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

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Chapter 1. The azobenzene molecules: photochromic materials and photodeformationeffects 12azo-containingthin film!"p-polar.(a)(b)pastel-00527388, version 1 - 19 Oct 2010(c)Figure 1.5: Photoinduced SRG on an azo-containing thin film under p-polarized light.(a) Scheme of the inscription experiment by projection of an interference pattern; (b)shear-force image of the SRG growth (3D representation); (c) SEM image of the SRG.The study of the SRG formation has been performed using indirect methods such asdiffraction efficiency measurements, and the actual surface relief pattern was measuredonly by ex-situ topography imaging such as AFM. Various parameters have been foundto contribute to the photoresponse of the material, some can be related to properties ofthe material, others are characteristic of the illumination.Material The presence of the azobenzene group seems to be a necessary condition forthe SRG formation. Several studies [18, 32] have shown that the substitution withother chromophores inhibits any photodeformation. Indeed, the concentration andthe type of incorporation (host/guest, grafting, etc.) of the dye in the materialaffects the deformation efficiency [2].an important role.Also the matrix structure seems to haveIt has been demonstrated [33] that in polymeric matrices,the increase of the polymer chain molecular weight slows down the deformationdynamics. Finally, it has been observed [34] that the presence of a thin polymericlayer (≃ 1 nm) on the sample surface diminishes the photodeformation amplitude,which is completely inhibited for a layer thickness of 20 nm. This seems to indicatethat a free surface is a necessary condition.Illumination The photodeformation can be induced using rather low field power densities,of the order of the mW/mm 2 . It has been observed that the deformation grows
Chapter 1. The azobenzene molecules: photochromic materials and photodeformationeffects 13with the exposure light dose, either under illumination with interfering beams[10, 16–18] and single focused beams [35, 36]. On the other hand, in relation tothe light field polarization configuration various different deformation mechanismsseem to be activated, which result in different SRG growth amplitudes, speed andspatial distribution with respect to the optical intensity.Since the light field polarization has been observed to be a critical parameter influencingthe photodeformation phenomena, we will briefly resume the principal observations sofar reported under various polarization configurations.1.3.1.1 The dependence on the polarizationpastel-00527388, version 1 - 19 Oct 2010Several studies have shown that the SRG formation is very weak under s-polarizedlight, while a higher efficiency is observed using p-polarized light [5, 10, 16, 37, 38].In particular, under projection of an intensity gradient, by illumination either with afocused gaussian beam or with an interference pattern, the resulting deformation is inπ − phase with the illumination: reliefs are formed in the dark areas, while grooves areinduced in the bright zones. Under illumination with a polarization interference pattern(±45 ◦ or orthogonally circularly polarized interfering beams) the highest deformationefficiency is observed [37–39]. This indicates that an intensity gradient is not a necessarycondition, while the effect of the polarization seems preponderant. On the other hand, aquite weak deformation is observed under an s- and a p-polarized interfering beams withthe formation of a double period SRG. Very recently, it has been demonstrated, using thesuperimposition of an interference pattern to an homogeneous, incoherent p-polarizedassisting beam, that the deformation efficiency obtained under s-polarized interferenceis increased [10], with a relief growth in phase with the light field [40]. This supportsthe critical influence of the light field polarization on the photodeformation phenomena.1.3.1.2 Photodeformation mechanisms: interpretationsThe understanding of the mechanisms at the origin of these phenomena is still debated.The observation of a deformation in π − phase with the illumination has suggestedthe existence of a mechanism of matter migration, due to the photoisomerization ofthe azobenzene molecules induced in the enlightened zones [2]. The mechanical workdone during each interconversion would produce an overall transport of matter, whichis accumulated in the darker zones, where the molecules are not photoactivated. Thisinterpretation is the most shared, even if hardly compatible with the high efficiency observedunder irradiation with a polarization interference pattern, and several modeling
Page 1 and 2: Thèse de doctorat de l’Ecole Pol
Page 3 and 4: AcknowledgementsAt the beginning it
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Page 7: ContentsAbstractiAcknowledgementsii
Page 10 and 11: List of Figures1.1 The trans/cis ph
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Page 18 and 19: Introduction 2In this work, we stud
Page 20 and 21: Bibliography[1] A. Natansohn, P. Ro
Page 22 and 23: Introduction 6azobenzene-containing
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Page 26 and 27: Chapter 1. The azobenzene molecules
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dose (mJ!mm )(a)dose (mJ!mm )(b)dos
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Chapter 3. Microscopic mechanisms a
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TOPOGRAPHYgratamplxxphotoexpansion
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pmmaChapter 3. Microscopic mechanis
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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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