41 research outputs found
Spatial wave intensity correlations in quasi-one-dimensional wires
Spatial intensity correlations between waves transmitted through random media
are analyzed within the framework of the random matrix theory of transport.
Assuming that the statistical distribution of transfer matrices is isotropic,
we found that the spatial correlation function can be expressed as the sum of
three terms, with distinctive spatial dependences. This result coincides with
the one obtained in the diffusive regime from perturbative calculations, but
holds all the way from quasi-ballistic transport to localization. While
correlations are positive in the diffusive regime, we predict a transition to
negative correlations as the length of the system decreases.Comment: 10 pages, 3 figures. Submitted to Physical Review Letter
Effect of long range spatial correlations on the lifetime statistics of an emitter in a two-dimensional disordered lattice
The effect of spatial correlations on the Purcell effect in a bidimensional
dispersion of resonant nanoparticles is analyzed. We perform extensive
calculations on the fluorescence decay rate of a point emitter embedded in a
system of nanoparticles statistically distributed according to a simple 2D
lattice-gas model near the critical point. For short-range correlations (high
temperature thermalization) the Purcell factors present a long-tailed statistic
which evolves towards a bimodal distribution when approaching the critical
point where the spatial correlation length diverges. Our results suggest
long-range correlations as a possible origin of the large fluctuations of
experimental decay rates in disordered metal films.Comment: 6 pages, 5 figure
Broadband telecom transparency of semiconductor-coated metal nanowires: more transparent than glass
Metallic nanowires (NW) coated with a high permittivity dielectric are
proposed as means to strongly reduce the light scattering of the conducting NW,
rendering them transparent at infrared wavelengths of interest in
telecommunications. Based on a simple, universal law derived from
electrostatics arguments, we find appropriate parameters to reduce the
scattering efficiency of hybrid metal-dielectric NW by up to three orders of
magnitude as compared with the scattering efficiency of the homogeneous
metallic NW. We show that metal@dielectric structures are much more robust
against fabrication imperfections than analogous dielectric@metal ones. The
bandwidth of the transparent region entirely covers the near IR
telecommunications range. Although this effect is optimum at normal incidence
and for a given polarization, rigorous theoretical and numerical calculations
reveal that transparency is robust against changes in polarization and angle of
incidence, and also holds for relatively dense periodic or random arrangements.
A wealth of applications based on metal-NWs may benefit from such invisibility
Optical amplification enhancement in photonic crystals
Improving and controlling the efficiency of a gain medium is one of the most
challenging problems of laser research. By measuring the gain length in an opal
based photonic crystal doped with laser dye, we demonstrate that optical
amplification is more than twenty-fold enhanced along the Gamma-K symmetry
directions of the face centered cubic photonic crystal. These results are
theoretically explained by directional variations of the density of states,
providing a quantitative connection between density of the states and light
amplification
Interaction Effects on the Magneto-optical Response of Magnetoplasmonic Dimers
The effect that dipole-dipole interactions have on the magneto-optical (MO)
properties of magnetoplasmonic dimers is theoretically studied. The specific
plasmonic versus magnetoplasmonic nature of the dimer's metallic components and
their specific location within the dimer plays a crucial role on the
determination of these properties. We find that it is possible to generate an
induced MO activity in a purely plasmonic component, even larger than that of
the MO one, therefore dominating the overall MO spectral dependence of the
system. Adequate stacking of these components may allow obtaining, for specific
spectral regions, larger MO activities in systems with reduced amount of MO
metal and therefore with lower optical losses. Theoretical results are
contrasted and confirmed with experiments for selected structures
Statistical Scattering of Waves in Disordered Waveguides: from Microscopic Potentials to Limiting Macroscopic Statistics
We study the statistical properties of wave scattering in a disordered
waveguide. The statistical properties of a "building block" of length (delta)L
are derived from a potential model and used to find the evolution with length
of the expectation value of physical quantities. In the potential model the
scattering units consist of thin potential slices, idealized as delta slices,
perpendicular to the longitudinal direction of the waveguide; the variation of
the potential in the transverse direction may be arbitrary. The sets of
parameters defining a given slice are taken to be statistically independent
from those of any other slice and identically distributed. In the
dense-weak-scattering limit, in which the potential slices are very weak and
their linear density is very large, so that the resulting mean free paths are
fixed, the corresponding statistical properties of the full waveguide depend
only on the mean free paths and on no other property of the slice distribution.
The universality that arises demonstrates the existence of a generalized
central-limit theorem.
Our final result is a diffusion equation in the space of transfer matrices of
our system, which describes the evolution with the length L of the disordered
waveguide of the transport properties of interest. In contrast to earlier
publications, in the present analysis the energy of the incident particle is
fully taken into account.Comment: 75 pages, 10 figures, submitted to Phys. Rev
Conductance Fluctuations in Disordered Wires with Perfectly Conducting Channels
We study conductance fluctuations in disordered quantum wires with unitary
symmetry focusing on the case in which the number of conducting channels in one
propagating direction is not equal to that in the opposite direction. We
consider disordered wires with left-moving channels and right-moving
channels. In this case, left-moving channels become perfectly conducting,
and the dimensionless conductance for the left-moving channels behaves as
in the long-wire limit. We obtain the variance of in the
diffusive regime by using the Dorokhov-Mello-Pereyra-Kumar equation for
transmission eigenvalues. It is shown that the universality of conductance
fluctuations breaks down for unless is very large.Comment: 6 pages, 2 figure
Strong magnetic response of submicron Silicon particles in the infrared
High-permittivity dielectric particles with resonant magnetic properties are
being explored as constitutive elements of new metamaterials and devices in the
microwave regime. Magnetic properties of low-loss dielectric nanoparticles in
the visible or infrared are not expected due to intrinsic low refractive index
of optical materials in these regimes. Here we analyze the dipolar electric and
magnetic response of loss-less dielectric spheres made of moderate permittivity
materials. For low material refractive index there are no sharp resonances due
to strong overlapping between different multipole contributions. However, we
find that Silicon particles with refractive index 3.5 and radius approx. 200nm
present a dipolar and strong magnetic resonant response in telecom and
near-infrared frequencies, (i.e. at wavelengths approx. 1.2-2 micrometer).
Moreover, the light scattered by these Si particles can be perfectly described
by dipolar electric and magnetic fields, quadrupolar and higher order
contributions being negligible.Comment: 10 pages, 5 figure
Optical parametric oscillation with distributed feedback in cold atoms
There is currently a strong interest in mirrorless lasing systems, in which
the electromagnetic feedback is provided either by disorder (multiple
scattering in the gain medium) or by order (multiple Bragg reflection). These
mechanisms correspond, respectively, to random lasers and photonic crystal
lasers. The crossover regime between order and disorder, or correlated
disorder, has also been investigated with some success. Here, we report
one-dimensional photonic-crystal lasing (that is, distributed feedback lasing)
with a cold atom cloud that simultaneously provides both gain and feedback. The
atoms are trapped in a one-dimensional lattice, producing a density modulation
that creates a strong Bragg reflection with a small angle of incidence. Pumping
the atoms with auxiliary beams induces four-wave mixing, which provides
parametric gain. The combination of both ingredients generates a mirrorless
parametric oscillation with a conical output emission, the apex angle of which
is tunable with the lattice periodicity