Samuel Albert
Laboratoire des nanostructures, ISIS – Institut de Science et d’Ingénierie Supramoléculaires, Université de Strasbourg

” Chiral thermodynamics and optical forces ”
Optical trapping systems are an outstanding tool as a model system to study the dynamics and thermodynamics of stochastically driven phenomena. At the same time, chirality is ubiquitous in nature and has strong potential applications. The work I will present stems from the recent discovery of chiral optical forces and aims to provide a framework for using optical traps to better understand the dynamics and thermodynamics of stochastic chiral nanoparticles.

I will present an optomechanical model that describes the stochastic motion of an overdamped chiral nanoparticle diffusing in the optical bistable potential formed in the standing-wave of two counter-propagating Gaussian beams. Chiral optical environments can be induced in the standing-wave with no modification of the initial bistability by controlling the polarizations of each beam. Under this control, optical chiral densities and/or optical chiral fluxes are generated, associated respectively with reactive and dissipative chiral optical forces exerted on the diffusing chiral nanoparticle. This optomechanical chiral coupling bias the thermodynamics of the thermal activation of the barrier crossing, in ways that depend on the nanoparticle enantiomer and on the optical field enantiomorph.

I will show that reactive chiral forces, being conservative, contribute to a global, enantiospecific, change of the Helmholtz free energy bistable landscape. In contrast, when the chiral nanoparticle is immersed in a dissipative chiral environment, the symmetry of the bistable potential is broken by non-conservative chiral optical forces. In this case, the chiral electromagnetic fields continuously transfer, through dissipation, mechanical energy to the chiral nanoparticle. This modifies the chiral non equilibrium steady-state by exchanging entropy with the thermal bath. This yields chiral deracemization schemes that can be explicitly calculated within the framework of our model.

Finally, we use three-dimensional stochastic simulations to confirm and further illustrate the thermodynamic impact of chirality. I will also show how these simulations allow us to study the relation between the dynamics and the thermodynamics of our stochastic system.

Our results reveal how chiral degrees of freedom both of the nanoparticle and of the optical fields can be transformed into true thermodynamics control
parameters, thereby demonstrating the significance of optomechanical chiral coupling in stochastic thermodynamics. They pave the way for new experiments probing chirality in stochastic processes.