Déformation photoinduite dans les films minces contenant des ...

Chapter 3. Microscopic mechanisms at the origin of the photo-induced deformation inazobenzene-containing thin films 87Assist. Interf. Photoexpansion Matter migration Matter migr.pol. pol. in phase π − phase in phase π − phase enhancementS S ✔ ✕ ✕ ✕ ✕P P ✔ ✕ ✕ ✔ ✕P S ✔ ✕ ✔ ✕ ✔S P ✔ ✕ ✕ ✔ ✔Table 3.2: Polarization dependence of the assisted photoinduced deformation phenomena.the photoresponses for the two orthogonal configurations p-assisted s interference ands-assisted p interference have opposite spatial phases. This can be clearly seen in Figure3.23 where we show that, for an identical intensity profile, matter is driven (a)towards or (b) away from the bright fringes. It is remarkable that in both cases matteraccumulates in the zones illuminated with a E ⊥ field component, while it is driven awaypastel-00527388, version 1 - 19 Oct 2010from the zones illuminated with a E ‖ field component 15 .By spatially modulating in real-time the interference pattern polarization and keepingconstant its intensity profile, we expect to obtain an oscillating surface deformation,that is, a reversible, alternating, directional mass migration.3.4.4 Directional reversible matter transport driven by the polarizationof the lightWe use the experimental setup described in Figure 3.24. A half-wave plate calibratedat our working wavelength (blue, λ = 473 nm) is mounted on a motorized rotation,which is used to switch the polarization of the interfering beams between the s and thep states. Another halfwave plate is used to correct the phase, after the beam-splitter, ofthe reflected beam with respect to the transmitted beam. In fact, we measure an opticalphase shift switching from p- to s-polarized interfering beams, which is probably due toa different path length of the reflected and the transmitted beams at the interface of thebeam-splitter. In this way, when changing the polarization, the spatial distribution ofthe dark and bright zones will remain constant, for s and p polarization configurations.A p-polarized beam in normal incidence is used to assist the interfering beams 16 .In Figure 3.25 we report the photodeformation obtained on a sol-gel sample, 200 nmthick. At t = 0 the exposure to s-polarized interfering beams induces the photoexpansion15 For the definition of E ⊥ and E ‖ see section 3.3.2.16 The most efficient configuration for this experiment would be to switch between p-assisted s interferenceand s-assisted p interference. Due to the lack of a second motorized rotation to change thepolarization of the assisting beam, we used a fixed p-polarized assisting beam, which only slightly reducesthe matter migration efficiency in the short time range (see Figure 3.21).