminatebetween the plastic and the elastic contributions of the photodeformation we havemeasured the amplitude of the SRG that forms subsequently to a pre-irradiation of thematerial with a uniform laser beam. In this way, the matrix photoexpansion is first generatedover the whole enlightened area. Then, while projecting an interference patternhaving the same polarization, the photoexpansion is further induced only in the brightfringes, giving rise to the formation of a SRG. For different pre-irradiation doses, theamplitude of the reliefs should vary as a function of the residual deformability of thematerial. In this sense, the projection of an interference pattern is used to quantify thecontribution of the plastic deformation to the SRG formation.So, we have performed the same experiment as presented in Figure 3.1 but for differentpre-irradiation doses. In Figure 3.2 we show the photodeformation results obtained onthe same sol-gel Si-DR1 sample for different uniform pre-irradiation doses using a singlebeam. The experience is performed in two steps, as schematized in Figure 3.2(a-b).First, we pre-irradiate the sample, during a time t, using only the right beam. Thisinduces a uniform matrix photoexpansion. Then, we wait 20 s, allowing eventually arelaxation of the deformation. Subsequently, we turn on both beams to project theinterference pattern. The amplitude of the SRG obtained during the interference patternprojection is shown in Figure 3.2(c) for both s (top graph) and p (bottom graph)polarizations. For different pre-irradiation doses, the subsequent projection of an interferencepattern always induces matrix photoexpansion in the bright fringes. However,the amplitude of this deformation decreases for increasing values of the pre-irradiationdose. The same behavior is observed for both polarizations.Note that, with p-polarized light, subsequently to the photoexpansion, matter migration1 We consider only these two cases as we do not need to describe other spatial relationships.