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Application d'un imageur polarimétrique multimodal pour l'étude de la réponse optique de milieux et de microstructures diffusantes

Abstract : The work carried out during this thesis was aimed to study the interaction of polarized light from the scattering media and particles. This work is part of a strong collaborative context between the LPICM and various private and public laboratories. A wide variety of aspects have been treated deeply, including instrumental development, advanced numerical simulation and the creation of measurement protocols for the interpretation of complex data.The instrumental part of the thesis was devoted to the development of an innovative instrument, suitable for taking polarimetric images at different scales (from millimeters to microns) that can be quickly reconfigured to offer different imaging modes of the same sample. The two main aspects that characterize the instrument are i) the possibility of obtaining real polarimetric images of the sample and the angular distribution of light scattered by an illuminated zone whose size and position can be controlled, ii) the total control of the polarization state, size and divergence of the beams. These two aspects are not united on any other commercial or experimental apparatus today.The first object of the study using the multimodal imaging polarimeter was to study the effect of the thickness from a scattering medium on its optical response. In medical imaging, there is a broad consensus on the benefits of using different polarimetric properties to improve the effectiveness of optical screening techniques for different diseases. Despite these advantages, the interpretation of the polarimetric responses in terms of the physiological properties of tissues has been obscured by the influence of the unknown thickness of the sample.The objective of the work was, therefore, to better understand the dependence of the polarimetric properties of different scattering materials with the known thickness. In conclusion, it is possible to show that the polarimetric properties of the scattering media vary proportionally with the optical path that the light has traveled inside the medium, whereas the degree of polarization depends quadratically on the optical path. This discovery could be used to develop a method of data analysis that overcomes the effect of thickness variations, thus making the measurements very robust and related only to the intrinsic properties of the samples studied.The second object of study was to study the polarimetric responses from particles of micrometric size. The selection of the particles studied by analogy to the size of the cells that form the biological tissues, and which are responsible for the dispersion of light. By means of the polarimetric measurements, it has been discovered that when the microparticles are illuminated with an oblique incidence with respect to the optical axis of the microscope, they appear to behave as if they were optically active. Moreover, it has been found that the value of this apparent optical activity depends on the shape of the particles. The explanation of this phenomenon is based on the appearance of a topological phase of the beam. This topological phase depends on the path of the light scattered inside the microscope. The unprecedented observation of this topological phase has been done by the fact that the multimodal polarimetric imager allows illumination of the samples at the oblique incidence. This discovery can significantly improve the efficiency of optical methods for determining the shape of micro-objects.
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Thomas Yoo. Application d'un imageur polarimétrique multimodal pour l'étude de la réponse optique de milieux et de microstructures diffusantes. Instrumentation and Detectors [physics.ins-det]. Université Paris Saclay (COmUE), 2018. English. ⟨NNT : 2018SACLX106⟩. ⟨tel-02445525⟩

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