Collective Motional Temperature

We introduce the concept of collective motional temperature and analyze its time evolution in a dense system of interacting nanoparticles. Using an experimental approach that provides the spatial and temporal resolution necessary to study nonequilibrium phenomena in large-scale dynamic systems, we examine the colloidal medium under the simultaneous influence of gravity and an external optical field. The optical field is dynamically modified by the colloidal particles and therefore mediates their many-body interactions. We find that the time evolution of the number density of the particles is mainly affected by the action of gravity and radiation pressure, while the collective motional temperature is determined by the dynamic optical speckle field. We show that, in agreement with recent studies on one isolated particle, the effective heating and cooling times are not equal for an interacting many-body system, either. We also show how the collective motional temperature depends on the intensity of the external optical field.