Exploring the Dynamics of Strongly Scattering Nano-Particles in Optical Traps

Since they were first reported in 1970, optical tweezers have proved to be a powerful tool for control and positioning of micro-and nanoscale particles and for the understanding the physical processes that drive them. Optical tweezers have been applied across a wide range of disciplines and led to exciting discoveries in biology, material science, condensed matter physics and more. A particle trapped by a laser beam will experience  a force that is dependent on laser beam’s intensity and phase gradient as well as on the dielectric coefficient of the particle itself. Thus, by controlling the amplitude and phase of the incident beam one can control the behavior and dynamics of a trapped particle. When multiple particles are captured in the trap, especially when the particles are strongly scattering (e.g. a plasmonic nanoparticle), we observe strong inter-particle interactions that reshape the trapping potential.

In this talk, I will discuss experimental results that demonstrate how the inter-particle scattering profoundly affects the behavior of trapped Ag nanoparticles, and how it can give rise to surprising physical phenomena. I will demonstrate how the introduction of angular momentum in the form of circularly polarized light can bind particle ensembles and cause them to rotate in a direction that is determined by inter-particle separation and dictated by interference effects. I will also show how nanoparticle dimers composed of dissimilar particles that are trapped in a uniform 1D potential, experience “non-reciprocal forces” resulting in super-diffusive motion of the center of mass of the bound pair. This unintuitive motion, which is a result of imbalance in the light scattered from the dimer is an observation of linear momentum transfer.