Déformation photoinduite dans les films minces contenant des ...

Chapter 3. Microscopic mechanisms at the origin of the photo-induced deformation inazobenzene-containing thin films 79leads to a rapid small decrease in the grating amplitude. As exemplified in Figure 3.16,this is probably due to the matrix photoexpansion in the zones that were not previouslyirradiated (i.e. the zones corresponding to the dark fringes) which now are exposed tothe assisting beam. Instead, the zones exposed to the bright fringes experiment a weakerphotoexpansion, as they have already been exposed to light and, for what we have shownin the previous section, the pre-irradiation dose is sufficient to considerably reduce thephotoresponse. Then, the net gratings amplitude due to the photoexpansion is smaller,as experimentally observed.OPTICSlight intensityxxpastel-00527388, version 1 - 19 Oct 2010TOPOGRAPHYTOPOGRAPHYSRGamplitudelight intensitygratingsamplitudephotoexpansion(a)photoexpansionFigure 3.16: Photoexpansion using an assisting beam.(a) Light present in the brightfringes of the interference pattern induces the photoexpansion of the matrix, hence theformation OPTICS of the gratings. (b) When an assisting beam is superposed the previously nonirradiated zones undergo photexpansion as well, leading to a diminution in the gratingamplitude.xxSubsequently to the grating amplitude decrease, a relief growth is observed in phasewith the grating pattern previously induced by the photoexpansion, which means thatthe growth is also in phase with the bright fringes of the interference pattern.xlarge amplitude and slow rate (0.2 nm/s) of this deformation process with respect tophotoexpansion are characteristic of the photoexpansionmatter migration phenomena matter [9]. migration It is extremelyremarkable that in this case matter migrates towards the enlightened zones. A similareffect has been reported, to our knowledge, only once [8], but with no direct correlatedlight intensityto the matter migration commonly observed using p-polarized light, where matter iss sp ptransported OPTICS away from the brightest areas towards the dark ones. It indicates thatthe difference between the molecular activityp p p(i.pe.pphotoisomerzation)s s sinduceds sin thebrighter and the darker zones cannot be the only one cause at the origin of the matterp-assisteds-assisteds interference p interference(a)measurements of the optical pattern and of the surface relief.This photo-induced mass transport regime arises in the opposite direction with respectxx(b)(b)xxThexphotoexpan(c)(a)(b)