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Silveirinha M.G.,Telecommunications Institute of Portugal
Physical Review B - Condensed Matter and Materials Physics | Year: 2016

Electronic topological insulators are one of the breakthroughs of 21st century condensed matter physics. So far, the search for a light counterpart of an electronic time-reversal invariant topological insulator has remained elusive. This is due to the fundamentally different natures of light and matter and the different spins of photons and electrons. Here, it is shown that the theory of electronic topological insulators has a genuine analog in the context of light wave propagation in time-reversal invariant continuous materials. We introduce a gauge invariant Z2 index that depends on the global properties of the photonic band structure and is robust to any continuous weak variation of the material parameters that preserves the time-reversal invariance. A nontrivial Z2 index has two possible causes: (i) the lack of smoothness of the pseudo-Hamiltonian in the k→ limit and (ii) the entanglement between positive and negative frequency eigenmode branches. In particular, it is proven that electric-type plasmas and magnetic-type plasmas are topologically inequivalent for a fixed wave polarization. We propose a bulk-edge correspondence that links the number of edge modes with the topological invariants of two continuous bulk materials and present detailed numerical examples that illustrate the application of the theory. © 2016 American Physical Society.


Silveirinha M.G.,Telecommunications Institute of Portugal
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

I characterize the spatial optical solitons supported by arrays of metallic nanowires embedded in Kerr-type material. The array of nanowires is described using an effective medium model and is regarded as a continuous medium. I show that the conditions necessary for the formation of spatial solitons are radically different in presence of the nanowires. In particular, within the effective medium model, spatial solitons are allowed in the nanowire material only when the host material is a "self-defocusing" material. It is proven that the characteristic soliton beamwidth is related to the degree of hyperbolicity of the isofrequency surfaces of the photonic states and that a sufficiently strong electric field amplitude may enable subwavelength solitary waves. © 2013 American Physical Society.


Silveirinha M.G.,Telecommunications Institute of Portugal
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2013

We quantize the macroscopic electromagnetic field in a system of nondispersive polarizable bodies moving at constant velocities possibly exceeding the Cherenkov threshold. It is shown that in general the quantized system is unstable and neither has a ground state nor supports stationary states. The quantized Hamiltonian is written in terms of quantum harmonic oscillators associated with both positive and negative frequencies, such that the oscillators associated with symmetric frequencies are coupled by an interaction term that does not preserve the quantum occupation numbers. Moreover, in the linear regime the amplitudes of the fields may grow without limit provided the velocity of the moving bodies is enforced to be constant. This requires the application of an external mechanical force that effectively pumps the system. © 2013 American Physical Society.


Maslovski S.I.,Telecommunications Institute of Portugal
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2011

The Casimir-Lifshitz interaction emerging from the relative movement of layers in stratified dielectric media (e.g., nonuniformly moving fluids) is considered. It is shown that such movement may result in a repulsive Casimir-Lifshitz force exerted on the layers, with the simplest possible structure consisting of three adjacent layers of the same dielectric medium at zero temperature, where the middle one is stationary and the other two are sliding along a direction parallel to the interfaces of the layers. © 2011 American Physical Society.


Iordache M.-D.,Flemish Institute for Technological Research | Bioucas-Dias J.M.,Telecommunications Institute of Portugal | Plaza A.,University of Extremadura
IEEE Transactions on Geoscience and Remote Sensing | Year: 2014

Sparse unmixing has been recently introduced in hyperspectral imaging as a framework to characterize mixed pixels. It assumes that the observed image signatures can be expressed in the form of linear combinations of a number of pure spectral signatures known in advance (e.g., spectra collected on the ground by a field spectroradiometer). Unmixing then amounts to finding the optimal subset of signatures in a (potentially very large) spectral library that can best model each mixed pixel in the scene. In this paper, we present a refinement of the sparse unmixing methodology recently introduced which exploits the usual very low number of endmembers present in real images, out of a very large library. Specifically, we adopt the collaborative (also called 'multitask' or 'simultaneous') sparse regression framework that improves the unmixing results by solving a joint sparse regression problem, where the sparsity is simultaneously imposed to all pixels in the data set. Our experimental results with both synthetic and real hyperspectral data sets show clearly the advantages obtained using the new joint sparse regression strategy, compared with the pixelwise independent approach. © 1980-2012 IEEE.


