The forces acting on optically trapped particles are commonly assumed to be conservative. Nonconservative scattering forces induce toroidal currents in overdamped liquid environments, with negligible effects on position fluctuations. However, their impact in the underdamped regime remains unexplored. Here, we study the effect of nonconservative scattering forces on the underdamped nonlinear dynamics of trapped nanoparticles at various air pressures. These forces induce significant low-frequency position fluctuations along the optical axis and the emergence of toroidal currents in both position and velocity variables. Our experimental and theoretical results provide fundamental insights into the functioning of optical tweezers and a means for investigating nonequilibrium steady states induced by nonconservative forces.
Time domain thermoreflectance (TDTR) is a timeresolved technique aiming at evaluating electron and phonon temperatures. After a few picoseconds, when the thermal equilibrium is reached, the lattice temperature and the electron temperature are equal and an equilibrium thermoreflectance coefficient can be evaluated. In this work, we show that, whatever the probe wavelength, the thermoreflectance signal at equilibrium measured on a 50 nm gold transducer is proportional to the incident pump fluence. The thermoreflectance coefficient can then be identified for each probe wavelength, and lattice temperature variations can be deduced. At short time scales, the electrons can reach much higher temperatures than phonons and the thermoreflectance signal is mainly driven by the electronic contribution. In this nonequilibrium regime, the thermoreflectance signal amplitude is not linear with the incident pump fluence and does not follow the expected electron temperature variation. We have, however, identified a specific probe wavelength (490 nm) for which the electron thermoreflectance coefficient can be estimated. The TDTR technique then becomes finally a self-calibrated electron and phonon ultrafast thermometer.
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