Subwavelength particles in an inhomogeneous light field: optical forces associated with the spin and orbital energy flows

We analyse 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, on the basis of the dipole model, and numerically. The three sources of the field mechanical action-the energy density gradient and the orbital and spin parts of the energy flow-differ in their ponderomotive mechanisms, and their physical nature manifests itself in the dependence of the optical force on the particle radius a. If a << lambda (the radiation wavelength), the optical force behaves as a(nu), and integer nu 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 quadrupole moments (if the particle is polarizable electrically but not magnetically or vice versa). In principle, the spin and orbital currents can be measured separately through the probe particle motion, employing a special choice of particles with the necessary magnetic and/or electric properties.