Iordache M.-D.,University of Extremadura | Bioucas-Dias J.M.,Telecommunications Institute of Portugal | Plaza A.,University of Extremadura
IEEE Transactions on Geoscience and Remote Sensing | Year: 2011

Linear spectral unmixing is a popular tool in remotely sensed hyperspectral data interpretation. It aims at estimating the fractional abundances of pure spectral signatures (also called as endmembers) in each mixed pixel collected by an imaging spectrometer. In many situations, the identification of the endmember signatures in the original data set may be challenging due to insufficient spatial resolution, mixtures happening at different scales, and unavailability of completely pure spectral signatures in the scene. However, the unmixing problem can also be approached in semisupervised fashion, i.e., by assuming that the observed image signatures can be expressed in the form of linear combinations of a number of pure spectral signatures known in advance (e.g., spectra collected on the ground by a field spectroradiometer). Unmixing then amounts to finding the optimal subset of signatures in a (potentially very large) spectral library that can best model each mixed pixel in the scene. In practice, this is a combinatorial problem which calls for efficient linear sparse regression (SR) techniques based on sparsity-inducing regularizers, since the number of endmembers participating in a mixed pixel is usually very small compared with the (ever-growing) dimensionality (and availability) of spectral libraries. Linear SR is an area of very active research, with strong links to compressed sensing, basis pursuit (BP), BP denoising, and matching pursuit. In this paper, we study the linear spectral unmixing problem under the light of recent theoretical results published in those referred to areas. Furthermore, we provide a comparison of several available and new linear SR algorithms, with the ultimate goal of analyzing their potential in solving the spectral unmixing problem by resorting to available spectral libraries. Our experimental results, conducted using both simulated and real hyperspectral data sets collected by the NASA Jet Propulsion Laboratory's Airborne Visible Infrared Imaging Spectrometer and spectral libraries publicly available from the U.S. Geological Survey, indicate the potential of SR techniques in the task of accurately characterizing the mixed pixels using the library spectra. This opens new perspectives for spectral unmixing, since the abundance estimation process no longer depends on the availability of pure spectral signatures in the input data nor on the capacity of a certain endmember extraction algorithm to identify such pure signatures. © 2011 IEEE.


Atanasov V.,Telecommunications Institute of Portugal | Saxena A.,Los Alamos National Laboratory
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

It is shown that for monolayer graphene electrons are confined on a perfect two-dimensional surface. The implications for the electronic properties of corrugated graphene are discussed in view of a derivation of the constrained relativistic dynamics for the massless carriers in two dimensions in accord with the Heisenberg uncertainty principle. Surface curvature is related to a series of phenomena with practical applications such as curvature induced p-n junctions, band-gap opening, and decoherence. We also establish a bending free energy by treating graphene as a soft electronic membrane. © 2010 The American Physical Society.


Silveirinha M.G.,Telecommunications Institute of Portugal
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

We critically analyze the anomalies in the frequency dispersion of the magnetic permeability, showing that it may be sometimes-without contradicting causality-inconsistent with the Kramers-Kronig relations for passive materials, as formulated by Landau and Lifshitz, even at extremely low frequencies where the permeability has definitely a very precise physical meaning. This suggests that in general the permeability may not satisfy the Kramers-Kronig formulas for passive materials, and an alternative set of relations to link the real and imaginary parts of the permeability in the frequency region where the permeability retains its physical meaning is proposed. © 2011 American Physical Society.


Silveirinha M.G.,Telecommunications Institute of Portugal
New Journal of Physics | Year: 2014

Here, we develop a comprehensive quantum theory for the phenomenon of quantum friction. Based on a theory of macroscopic quantum electrodynamics for unstable systems, we calculate the quantum expectation of the friction force at zero temperature, and link the friction effect to the emergence of system instabilities related to the Cherenkov effect. These instabilities may occur due to the hybridization of particular guided modes supported by the individual moving bodies, and selection rules for the interacting modes are derived. It is proven that the quantum friction effect can take place even when the interacting bodies are lossless and made of nondispersive dielectrics. © 2014 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.


Estima J.O.,University of Coimbra | Estima J.O.,Telecommunications Institute of Portugal | Marques Cardoso A.J.,Telecommunications Institute of Portugal | Marques Cardoso A.J.,University of Beira Interior
IEEE Transactions on Industrial Electronics | Year: 2013

Three-phase inverters are currently utilized in an enormous variety of industrial applications, including variable-speed ac drives. However, due to their complexity and exposure to several stresses, they are prone to suffer critical failures. Accordingly, this paper presents a novel diagnostic algorithm that allows the real-time detection and localization of multiple power switch open-circuit faults in inverter-fed ac motor drives. The proposed method is quite simple and just requires the measured motor phase currents and their corresponding reference signals, already available from the main control system, therefore avoiding the use of additional sensors and hardware. Several experimental results using a vector-controlled permanent-magnet synchronous motor drive are presented, showing the diagnostic algorithm effectiveness, its relatively fast detection time, and its robustness against false alarms. © 1982-2012 IEEE.

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