Abstract
We present a study of the optical force on a small particle with both electric and magnetic response, immersed in an arbitrary non-absorbing medium, due to a generic incident electromagnetic field. Expressions for the gradient force, radiation pressure and curl components are obtained for the force due to both the electric and magnetic dipoles excited in the particle. In particular, for the magnetic force we tentatively introduce the concept of curl of the spin angular momentum density of the magnetic field, also expressed in terms of 3D generalizations of the Stokes parameters. From the formal analogy between the conservation of momentum and the optical theorem, we discuss the origin and significance of the self-interaction force between both dipoles; this is done in connection with that of the angular distribution of scattered light and of the extinction cross section.
Highlights
Light carries energy and both linear and angular momenta that can be transferred to atoms, molecules and particles
We present a study of the optical force on a small particle with both electric and magnetic response, immersed in an arbitrary nondissipative medium, due to a generic incident electromagnetic field
We have analyzed the different contributions to the optical force on a small dipolar magnetic particle, immersed in an arbitrary non-dissipative medium, with both electric and magnetic response to an arbitrary external electromagnetic field
Summary
Light carries energy and both linear and angular momenta that can be transferred to atoms, molecules and particles. In analogy with electric dipoles, the optical forces on a magnetic dipole are shown to present a non-conservative contribution proportional to the curl of the spin angular momentum density of the magnetic field These curl forces can be expressed in terms of the three-dimensional Stokes parameters as discussed in Sec 4.1. We know that according to the theorem of momentum conservation from which Eq (1) derives, the same result Eq (2) should be obtained by choosing in Eq (1) any surface S enclosing the particle The advantage of this approach is that it shows the formal analogy both between the mathematical expression of the force and of the optical theorem, as well as between their respective derivations. The sum of Eq (12) and Eq (13) coincides with Eq (2)
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