Special AMO Seminar: Laure Mercier, Institut NEEL CNRS/UGA, “Multimode nanomechanics in conservative and non-conservative force fields”

Force sensors based on nanomechanical oscillators do not only exhibit exceptional sensitivities, they are also very readily affected by the vectorial character of the investigated force fields. This particularly holds for nearly-degenerate multidimensional oscillators. In the context of force measurement, we study the phenomenology of the mechanical dressing of a 2D oscillator - a singly clamped SiC nanowire oscillating along two transverse directions with quasi-degenerated eigenfrequencies - by a 2D force field. Our analytical model for this dressing reveals a new physical richness specific of dimensions greater than one. To investigate these predictions experimentally, we developed an optical method to measure all the components of the nanowire motion in 2D. This concept was first proved by mapping out the 4 vectorial components of a 2D electrostatic force field. All predicted signatures were observed, among which the rotation the mechanical polarizations basis due to shear components of the force field. One component was however missing from this field: the non-conservative, rotational component. We then adapted the system to characterize the static, non-zero rotational force field applied by a laser beam focused on the nanowire. Dramatic modifications of both the probe thermal noise and driven dynamics of the nanowire were observed, as the eigenmodes lose their initial orthogonality. The system is driven out of equilibrium by the non-conservative force, which results in a violation of the fluctuation-dissipation relation. We then approached the thermodynamic description of this system from a geometrical angle and derived a patch of the FDR for multidimensional rotationally-coupled systems. Finally, we suggest that the system can be used both as an ultrasensitive measurement technique for imaging 2D force fields and as a simple platform to test stochastic thermodynamic equalities out of equilibrium.