DÃ©formation photoinduite dans les films minces contenant des ...

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Chapter 3. Microscopic mechanisms at the origin of the photo-induced deformation inazobenzene-containing thin films 98Bessel beam (e-f) the distribution is inverted. In the particular case of an azimuthallypolarized Bessel beam (i-j), the ⃗ E ⊥ component is zero.In Figure 3.31(c,g,k) we report the results that have been obtained on a PMMA-DR1 thin film having a thickness of 100 nm, using a 200 mJ/mm 2 light dose, in thethree polarization configurations, respectively. Under radial and circular polarizations,a relief growth is observed always in correspondence of the ⃗ E ⊥ component, while matteris driven away from the ⃗ E ‖ component. This is the same behavior that we have observedin our experiments, confirming that the orientation of the matter migration is strictlydependent on the spatial distribution of the field polarization.pastel-00527388, version 1 - 19 Oct 2010Only in the case of an azimuthally polarized Bessel beam (i-l), the observed matteraccumulation is associated to the E ⃗ ‖ component. In this case an extremely weak deformationis produced (≃ 2 nm). We note that in this polarization configuration onlythe E ⃗ ‖ component is present, since the E ⃗ ⊥ = 0, so the amplitude of the spatial modulationof the polarization E pol = 0. This is the symmetric configuration with respectto the s-polarized interference that we have used in our experiments, where E ⃗ ‖ = 0 andE pol = 0. In this case, we observe also a weak relief growth in phase with the lightintensity, which is due to the matrix photoexpansion. This suggests that the matteraccumulation observed in the bright zones using an azimuthally polarized Bessel beamcould be due to the matrix photoexpansion as well.Local probe illumination and apertureless SNOMIn the experiments involving near-field illumination through the aperture of a metallizedoptical fiber (≃ 40 nm aperture [21]), matter accumulation is observed in the enlightenedzones ([4, 22]). The irradiating light field is transversally polarized ( E ⃗ ‖ ) and farfrom the tip apex (in the bulk and in the periphery) the light polarization is essentiallytransversal. However, a longitudinal component along the z axis ( E ⃗ ⊥ ) is present in thevicinity of the tip apex [18]. So, matter accumulates towards the area where the E ⃗ ⊥ ismaximum and away from the transversally polarized light, which is the same behaviorthat we observe in our experiments.Similar results have been obtained in the experiments using irradiation by aperturelessSNOM [23, 24]. When p-polarized light is concentrated, in near field, using a laser beamincident on a metal coated tip brought close to the sample’s surface, the growth of arelief is observed in the vicinity of the tip. When s-polarized light illuminates the tip,the effect is qualitatively similar, but much weaker then using p-polarized light. This isalso coherent with our observations, since in this case the light field has almost only one
Chapter 3. Microscopic mechanisms at the origin of the photo-induced deformation inazobenzene-containing thin films 99component, along the s direction.We conclude that the photodeformation phenomena in sol-gel Si-DR1 thin films aredependent not only on the spatial distribution of the light intensity but also on thedistribution of the field polarization. In particular, the latter governs the efficiencyand the orientation of a directional reversible matter migration process. Accordingto the field spatial distribution, this can induce matter accumulation towards thezones where the field polarization is perpendicular to the intensity gradient, or candrive matter away from the field components parallel to the intensity gradient. Theseresults are in agreement with several observations reported in the far and in the nearfield and allow to coherently interpret a certain number of unexplained phenomena.pastel-00527388, version 1 - 19 Oct 20103.5 Matter migration phenomena in PMMA-DR13.5.1 Matter migration under s- and p-polarized lightGenerally, photoinduced deformation in azo-containing materials are performed in polymermatrices like PMMA. The results reported so far show that under s-polarized interferencevery weak photoinduced deformations occur. This is consistent with ourobservations in sol-gel Si-DR1 thin films and we may interpret the weak photoinduceddeformation has being due to matrix photoexpansion. However, in a previous study [15]photoinduced matter migration has been observed under the exposure to s-polarizedinterference.We have used the techniques presented in the previous sections to study the photomechanicalphenomena in the PMMA-DR1 material described in section 2.2.3. In Figure3.32 we show the photoinduced deformation obtained on a PMMA-DR1 sample180 nm thick under irradiation with an interference pattern using a very weak powerdensity, 70 µW/mm 2 per beam (blue laser, λ = 473 nm), for both s and p polarizations26 . In (a-b) we show the optical and topography images, obtained scanning overthe same line with a range of 5 µm, of the first ≃ 200 s of irradiation using s-polarizedlight. In (e-top) is plotted the average transmitted light intensity corresponding to (a)while in (e-bottom) is reported the average profile evolution corresponding to (b). Thesame is shown for p-polarized light (c-d) and (f).At t = 0, turning the laser on, we observe in both cases the growth of a weak relief(≃ 1 nm) in phase with the bright fringes of the interference pattern, which can be26 The SNOM signal of the optical image for the p polarization is very weak, which might be due tothe anistropic sensibility of the tip for s and p polarizaitions, which is exalted at low light intensitites.
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Thèse de doctorat de l’Ecole Pol
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AcknowledgementsAt the beginning it
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vpastel-00527388, version 1 - 19 Oc
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ContentsAbstractiAcknowledgementsii
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List of Figures1.1 The trans/cis ph
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List of Figuresxiipastel-00527388,
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Introduction 2In this work, we stud
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Bibliography[1] A. Natansohn, P. Ro
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Introduction 6azobenzene-containing
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"##$%&'()')*#'+(%,-.(/#'011'2'"(%)(
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Chapter 1. The azobenzene molecules
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Chapter 2. Experimental setup and s
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Chapter 5. Nanostructured hybrid sy
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Conclusionspastel-00527388, version
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Bibliography[1] Chi-Ming Che et Al.
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Appendix A 156⎛ ⎞ ⎛n g sinθ
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Appendix A 158hence for each compon
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Appendix BPhotoisomerization dynami
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Appendix B 162( 1Since λ 2 = −(
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Appendix C 164L = 5000 nmu = 1 nmL!
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Appendix D 166n. light power densit
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Bibliography 170azobenzene-containi
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Bibliography 172as estimated from a
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Bibliography 174[53] C. Barrett, A.
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