Optical force and torque on single and aggregated spheres_ the trapping issue

Radiation force and radiation torque stem from the conservation theorems governing the interaction between radiation and particles. As a result, even simple plane waves may yield force and torque that are nonnegligible when studying the dynamics of particles. We show how these forces and torques can be put into formulas of practical use starting with the expansion of the electromagnetic field in a series of vector multipole fields, and adapt the formalism for particles that are fairly well modeled as single or aggregated spheres. Then, we extend the formalism to deal with the case the field is a highly focalized laser beam, that, as is well known, may trap particles within the focal region. Finally, we present our calculations performed by the theory we exposed, finding a convincing concordance as long as comparison can be made with available experimental data.