Déformation photoinduite dans les films minces contenant des ...

Chapter 3. Microscopic mechanisms at the origin of the photo-induced deformation inazobenzene-containing thin films 92experiments with different assisting beam intensities. In order to vary the polarizationmodulation from zero we consider the case of p-assisted s-polarized interference for whichthere is no modulation of the polarization when the assisting beam is off (see 3.4.1).In Figure 3.27 we show the deformation kinetics obtained on a sol-gel sample 19 (quantitativedata are reported in Table 3.3) for different power densities of the p-polarizedassisting beam, while the s-polarized interfering beams have always the same powerdensity of 1 mW/mm 2 per beam. In this experiment, the interfering and the assistingbeams are switched on simultaneously at t = 0, in order to measure the full evolution ofthe assisted photoinduced deformation. The growth rate is the slope of the deformationkinetics: for the matrix photoexpansion mechanism it is measured at t = 0, while forthe mass migration mechanism it is measured over the first 100 s of irradiation, duringwhich the deformation amplitude is quasi-linear.pastel-00527388, version 1 - 19 Oct 2010With no assisting beam (Figure 3.27(a)), the photoinduced deformation is only dueto photoexpansion, which is characterized by a high rate and a low efficiency with a maximumdeformation of ≃ 7 nm. Using 0.4 mW p-polarized assisting beam (Figure 3.27(b))is sufficient to activate the matter migration with a low SRG growth rate of 0.04 nm/s.In this case, as the light intensity contrast 20 is ≃ 90%, we measure a smaller deformationdue to the photoexpansion (≃ 4 nm). When increasing the intensity of the assistingbeam (hence reducing the contrast), the photoexpansion contribution to the deformationvanishes (it almost disappears in (d) where the optical contrast is ≃ 78%), while mattermigration is more and more efficient (d-f). At the maximum power density availablefor the assisting beam, 2.1 mW/mm 2 , where the ratio between the assisting and theincident beams power densities is ≃ 1, we obtain a higher SRG growth rate of 2.1 nm/s.Let us define the spatial modulation of the polarization as E pol = min{E‖ 0 , E0 ⊥ } (byanalogy with section 3.3.2). The parallel component is provided only by the assistingbeam 21 (E‖0 = √ I assist ) while the perpendicular component is given by the interferingbeams (E⊥ 0 = √ I interf ). For assisting beam intensities smaller than the intensity interferencepattern amplitude, we have: E pol = E‖ 0 = √ I assist . So, we expect that the efficiencyof the SRG formation increases as √ I assist .In Figure 3.28 we compare the SRG growth rate experimentally observed (□) andthe corresponding values of √ I assist .The close correlation between the experimental19 In this experiment we used a 450 nm thick sol-gel sample, as we wanted to better discriminatebetween the photoexpansion and the matter migration mechanisms. As shown in [9], the photoexpansionmechanism is dependent on the thickness, while [15] matter migration efficiency doesn’t significantlychange using a 200 nm or a 450 nm thick sample.20 Defined asImax − IminI max, where I max and I min are the maximum and the minimum intensity valuesof the total light field.21 We recall that the p-polarized assisting beam is in normal incidence on the sample.