Phase and polarization pulse shaping for nonlinear microscopy – NLO-shaping

This proposal in the Mosaic group in the Fresnel Institute (Marseille), concerns the development of a new nonlinear microscopy technique based on the spectral coherent control, in phase and polarization, of the optical excitation. This technique will be dedicated to the observation of the dynamic of local orientational orders in molecular and biomolecular samples, from ordered organic crystalline systems to the living cell. The goal of this work is to answer to the need of a more refined knowledge of molecular arrangements in processus where they are crucial, such as the dynamics of crystals formation or the characteristics of lipid domains formation in cell membranes contributing to cell recognition. This experiment will be mainly based on two-photon (second harmonic generation: SHG) and three-photon (coherent anti-stokes Raman scattering: CARS) nonlinear processes, chosen for the complementary information they bring on molecular systems. These phenomena are also well known in the team responsible for this project, through the benefit of experience I gained in my previous works on SHG and of the Mosaic group in CARS. The originality of this experience is to combine on the one hand recent advances in terms of multiphotonic microscopy and on the other hand in coherent control. In particular, it is recognized today that a polarimetric measurement of nonlinear responses is rich in information on molecular organization, and that the spectral phase profile of a short pulse can greatly influence the nonlinear processes efficiency. In this experience, we benefit from an excitation of molecular systems by a large spectrum (~120nm) from an ultra-short laser pulse (~10fs). The coherent control will be based on the shaping of the spectral profile of the pulses, in phase and polarization, using a spatial light modulator after spectral dispersion. This set-up allow to (i) optimise the excitation pathways of molecules by multiphotonic intra-pulse interferences, and (ii) profit from a polarization shaping of the excitation for a direct read-out, local and in real time, of complex molecular orientational distributions. The studied samples will be first based on simple symmetries such as in crystals, then on more complex structures such as in molecular monolayers and lipid bilayers. These structures will be doped with active molecules defined in collaboration with chemists in ENS Lyon. The last part of the project concerns the application of this imaging technique to cell membranes with the study of the dynamics of molecular order within lipid domains. This last part will be developed in collaboration with the Centre d'Immunologie de Marseille Luminy (CIML).