7,842 research outputs found
Anderson Photon-Phonon Colocalization in Certain Random Superlattices
International audienceFundamental observations in physics ranging from gravitational wave detection to laser cooling of a nanomechanical oscillator into its quantum ground state rely on the interaction between the optical and the mechanical degrees of freedom. A key parameter to engineer this interaction is the spatial overlap between the two fields, optimized in carefully designed resonators on a case-by-case basis. Disorder is an alternative strategy to confine light and sound at the nanoscale. However, it lacks an a priori mechanism guaranteeing a high degree of colocalization due to the inherently complex nature of the underlying interference processes. Here, we propose a way to address this challenge by using GaAs=AlAs vertical distributed Bragg reflectors with embedded geometrical disorder. Because of a remarkable coincidence in the physical parameters governing light and motion propagation in these two materials, the equations for both longitudinal acoustic waves and normal-incidence light become practically equivalent for excitations of the same wavelength. This guarantees spatial overlap between the electromagnetic and displacement fields of specific photon-phonon pairs, leading to strong light-matter interaction. In particular, a statistical enhancement in the vacuum optomechanical coupling rate, g o , is found, making this system a promising candidate to explore Anderson localization of high frequency (∼20 GHz) phonons enabled by cavity optomechanics. The colocalization effect shown here unlocks the access to unexplored localization phenomena and the engineering of light-matter interactions mediated by Anderson-localized states
Intensity Distribution of Modes in Surface Corrugated Waveguides
Exact calculations of transmission and reflection coefficients in surface
randomly corrugated optical waveguides are presented. As the length of the
corrugated part of the waveguide increases, there is a strong preference to
forward coupling through the lowest mode. An oscillating behavior of the
enhanced backscattering as a function of the wavelength is predicted. Although
the transport is strongly non isotropic, the analysis of the probability
distributions of the transmitted waves confirms in this configuration
distributions predicted by Random Matrix Theory for volume disorder
Performance of global retention models in the optimisation of the chromatographic separation (I): Simple multi-analyte samples
Conventional retention models lead to accurate descriptions of the elution behaviour from the fitting of data for single solutes or from a set of solutes, one by one. However, the simultaneous fitting of several solutes through a regression process that separates the contributions of column and solvent from those of each solute is also possible. The result is a global retention model constituted by a set of equations with some common parameters (those associated with column and solvent), whereas others, specific to each solute, differ for each equation. This work explores the possibilities, advantages, and limitations of global models when they are applied to the optimisation of chromatographic resolution. A set constituted by 13 drugs (diuretics and β-blockers) and a training experimental design of seven multi-linear gradients are considered. Since standards for all compounds were available, the optimisation based on global models could be compared with the conventional optimisation, which is based on individual models. In their current state, global models do not predict changes in elution order, but they do allow for incorporating additional solutes (e.g., new analytes or matrix peaks) with only one new experiment. This possibility is explored by extending the model for the 13 analytes to include 26 peaks associated with a contamination in the injector. The combination of individual and global models allows an optimisation where the effects of matrix peaks on the separation of analytes can be integrated.Fil: Peiró Vila, P.. Universidad de Valencia; EspañaFil: Villamonte, María Daniela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Biológicas. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Biológicas; ArgentinaFil: Luján Roca, I.. Universidad de Valencia; EspañaFil: Torres Lapasió, J.R.. Universidad de Valencia; EspañaFil: García Alvarez Coque, M. C.. Universidad de Valencia; Españ
Engineering nanoscale hypersonic phonon transport
Controlling the vibrations in solids is crucial to tailor their mechanical
properties and their interaction with light. Thermal vibrations represent a
source of noise and dephasing for many physical processes at the quantum level.
