Hight symmetry orders probed by polarized coherent Anti Stokes Raman Scattering and Four Wave Mixing
Résumé
Molecular organization at the submicron scale is of a great interest in crystals, bio-mineral structures or tissues, in which pathologies (skin lesion, neurodegenerative diseases, etc) can induce strong modifications in proteins' organization. In this context, Second Harmonic Generation (SHG), Four Wave Mixing (FWM) and Coherent Raman Scattering (CARS), are powerful investigative tools to image specific objects without the need of labelling or staining. While numerous studies have been undertaken using polarization resolved second order nonlinear optical microscopy which is only specific to non-centrosymmetric organizations [1], higher order effects have been less explored. FWM microscopy and its resonant counterpart CARS can be of a great utility as label free diagnosis tools benefiting from less constraining symmetry rules [2]. In this work, we implement incident polarizations tuning in FWM and CARS to probe molecular order of the sample, using a generic method to read-out symmetry information. The polarization resolved FWM/CARS signal is decomposed in Fourier series which allows fast data processing. Then, a generic model is used to retrieve the symmetry orders of the molecular orientational distribution present in the excited volume. This method does not require a priori knowledge of the organization structure and, in contrast to SHG [3], provides quantitatively its second and fourth order symmetries. Such high order determination can be advantageously used for a more refined analysis of supra-molecular assemblies in complex media. We demonstrate the potential of this technique on different systems, from crystalline to less organized soft matter (Fig.1). In crystals, we confirm the possibility to use this technique as a symmetry read-out with a visible correlation between the retrieved information and unit-cell structure. In multilamellar vesicles (MLV), we demonstrate that the global lipid organization (FWM) and the C-H bonds (resonant CARS) organizations have distinct orientational degrees of freedom.