Abstract
In this paper we analyze the non-conservative forces on magneto-dielectric particles in special configurations where the scattering force is not proportional to the average value of the Poynting vector. Based on these results, we revisit the concept of electromagnetic momentum density.
Highlights
After the pioneering experimental work by Ashkin and coworkers [1], the mechanical action of light on small particles has been extensively used to trap and manipulate small particles [2, 3, 4] or to induce significant optical forces between particles [5, 6, 7]
Light forces on small dielectric particles are described as the sum of the dipole or gradient force and the radiation pressure or scattering force [15, 16]
By summarizing the results presented in this paper, we may say that, even with a null average value of the Poynting vector, a dielectric particle experiences a pure scattering force given by n o hF i r ⇥ hL i while a magnetic particle, with zero Poynting vector, experiences a scattering force given by n o hF i r ⇥ hL i
Summary
After the pioneering experimental work by Ashkin and coworkers [1] , the mechanical action of light on small particles has been extensively used to trap and manipulate small particles [2, 3, 4] or to induce significant optical forces between particles [5, 6, 7]. For non-homogeneous waves there is an additional contribution to the scattering force [17], which has been shown to be a non-conservative force proportional to the curl of the spin angular momentum of the light field [18] This curl term maybe interpreted not like an extra force, but like the absence of the spin component in the momentum transferred, which is given by the orbital component of the Poynting vector [19, 20, 21]. Forces coming from the Poynting vector and spin forces, must be considered in order to understand the scattering forces in the focal volume of microscope objectives [22, 23, 24, 25] These non-homogeneous light fields may be produced by an appropriate manipulation of interfering laser beams. We will discuss about the value of the linear momentum transferred to the particle, based on the total scattering forces
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