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

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Chapter 3pastel-00527388, version 1 - 19 Oct 2010Microscopic mechanisms at theorigin of the photo-induceddeformation inazobenzene-containing thin films3.1 IntroductionIt is widely established [1–7] that the surface relief grating (SRG) growth efficiency inducedunder polarized interference in azo-containing thin films strongly depends on thelight polarization. In particular, higher efficiencies have been observed in presence ofa polarization grating. Also, as recently shown, the SRG growth efficiency can be enhancedby combining an assisting beam with the interfering beams [3, 8]. The influenceof the polarization configuration on the SRG growth efficiency has been deduced byindirect techniques, such as diffraction efficiency measurements. However, the nature ofthe deformation and its dependence on the spatial distribution of the light field has notbeen completely established.In this chapter, we will study the SRG inscription in thin films containing azobenzenederivatives. We use the direct techniques described in Chapter 2, which allow usto correlate the topography and the optical measurements in situ, in real-time and atnanoscale. These techniques have already allowed to identify two types of photodeformationprocesses: a local matrix photoexpansion due to the free volume increase necessaryfor the azobenzene photoisomerization, and a photo-induced mass transport whose relationto the azobenzene photoisomerization is still debated.Our aim is the comprehension of the mechanisms at the origin of these photodeformation54
Chapter 3. Microscopic mechanisms at the origin of the photo-induced deformation inazobenzene-containing thin films 55phenomena. First, we study the photodeformation produced by matrix photoexpansionand we evidence a plastic and an elastic contributions. Second, we study the polarizationand intensity effects on the matter migration phenomena, in both sol-gel Si-DR1 andPMMA-DR1 samples. We identify the coexistence of two distinct microscopic mechanisms,one driven by the light polarization distribution and the other one by the lightintensity pattern. We then demonstrate that the relative efficiency of these mechanismsdepends on the nature of the matrix bearing the chromophore. This leads to coherentlyexplain several observations in the far and in the near field, not completely understoodup to now.pastel-00527388, version 1 - 19 Oct 20103.2 Elastic and plastic matrix deformation induced by theazobenzene photoisomerizationIt has been shown in a previous work [9] that the first stages of the SRG formationunder illumination are characterized by a very fast deformation of moderate amplitude.This deformation process is attributed to a matrix photoexpansion, resulting from theincrease of the molecular free volume due to the photoinduced interconversion betweenthe trans and the cis isomers. Photoexpansion has been found to depend on the lightintensity and to arise for any polarization state of the exciting light.Figure 3.1 shows the first stages of the SRG formation kinetics measured with near-fieldtechniques upon illumination with an s- and a p-polarized interference patterns. Thesample is a sol-gel Si-DR1 film having a thickness of 200 nm. It is illuminated throughthe substrate by two interfering beams (Figure 3.1(a,e)) having a 1 mW/mm 2 powerdensity and a wavelength λ = 473 nm, with an angle of incidence at the sample surfaceθ = 16.5 ◦ , in air. The interference pattern obtained in this configuration has a spatialperiod of 830 nm. It is described in detail in Appendix A for the different configurationsof the light polarization, in particular, for the s-polarization, which correspondsto the electric field along the y-axis, and for the p-polarization, which corresponds tothe electric field parallel to the incident plane. In (b-c) we show the optical and topographicalimages simultaneously obtained by Scanning Near-field Optical Microscopyand Shear-force microscopy, respectively, under illumination with an s-polarized interferencepattern. The horizontal axis corresponds to the x-axis, while on the verticalaxis we represent the time t. The corresponding time-evolution of the SRG amplitudeis reported in (d). The same is shown in (f-h) for the experiment performed underp-polarized light. The time-evolution of the SRG amplitude is directly measured bythe topography profile along a scanning line across the interference pattern. Over thescanning width, N interference periods are measured, each one producing a dark and
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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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Page 22 and 23: Introduction 6azobenzene-containing
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pmmaChapter 3. Microscopic mechanis
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Chapter 3. Microscopic mechanisms a
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Chapter 4. Photomechanical response
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sg+goldChapter 4. Photomechanical r
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P-250s-4VChapter 4. Photomechanical
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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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Bibliography[1] A. Natansohn, P. Ro
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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[1] A. Natansohn, P. Ro
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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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