12 Juillet – Thesis defense - Lorenzo Mauro

14 h30 Amphi 3 / Building A9 (University of Bordeaux - Talence campus)

Charge-transfer chemical reactions and chiral discrimination in electromagnetic Fabry-Pérot cavities.

The first part of the thesis concerns an investigation on the chemical reactivity of molecular populations confined inside a nanofluidic Fabry-Pérot cavity. Due to strong light-matter interactions developing between a resonant electromagnetic cavity mode and the electric dipole moment of the confined molecules, a collective Polariton excitation is formed. The former gets dressed by environmental vibrational and rotational degrees of freedom of the solvent. We call the resulting Polariton dressed by its cloud of environmental excitation a “Reacton”, since it further undergoes chemical reactions. We characterize how the Reacton formation modifies the kinetics of a photoisomerization chemical reaction involving an elementary charge-transfer pro-
cess. We show that the reaction driving force and reorganization energy are both modulated optically by the reactant concentration, the vacuum Rabi splitting, and the detuning between the Fabry-Pérot cavity frequency and targeted electronic transition. Finally, by computing the ultrafast picosecond dynamics of the whole photochemical reaction, we predict that, despite optical cavity losses and solvent-mediated nonradiative relaxation, measurable signatures of the reacton formation can be found in state-of-the-art pump-probe experiments.
The second part focuses on the chiral discrimination in Fabry-Pérot cavities. A Fabry-Pérot interferometer filled with chiral molecules has a well defined Differential Circular Transmission (DCT) signal under normal illumination of circularly polarized light. Based on this figure of merit, we firstly provide analytical and numerical evidence that traditional Fabry-Pérot interferometers cannot enhance the chiroptical response of molecules, because the mirrors perfectly convert the circular polarization of light rays. We hence propose and model an helicity-preserving cavity, with chiral mirrors, satisfying time-reversal symmetry. The empty cavity, made by the modelled chiral mirrors, generates a spectral helicity-preserving region which is not available with a traditional interferometer. The region breaks, in a small frequency range, the perfect internal conversion of polarized light and thus it enables to discriminate the otherwise achiral cavity light modes. Polaritons which are generated at resonance with the preserving region thus inheritates a partial chiral character. By subtracting the helicity-preserving cavity contribution to the total DCT, we show that our proposed setup is up to two orders of magnitude more sensitive with respect to the chiroptical response of the isolated molecules. We reveal that the registered enhancement is consistent even for an extremely weak molecular chiroptical responses, and that our setup can enhance the chiral signal of molecules either in the weak or in the strong coupling (Polaritonic) regime.

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