  As a first approximation, the forces acting on optically trapped particles
are commonly assumed to be conservative. The influence of the nonconservative
force has been shown to be negligible in overdamped liquid environments.
However, its impact in the underdamped regime remains unexplored. Here, we
experimentally study the combined effects of gradient and scattering forces on
the nonlinear dynamics of trapped particles at various air pressures, leading
to significant low-frequency noise excess along the optical axis and the
emergence of toroidal Brownian vortices. Our results supported, by numerical
simulations and a theoretical model, provide fundamental insights into the
functioning of optical tweezers and a means for investigating nonequilibrium
steady states in the presence of nonconservative forces.
