Déformation photoinduite dans les films minces contenant des ...

Chapter 3. Microscopic mechanisms at the origin of the photo-induced deformation inazobenzene-containing thin films 76pastel-00527388, version 1 - 19 Oct 2010Our results address a fundamental problem in the comprehension of the mechanism atthe origin of the photodeformation phenomena observed in azo-containing materials. Itis well established [1] that these phenomena depend on the photoisomerization of theazobenzene units. The rate of the isomerization events is dependent on the intensityof the light field, which has been so far considered as prevalently governing the photodeformation.Indeed, we observe the maximum of the SRG growth efficiency whenthe light field intensity is constant and the spatial modulation of the polarization ismaximum. On the other hand, when the light polarization is not spatially modulatedthe SRG growth efficiency drops to its minimum, even if the light intensity is spatiallymodulated. This indicates a preponderant effect of the light field rather than of the lightintensity. Our findings explain the highest SRG inscription efficiency observed so far inthe experiments performed using a polarization interference pattern (±45 ◦ or circularlyleft and right) [3, 12–14]. Note that a polarization interference pattern is obtained byorthogonally polarized interfering beams. When using s- and a p-polarized interferingbeams, a polarization pattern of constant intensity is also obtained, but the deformationefficiency vanishes [15]. This is due to the fact combining s- and a p-polarized beamsleads to a spatial modulation of the polarization between E ⃗ ⊥ and the E ⃗ ‖ componentsequal to zero: E pol = 0, according to eq. 3.28.Let us remark that, since mass transport is governed by the polarization spatial distribution,it is not straightforward to relate the rate of photoisomerization events with thematter migration efficiency. Another interesting remark is that the deformation amplitudesign is always positive (see Figure 3.7). By definition (see Figure 3.6) this meansthat the SRG growth is always in phase with the s-polarized interference pattern. In ourexperiments the light field is constituted by the superposition of an s-polarized and a p-polarized interference patterns. In particular, as shown in Figure 3.10, the s-polarizedfield component (i.e. polarized along the y-axis) is superimposed to the z-polarized fieldcomponent, while the x-polarized field component is Λ/2 spatially shifted 11 . So, matteris driven towards the superimposed y and z components (the E ⊥ field component), whileit is driven away from the x-polarized component (the E ‖ field component).In order to verify the influence of the light polarization on the directionality of the mattermigration we should project a fixed intensity profile and change the polarization ofthe light. This can be obtained by supporting to the two interfering beams with anassisting beam, which is the object of the next section.11 Where Λ ≃ 830 nm is the interference pattern spatial period.