Conservative and Nonconservative Torques in Optical Binding

Abstract

We show that in the canonical case of two lossless spheres that are electromagnetically coupled there is interplay between conservative and nonconservative forces, which is controlled by the polarization of the bounding field. We demonstrate that this phenomenon leads to new mechanisms to induce torques on spherically symmetric, optically isotropic, and lossless objects. The electromagnetic interaction can be exploited to apply orbital torque about the mutual center of mass of the bounded spheres as well as spin around the individual axes. When the incident field is linearly polarized, the torques are mostly conservative and affect only transient behaviors while for circularly polarized fields, the torques are entirely nonconservative, resulting in steady rotations. Means to control the magnitudes of orbital and spin torques are presented and applications to nanorotator machines are discussed.

Optical torque: Electromagnetic spin and orbital-angular-momentum conservation laws and their significance

“…Notice on comparing (30) and (31) with (26) and (27), the two additional terms in (30) due to the spatial structure of the incident field when this is not a plane wave. This term accounts for an extrinsic torque component.…”

“…This term accounts for an extrinsic torque component. Also, the second and third terms of (30) have an analogy with those of the dipolar component of the electromagnetic force, whereas those contained in < Γ 0 > keep it with those of the Lorentz's component of the optical force. This duality between the E and B vectors is also evident by comparing (29) with the expression of the electric (e) plus magnetic (m) forces on a magnetodielectric dipolar particle [54].…”

“…(17), governed by the conservation of the spin angular momentum, is given by the sum of (24) plus (29) or (30). This torque has the recoil component L s added to L ′ , which is as necessary to describe the dynamics as the above referenced F e−m component of the force .…”

“…Observations are more difficult to control and to quantitatively interpret with existing models. With few exceptions [34,36], most experimental [27,28] and theoretical [29][30][31][32][33][34]53] studies employ a static formulation, (perhaps following the path of pioneering work [21]), which was shown [37] to be incomplete and not compatible with energy and angular momentum conservation. Only for extremely small (i.e.…”

Interparticle Interactions: Energy Potentials, Energy Transfer, and Nanoscale Mechanical Motion in Response to Optical Radiation

“…These optically induced interparticle interactions give rise to forces and torques, usually described as optical binding although the forces are not necessarily attractive in form, which are the subject of particularly interesting recent research. 10,[37][38][39][40][41][42][43][44][45] The phenomenon has increasingly been advocated as a tool for the optical manipulation and configuration of particles, and many optically induced arrays have been observed experimentally. 8,46 In optical binding, particles 1 Following the optical binding process, which is described by forward Rayleigh scattering, it seems natural to consider the non-forward case where the energy states of the two particles also remain unchanged.…”