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Imagerie nanométrique ultra-rapide par diffraction cohérente de rayonnement XUV produit par génération d'harmoniques d'ordre élevés

Abstract : The aim of this dissertation is to develop new lensless single shot imaging technique in 2D and 3D with XUV harmonic sources which can be applied to study biological objects and phase objects. Firstly, we introduce the theory underlying lensless imaging techniques and we describe the methods used during this thesis to reconstruct the light field diffracted by the studied object. The imaging techniques are split in two catego- ries : iterative and holographic. The iterative methods reconstruct the phase of the diffracted wave front using constraints in the Fourier space and the reel space. With the holographic techniques, the phase is encoded directly in the interference fringes between the reference and the object within the diffraction pattern. We discuss the experimental parameters required to achieve an image reconstruction and we compare the respective advantages of the two types of method. Then, we describe the experimental parameters of the XUV beam produced by high harmonic generation (HHG) and we briefly explain the theory of the HHG. The next section discusses the para- meters the quality of the reconstructed image. We show how to improve the resolution and the signal to noise ratio using the HERALDO technique in the low flux regime. We then show the result of a new technique for the single shot characterization of the spatial coherence of XUV beams. Indeed, the spatial coherence is a critical parameter for coherent diffractive imaging techniques. Using a NRA of reference holes, we measure the spatial coherence for each distance between each pair of holes, without the knowledge of the intensity distribution on the sample. We show that the spatial coherence has a gaussian distribution and that its diameter varies according to the generation parameters of the harmonic beam. We also study quantitatively the effect of multi-shots accumulation of the diffraction pattern on the apparent coherence of the beam. We also show the result of phase object imaging using coherent diffractive imaging with a harmonic source. To our knowledge, this if the first time such result has been achieved. The rest of the dissertation present new lensless imaging 3D techniques using harmonic sources. The first of the last two experiments shown is a lensless single shot stereo 3D technique. It is the first one allowing a 3D reconstruction from a single acquisition, with a nanometer spatial resolution and a femtosecond temporal resolution, without using a priori knowledge of the sample studied. This method has a vast spectrum of appli- cation and is particularly interesting for the structural study of biological sample sensitive to radiation damage and for the study of non-reversible dynamical phenomena in 3D. Furthermore, this can easily be implemented in FELs and synchrotrons to reach even better spatial resolution. The second 3D experiment shown in this thesis is a proof of concept of cryptotomography using a high harmonic source in a low ux regime. To reconstruct the 3D volume of the sample, cryptotomographie uses diffraction pattern acquired for unknown sample orientations and therefore non-classified. The low flux regime used here simulate the flux of a harmonic source generated in the water window. We conclude from this experiment that, with the proper experimental conditions, the diffraction signal is sufficient to allow the classiffcation by orientation of the diffraction patterns. With enough diffraction pattern and angles of the sample recorded, we can achieve a 3D reconstruction of the sample. This result implies that the cryptotomography of biological objects using a water window harmonic source is possible.
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Submitted on : Thursday, December 14, 2017 - 11:06:15 PM
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Rémy Cassin. Imagerie nanométrique ultra-rapide par diffraction cohérente de rayonnement XUV produit par génération d'harmoniques d'ordre élevés. Optique [physics.optics]. Université paris sud 11, LRI; Paris Saclay, 2017. Français. ⟨tel-01664520⟩

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