The optical theorem for the conservation of the electromagnetic helicity: Its significance for molecular transfer and enantiomeric discrimination by dichroism

We put forward the physical meaning of the conservation equation for the helicity on scattering of an electromagnetic field with a generally magnetodielectric bi-isotropic dipolar object. This is the optical theorem for the helicity that, as we find, plays a role for this quantity analogous to that of the optical thorem for energy. We discuss its consequences for helicity transfer between molecules and for new detection procedures of circular dichroism based on ellipsometric measurements

The optical torque: Electromagnetic spin and orbital angular momenta conservation laws and their significance

The physics involved in the fundamental conservation equations of the spin and orbital angular momenta leads to new laws and phenomena that I disclose. To this end, I analyse the scattering of an electromagnetic wavefield by the canonical system constituted by a small particle, which I assume dipolar in the wide sense. Specifically, under quite general conditions these laws lead to understanding how is the contribution and weight of each of those angular momenta to the electromagnetic torque exerted by the field on the object, which is shown to consist of an extinction and a scattering, or recoil, part. This leads to an interpretation of its effect different to that taken up till now by many theoretical and experimental works, and implies that a part of the recoil torque cancels the usually called intrinsic torque which was often considered responsible of the particle spinning. In addition, I obtain the contribution of the spatial structure of the wave to this torque, unknown to this date, showing its effect in the orbiting of the object, and demonstrating that it often leads to a negative torque on a single particle, i.e. opposite to the incident helicity, producing an orbital motion contrary to its spinning. Furthermore, I establish a decomposition of the electromagnetic torque into conservative and non-conservative components in which the helicity and its flow play a role analogous to the energy and its flux for electromagnetic forces. I illustrate these phenomena with examples of beams, also showing the difficulties of some paraxial formulations whose fields do not hold the transversality condition

Comment on "Optical torque on small chiral particles in generic optical fields "

We comment on mistakes and inaccuracies of a paper by Chen et al. concerning the optical torque from generic optical fields on dipolar chiral particles, i.e. on those whose scattering is fully described by the first electric, magnetic and magnetoelectric Mie coefficients

The optical torque on small bi-isotropic particles

Most previous theoretical studies on the optical torque exerted by light on dipolar particles are incomplete. Here we establish the equations for the time-averaged optical torque on dipolar bi-isotropic particles. Due to the interference of scattered fields, it has a term additional to that commonly employed in theory and experiments. Its consequences for conservation of energy, angular momentum, and effects like negative torques, are discussed

Comment on "Poynting vector, orbital and spin momentum and angular momentum versus optical force and torque on arbitrary particle in generic optical fields"

We criticize the originality or correctness of some of the ideas and results recently reported by Ng et al. in arXiv:1511.08546

Fundamentals and model of resonance helicity and energy transfer between two magnetoelectric chiral particles

We establish a classical electrodynamic theory for the non-radiative transfer of field helicity (RHELT) and energy (RET) between a donor and an acceptor, both being dipolar, magnetoelectric and bi-isotropic, chiral in particular, with rotating excited dipoles. We introduce orientational factors that control this process. Also, a RHELT and RET interaction radius is put forward. The detection of RHELT adds a wealth of information contained in the helicity of the transferred fields, never used or established to date. The nature of these dipolar magnetoelectric bi-isotropic particles and/or molecules with induced dipoles possessing angular momentum, enriches the number of variables and associated effects. Hence the landscape involved in this transfer phenomenon, never explored before, is significantly broader than in conventional FRET. In this way, chiral interacting objects convey terms in the equations of transfer rate of helicity and energy that are discriminatory, so that one can extract information on their structural chirality handedness and polarization rotation. As such, not only the rate of electromagnetic helicity transfer, but also that of energy transfer may be negative, which for the latter means an enhanced emission from the donor in pressence of acceptor, a phenomenon which does not exist in conventional FRET. Importantly, both the RHELT and RET rates, as well as the RHELT interaction radius, are very sensitive to changes in the helicity, or state of polarization, of the illumination, as well as to the polarization of the excited electric and magnetic dipole moments of donor and acceptor. Finally, we introduce the observable quanties in terms of which one can obtain the transfer rates and interaction radii

Creation of Van der Waals, Casimir, and many more stochastic forces, with light radiation pressure via optics of randomly fluctuating sources

The mechanical action on matter of the electromagnetic field emitted by a fluctuating source is governed by its statistics. In particular, thermal sources and vacuum fluctuations exert on bodies those well-known Casimir (C) and Van der Waals (VdW) forces. However, we have recently demonstrated that partially coherent random electromagnetic fields emitted by tailored optical sources, induce a photonic force on particles which, in particular, may be equivalent to those of Van der Waals and Casimir

Partially coherent sources which produce the same far zone optical force as a laser beam

On applying a theorem previously derived by Wolf and Collett, we demonstrate that partially coherent Gaussian Schell model uctuating sources (GSMS) produce exactly the same optical forces as a fully coherent laser beam. We also show that this kind of sources helps to control the light-matter interaction in biological samples which are very sensitive to thermal heating induced by higher power intensities; and hence the invasiveness of the manipulation. This is a consequence of the fact that the same photonic force can be obtained with a low intensity GSMS as with a high intensity laser beam

Forces between a partially coherent fluctuating source and a magnetodielectric particle

We address the forces exerted by the electromagnetic field emitted by a planar uctuating source on dielectric particles that have arose much interest because of their recently shown magnetodielectric behavior. In this context, we analyze as a particular case the modification of the Casimir and Van der Waals forces. We study the effect of the source coherence length as well as the interplay between the force from the radiated field and that from the electric and magnetic dipoles induced on the particle. This allows a control of these interactions as well as of the weight and interference effects between the fields from both kinds of induced dipoles, in particular when large changes in their differential scattering cross section occur due to Kerker minimum forward or zero backward conditions; thus opening new paths to nanoparticle ensembling and manipulation. The influence of surface waves of the source is also studied

Time averaged total force on a dipolar sphere in an electromagnetic field

We establish the time averaged total force on a subwavelength sized particle in a time harmonic varying field. Our analysis is not restrictive about the spatial dependence of the incident field. We discuss the addition of the radiative reaction term in the polarizability in order to correctly deal with the scattering force. As a consequence and illustration, we assess the degree of accuracy of several polarizability models previously established.Comment: 3 pages, 2 figure