151 research outputs found
The role of the angular momentum of light in Mie scattering. Excitation of dielectric spheres with Laguerre-Gaussian modes
We present a method to enhance the ripple structure of the scattered
electromagnetic field in the visible range through the use of Laguerre-Gaussian
beams. The position of these enhanced ripples as well as their linewidths can
be controlled using different optical beams and sizes of the spheres.Comment: 10 pages, 4 figures, 1 table,
http://dx.doi.org/10.1016/j.jqsrt.2012.10.01
Vortex nucleation and evolution in parametric wave mixing
We predict a variety of new phenomena, that includes the spontaneous nucleation of multiple vortex twins, vortex rotation and drift, vortex-antivortex interaction and annihilation, and formation of quasi-aligned patterns of single-charge vortices. We consider cw light propagation in a bulk quadratic nonlinear crystal under conditions for type I second-harmonic generation. We restrict ourselves to up-conversion geometries with material and light conditions that yield negligible depletion of the pump fundamental frequency (FF) beam. Then, the second-harmonic (SH) beam is dictated by an inhomogeneous linear partial differential equation whose general solution can be obtained by means of the Green function approach. In the case of un-seeded geometries (i.e., no SH input light), and in absence of Poynting vector walk-off between the FF and SH beams, sum- and difference-charge arithmetic operations have been predicted and observed experimentally. However, a new range of phenomena is discovered in seeded geometries and with Poynting vector walk-off. In particular, in the case of seeded schemes without walk-off, our numerical and experimental investigations show the spontaneous nucleation of multiple-vortex twins. In such case, the number of vortices present in the SH beam and its total topological charge varies with the propagation distance inside the crystal.Peer ReviewedPostprint (published version
Azimuthal distinguishability of entangled photons generated in spontaneous parametric down-conversion
We experimentally demonstrate that paired photons generated in different
sections of a down-conversion cone, when some of the interacting waves show
Poynting vector walk-off, carry different spatial correlations, and therefore a
different degree of spatial entanglement. This is shown to be in agreement with
theoretical results. We also discuss how this azimuthal distinguishing
information of the down-conversion cone is relevant for the implementation of
quantum sources aimed at the generation of entanglement in other degrees of
freedom, such as polarization.Comment: 7 pages, 5 figures, submitted to Opt. Expres
Angular momentum-induced circular dichroism in non-chiral nanostructures
Circular dichroism (CD), i.e. the differential response of a system to left
and right circularly polarized light, is one of the only techniques capable of
providing morphological information of certain samples. In biology, for
instance, CD spectroscopy is widely used to study the structure of proteins.
More recently, it has also been used to characterize metamaterials and
plasmonic structures. Typically, CD can only be observed in chiral objects.
Here, we present experimental results showing that a non-chiral sample such as
a sub-wavelength circular nano-aperture can produce giant CD when a vortex beam
is used to excite it. These measurements can be understood by studying the
symmetries of the sample and the total angular momentum that vortex beams
carry. Our results show that CD can provide a wealth of information about the
sample when combined with the control of the total angular momentum of the
input field
Vortex evolution in parametric wave mixing
We investigate the evolution of vortex wave front dislocations in multiple-wave second-harmonic generation processes in quadratic nonlinear media. Vortices nested in finite-size host beams are shown to nucleate and to annihilate in pairs, and to move across the transverse wave front during the beam evolution. A closed-form model that holds under conditions of negligible-depletion of the pump beam is developed to describe the vortex dynamics in order to predict the number of vortices present in the wave fronts of the beams at any instance of the propagation. Results are compared with numerical simulations of the full governing equations and with experimental observations. Limitations of the model are outlined.Peer ReviewedPostprint (published version
On the transformations generated by the electromagnetic spin and orbital angular momentum operators
We present a study of the properties of the transversal "spin angular
momentum" and "orbital angular momentum" operators. We show that the "spin
angular momentum" operators are generators of spatial translations which depend
on helicity and frequency and that the "orbital angular momentum" operators
generate transformations which are a sequence of this kind of translations and
rotations. We give some examples of the use of these operators in light matter
interaction problems. Their relationship with the helicity operator allows to
involve the electromagnetic duality symmetry in the analysis. We also find that
simultaneous eigenstates of the three "spin" operators and parity define a type
of standing modes which has been recently singled out for the interaction of
light with chiral molecules. With respect to the relationship between "spin
angular momentum", polarization, and total angular momentum, we show that,
except for the case of a single plane wave, the total angular momentum of the
field is decoupled from its vectorial degrees of freedom even in the regime
where the paraxial approximation holds. Finally, we point out a relationship
between the three "spin" operators and the spatial part of the Pauli-Lubanski
four vector
Necessary symmetry conditions for the rotation of light
Two conditions on symmetries are identified as necessary for a linear
scattering system to be able to rotate the linear polarisation of light: Lack
of at least one mirror plane of symmetry and electromagnetic duality symmetry.
Duality symmetry is equivalent to the conservation of the helicity of light in
the same way that rotational symmetry is equivalent to the conservation of
angular momentum. When the system is a solution of a single species of
particles, the lack of at least one mirror plane of symmetry leads to the
familiar requirement of chirality of the individual particle. With respect to
helicity preservation, according to the analytical and numerical evidence
presented in this paper, the solution preserves helicity if and only if the
individual particle itself preserves helicity. However, only in the particular
case of forward scattering the helicity preservation condition on the particle
is relaxed: We show that the random orientation of the molecules endows the
solution with an effective rotational symmetry; at its turn, this leads to
helicity preservation in the forward scattering direction independently of any
property of the particle. This is not the case for a general scattering
direction. These results advance the current understanding of the phenomena of
molecular optical activity and provide insight for the design of polarisation
control devices at the nanoscale.Comment: 17 pages, 3 figure
Dual and anti-dual modes in dielectric spheres
We present how the angular momentum of light can play an important role to
induce a dual or anti-dual behaviour on a dielectric particle. Although the
material the particle is made of is not dual, i.e. a dielectric does not
interact with an electrical field in the same way as it does with a magnetic
one, a spherical particle can behave as a dual system when the correct
excitation beam is chosen. We study the conditions under which this induced
dual or anti-dual behaviour can be induced.Comment: 13 pages, 5 figure
Far-field measurements of vortex beams interacting with nanoholes
We measure the far-field intensity of vortex beams going through nanoholes.
The process is analyzed in terms of helicity and total angular momentum. It is
seen that the total angular momentum is preserved in the process, and helicity
is not. We compute the ratio between the two transmitted helicity components,
. We observe that this ratio is highly dependent on the helicity
() and the angular momentum () of the incident vortex beam in
consideration. Due to the mirror symmetry of the nanoholes, we are able to
relate the transmission properties of vortex beams with a certain helicity and
angular momentum, with the ones with opposite helicity and angular momentum.
Interestingly, vortex beams enhance the ratio as compared to
those obtained by Gaussian beams
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