Subwavelength particles in an inhomogeneous light field: Optical forces associated with the spin and orbital energy flows
We analyze the ponderomotive action experienced by a small spherical particle immersed in an optical field, in relation to the internal energy flows (optical currents) and their spin and orbital constituents. The problem is studied analytically, based on the dipole model, and numerically. Three sources of the field mechanical action - energy density gradient and the orbital and spin parts of the energy flow - differ by the ponderomotive mechanism, and their physical nature manifests itself in the optical force dependence on the particle radius a. If a is much less than the radiation wavelength, the optical force behaves as a^n and integer n can be used to classify the sources of the mechanical action. This classification correlates with the multipole representation of the field-particle interaction: The gradient force and the orbital-momentum force appear due to the electric or magnetic dipole moments per se, the spin-momentum force emerges due to interaction between the electric and magnetic dipoles or between the dipole and quadruple moments (if the particle is polarisable electrically but not magnetically or vice versa). In principle, the spin and orbital currents can be measured separately by the probe particle motion, employing the special choice of particles with necessary magnetic and/or electric properties.