Déformation photoinduite dans les films minces contenant des ...

TOPOGRAPHYgratamplxxphotoexpansion photoexpansionChapter 3. Microscopic mechanisms at the origin of the photo-induced deformation inazobenzene-containing thin films(a)(b)82OPTICSlight intensityx x xTOPOGRAPHYgratingsamplitudexxxphotoexpansion(a)matter migration(b)photoexpansion(c)pastel-00527388, version 1 - 19 Oct 2010Figure 3.18: Photoexpansion using an assisting beam: s-assisted p interference. (a)Light located in the bright fringess sof the interferenceppatternpinduces the photoexpansionof the matrix, hence the formation of SRG. (b) Matter migrates towards the darkOPTICSzones. (c) When an assisting p p beam p p is p superimposed s s tos the s s interfering beams the previouslynon irradiated zones undergo photexpansion xas well, leading x to an increase inthe SRG amplitude. p-assisteds-assisteds interference p interferencelight intensitycomparison of the two polarization(a)configurations in the(b)same conditions. We report thedeformation kinetics in Figure 3.19 13 .In the case of p-assisted s-polarized interference (Figure 3.19(a-c)),at t = 0 when PP+S,the 3 beams are turned on we observe a weak matrix photoexpansion (≃ 1 nm), dueto the weak light intensity contrast, followed by the same matter migration behavior aspreviously described: an efficient relief growth , with a SRG growth rate of ≃ 0.2 nm/s.For the s-assisted p interference configuration (Figure 3.19(d-f)), once the light excitationon, a similar weak matrix photoexpansion (≃ 1 nm) is observed in the enlightenedzones.Then, matter starts migrating away from the bright areas, towards the darkzones. In this case, the migration rate is ≃ 0.23 nm/s which is almost the double withrespect to the non assisted case (dotted curve).SS+Pno delayFigure 3.20 compares the growth kinetics obtained over a long exposure time for bothpolarization configurations. It is remarkable that, in the case of s-assisted p-interferingbeams, the acceleration of the grating formation observed is effective mainly in the first300 s. At long time, the growth reaches approximately the same efficiency as in the nonassisted case.13 The contrast of the optical image for the p-assisted s-polarized interference configuration is ratherpoor, which is probably due to an anisotropy of the tip.