3,025 research outputs found
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
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
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 "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
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
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
Spectral behavior of localized plasmon resonances in the near- and far-field regimes. Comment on The spectral shift between near- and far-field resonances of optical nano-antennas
In a recent paper by Menzel et al. (Opt. Exp. 22, 9971 (2014)), its authors
analyze the spectral red-shift effect of plasmonic resonances of metallic
nano-antennas between the far-field and near-field regimes. Here, we
demonstrate that their interpretation of this effect is done under the same
perspective as one recently reported in Langmuir 29, 6715 (2013); however, the
former is incomplete and needs some remarks and clarifications which require
additional relevant concepts and arguments of physical significance
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