Déformation photoinduite dans les films minces contenant des ...

Chapter 3. Microscopic mechanisms at the origin of the photo-induced deformation inazobenzene-containing thin films 84A (nm)(a)100806040200p-assist.s-interf.s-interf.20090311_1120090310_60 500 1000 1500TIME (s)A (nm)(b)0-20-40-60-80-100s-assist.p-interfp-interf.20090206_220090309_190 500 1000 1500TIME (s)pastel-00527388, version 1 - 19 Oct 2010Figure 3.20: Enhanced matter migration using simultaneous orthogonally polarizedassisted and interfering beams. (a) s-assisted p-interfering beams; b) p-assisted s-interfering beams: time evolution of the deformation amplitude (solid curve), the dottedcurve represents the deformation amplitude without the assisting beam. Laser powerdensity: 1 mW/mm 2 per beam (blue), 2.1 mW/mm 2 per beam (green); sample =sol-gel Si-DR1 ≃ 200 nm thick.3.4.3 Identically polarized beamsThe increased efficiency of the SRG formation process is obtained when the assistingbeam polarization is perpendicular to the interference pattern polarization. This is anecessary condition, since using the same polarization for the assisting beam and theinterference pattern has a detrimental effect.In Figure 3.21 we report the case of PP+Pp-assisted p-polarized interference. In (a-c) we activate the interfering beams at t = 0keeping the assiting beam off. As expected, we observe the matrix photoexpansion inthe bright fringes. Then matter begins to migrate towards the darker zones. As mattermigration starts, the assisting beam is switched on (t = 20 s) 14 . We observe a suddenjump in the deformation amplitude produced by the photoexpansion in the dark fringesof the interference pattern, as we did in the case of s-assisted p-polarized interference(Figure 3.18). Subsequently to the effect of the photoexpansion, the deformation kineticsretrieves a rate characteristic of the photoinduced matter migration. But it isslightly lower than the non assisted case, since the contrast between the dark and thebright fringes is lowered by the assisting beam.We repeat this experiment by activating the assisting and the interfering beams simultaneously(d-f). As the beams are turned on we observe a weaker matrix photoexpansion,which is expected since the contrast is weaker when the assisting beam is kept off at14 In this case we decided to turn the assisting beam on earlier with respect to the previos experiments,since we have seen that the greatest effects of the assisting beam are measured during the first 300 s ofmatter migration.