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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.

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.

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.

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.

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.

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