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 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.
TOPO0-3sA = h(x brights) ! h(x dark)0 1 2 3 4 5Chapter 3. Microscopic mechanisms at the origin of the photo-induced deformation inazobenzene-containing thin films 77x bright84OPTICSIntensity gratingAssisting beam intensity00 1 2 3 43 x dark0A = h(x bright) ! h(x dark)TOPO-30 1 2 3 4 5Figure 3.14: Deformation amplitude convention.3.4 Photoinduced SRG formation with an assisting beampastel-00527388, version 1 - 19 Oct 2010We investigate different polarization configurations, where an incoherent beam is superimposedto two interfering beams. In this way, we keep fixed the light intensity patternand we vary only the spatial distribution of the light polarization. In order to studythe directionality of the matter transport we use the intensity pattern spatial distributionas absolute spatial reference, by using the following convention for the deformationamplitude A:A = h(x bright ) − h(x dark ) (3.30)where x bright and x dark are defined in Figure 3.14. So, A > 0 indicates matter migrationtowards the brighter fringes, while A < 0 indicates matter transport towards the darkerfringes.3.4.1 p-assisted s-interfering beamsWe use a green 12 (λ = 532 nm) laser beam in normal incidence on the sample to assisttwo blue (λ = 473 nm) interfering beams. Figure 3.15(a) shows the setup in the caseof a p-assisted s-polarized interfering beams. In (b) are reported the time evolution ofthe optical input transmitted intensity I (top) and the corresponding grating formationdynamics (bottom - solid curve); the dotted curve indicates the deformation obtainedwithout the assisting beam, on the same sample in the same conditions. The completeoptical and the topography images are shown in (c) and (d).At t = 0 the s-interfering beams are turned on, keeping the assisting beam off. Thisproduces the rapid (0.4 nm/s), modest deformation amplitude (≃ 3 nm) characteristicof the matrix photoexpansion in the zones corresponding to the bright fringes, i. e. thedeformation is in phase with the interference pattern. Turning on the single green beam12 Laser 2000 TM , maximum nominal power density = 21 mW/mm 2SS+P
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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 4. Photomechanical response
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P-250s-4VChapter 4. Photomechanical
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Chapter 5Nanostructured hybrid syst
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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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