Non-conservative Optical Forces on Nanoparticles
May 1, 2012 - 11:00am
We will address some basic questions related to the light forces on small (Rayleigh) particles, which are usually described as the sum of two terms: the dipolar or gradient force and the scattering or radiation pressure force. The scattering force is traditionally considered proportional to the Poynting vector, which gives the direction and magnitude of the momentum flow. However, as we will show, when the light field has a non- uniform spatial distribution of spin angular momentum there is an additional non-conservative force term is proportional to the curl of the spin angular momentum of the light field.
The peculiar dynamics of gold and silver nanoparticles in an optical vortex lattice will be discussed. Radiation pressure in the vortex field (arising in the intersection region of two crossed optical standing waves) may lead to a giant acceleration of free diffusion. The unusual properties of the optical forces acting on particles with both electric and magnetic response will also be analyzed. We will focus on nanometer-sized spheres of conventional semiconductor materials, like Silicon (Si) or Germanium (Ge), which have extraordinary electric and magnetic optical properties in the infrared-telecom range of the electromagnetic spectrum.