One strategy to avoid these vibrations is to structure a solid such that it
possesses a phononic stop band, i.e., a frequency range over which there are no
available mechanical modes. Here, we demonstrate the complete absence of
mechanical vibrations at room temperature over a broad spectral window, with a
5.3 GHz wide band gap centered at 8.4 GHz in a patterned silicon nanostructure
membrane measured using Brillouin light scattering spectroscopy. By
constructing a line-defect waveguide, we directly measure GHz localized modes
at room temperature. Our experimental results of thermally excited guided
mechanical modes at GHz frequencies provides an eficient platform for
photon-phonon integration with applications in optomechanics and signal
processing transduction
Reflection and transmission of waves in surface-disordered waveguides
The reflection and transmission amplitudes of waves in disordered multimode
waveguides are studied by means of numerical simulations based on the invariant
embedding equations. In particular, we analyze the influence of surface-type
disorder on the behavior of the ensemble average and fluctuations of the
reflection and transmission coefficients, reflectance, transmittance, and
conductance. Our results show anomalous effects stemming from the combination
of mode dispersion and rough surface scattering: For a given waveguide length,
the larger the mode transverse momentum is, the more strongly is the mode
scattered. These effects manifest themselves in the mode selectivity of the
transmission coefficients, anomalous backscattering enhancement, and speckle
pattern both in reflection and transmission, reflectance and transmittance, and
also in the conductance and its universal fluctuations. It is shown that, in
contrast to volume impurities, surface scattering in quasi-one-dimensional
structures (waveguides) gives rise to the coexistence of the ballistic,
diffusive, and localized regimes within the same sample.Comment: LaTeX (REVTeX), 12 pages with 14 EPS figures (epsf macro), minor
change
Circular dichrosim in photoionization of H2 and D2
ABSTRACT: In this work, circular dichroism in H2 (D2) photoionization is studied in detail. We have selected several
photon energies for a case study: 19 eV for which only direct ionization to the 1s_g ionization channel is present, 27 eV where autoionization of Q1 doubly excited states takes place, and 32.5 eV for which autoionization from Q1 and doubly excited states and direct ionization to 1s_g and 2p_u channels strongly interfere. The latter case shows clear evidence of different behavior of the photoionization against radiation helicity
Circular dichroism in photoionization of H2
ABSTRACT: Circular dichroism is a consequence of chirality. However, nonchiral molecules can also exhibit it when the measurement itself introduces chirality, e.g., when measuring molecular-frame photoelectron angular distributions. The few such experiments performed on homonuclear diatomic molecules show that, as expected, circular dichroism vanishes when the molecular-frame photoelectron angular distributions are
integrated over the polar electron emission angle. Here we show that this is not the case in resonant dissociative ionization of H2 for photons of 30–35 eV, which is the consequence of the delayed ionization from molecular doubly excited states into ionic states of different inversion symmetry
The 1989 and 2015 outbursts of V404 Cygni: a global study of wind-related optical features
The black hole transient V404 Cygni exhibited a bright outburst in June 2015
that was intensively followed over a wide range of wavelengths. Our team
obtained high time resolution optical spectroscopy (~90 s), which included a
detailed coverage of the most active phase of the event. We present a database
consisting of 651 optical spectra obtained during this event, that we combine
with 58 spectra gathered during the fainter December 2015 sequel outburst, as
well as with 57 spectra from the 1989 event. We previously reported the
discovery of wind-related features (P-Cygni and broad-wing line profiles)
during both 2015 outbursts. Here, we build diagnostic diagrams that enable us
to study the evolution of typical emission line parameters, such as line fluxes
and equivalent widths, and develop a technique to systematically detect outflow
signatures. We find that these are present throughout the outburst, even at
very low optical fluxes, and that both types of outflow features are observed
simultaneously in some spectra, confirming the idea of a common origin. We also
show that the nebular phases depict loop patterns in many diagnostic diagrams,
while P-Cygni profiles are highly variable on time-scales of minutes. The
comparison between the three outbursts reveals that the spectra obtained during
June and December 2015 share many similarities, while those from 1989 exhibit
narrower emission lines and lower wind terminal velocities. The diagnostic
diagrams presented in this work have been produced using standard measurement
techniques and thus may be applied to other active low-mass X-ray binaries.Comment: Accepted for publication in MNRAS. 23 pages paper, plus a 9 pages
appendix with extra tables and figures. 18 figures are included in the paper
and 8 in the appendi
Stiff monatomic gold wires with a spinning zigzag geometry
Using first principles density functional calculations, gold monatomic wires
are found to exhibit a zigzag shape which remains under tension, becoming
linear just before breaking. At room temperature they are found to spin, what
explains the extremely long apparent interatomic distances shown by electron
microscopy.The zigzag structure is stable if the tension is relieved, the wire
holding its chainlike shape even as a free-standing cluster. This unexpected
metallic-wire stiffness stems from the transverse quantization in the wire, as
shown in a simple free electron model.Comment: 4 pages, latex, 5 figures, submitted to PR
Learning Unions of k-Testable Languages
A classical problem in grammatical inference is to identify a language from a
set of examples. In this paper, we address the problem of identifying a union
of languages from examples that belong to several different unknown languages.
Indeed, decomposing a language into smaller pieces that are easier to represent
should make learning easier than aiming for a too generalized language. In
particular, we consider k-testable languages in the strict sense (k-TSS). These
are defined by a set of allowed prefixes, infixes (sub-strings) and suffixes
that words in the language may contain. We establish a Galois connection
between the lattice of all languages over alphabet {\Sigma}, and the lattice of
k-TSS languages over {\Sigma}. We also define a simple metric on k-TSS
languages. The Galois connection and the metric allow us to derive an efficient
algorithm to learn the union of k-TSS languages. We evaluate our algorithm on
an industrial dataset and thus demonstrate the relevance of our approach
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