56 research outputs found
Excitation Mechanisms of Whispering Gallery Modes with Direct Light Scattering
AbstractIn optics, whispering gallery modes (WGMs) are modes of light that arise in cylindrically symmetric structures. Their intensity profile is strongly confined around the interface of the structure, and their Q‐factors are some of the highest ever measured with light. Here, the physical mechanisms governing the coupling of a tangential beam into a WGM of a microsphere are analytically demonstrated. For that, Mie theory and the symmetries of light are made use of. It is demonstrated that the coupling mechanism is not related to any evanescent tunnelling effect. Rather, it is shown that it has to do with the angular momentum matching between the available inner WGMs of the sphere and the angular momentum content of the incident beam. The model is valid for any homogeneous sphere, for any wavelength, and for any incident cylindrically symmetric beam, focused or not. It quantitatively predicts the optical coupling efficiency to the resonator for any tangential position of the incident beam. And it sketches four different regimes of interaction depending on the beam position with respect to the sphere, properly matching experimental resonance spectra observed with free‐space laser scattering
Purcell factor of Mie resonators featuring electric and magnetic modes
We present a modal approach to compute the Purcell factor in Mie resonators
exhibiting both electric and magnetic resonances. The analytic expressions of
the normal modes are used to calculate the effective volumes. We show that
important features of the effective volume can be predicted thanks to the
translation-addition coefficients of a displaced dipole. Using our formalism,
it is easy to see that, in general, the Purcell factor of Mie resonators is not
dominated by a single mode, but rather by a large superposition. Finally we
consider a silicon resonator homogeneously doped with electric dipolar
emitters, and we show that the average electric Purcell factor dominates over
the magnetic one
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
Control and characterization of nano-structures with the symmetries of light
Light beams can be symmetric under different transformations: translations,
rotations, mirror symmetries, duality transformations, etc. In this thesis, a
systematic way of characterizing these symmetries is presented. Then, it is
shown that light beams symmetric under different transformations can be used to
control light-matter interactions at the nano-scale. Particular applications
are developed, both theoretically and experimentally. Inducing a dual behaviour
on a non-dual sample, the excitation of high multipolar order resonances and
the measurement of circular dichroism using vortex beams are among them.Comment: PhD Thesis, Department of Physics and Astronomy, Macquarie
University. PhD Supervisor: Gabriel Molina-Terriz
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
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
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
Transverse multipolar light-matter couplings in evanescent waves
We present an approach to study the interaction between matter and evanescent
fields. The approach is based on the decomposition of evanescent plane waves
into multipoles of well-defined angular momentum transverse to both decay and
propagation directions. We use the approach to identify the origin of the
recently observed directional coupling of emitters into guided modes, and of
the opposite Zeeman state excitation of atoms near a fiber. We explain how to
rigorously quantify both effects, and show that the directionality and the
difference in excitation rates grow exponentially with the multipolar order of
the light-matter interaction. We also use the approach to study and maximize
the transverse torque exerted by an evanescent plane wave onto a given
spherical absorbing particle. The maximum occurs at the quadrupolar order of
the particle, and for a particular polarization of the plane wave. All the
obtained physical insights can be traced back to the two main features of the
decomposition of evanescent plane waves into transverse multipolar modes: A
polarization independent exponential dominance of modes with large transverse
angular momentum, and a polarization controlled parity selection rule.Comment: Last version with slight changes in the figures and tex
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