Surface solitons in two-dimensional chirped photonic lattices

We study surface modes in semi-infinite chirped two-dimensional photonic lattices in the frame- work of an effective discrete nonlinear model. We demonstrate that the lattice chirp can change dramatically the conditions for the mode localization near the surface, and we find numerically the families of surface modes, in linear lattices, and discrete surface solitons, in nonlinear lattices. We demonstrate that, in a sharp contrast to one-dimensional discrete surface solitons, in two-dimensional lattices the mode threshold power is lowered by the action of both the surface and lattice chirp. By manipulating with the lattice chirp, we can control the mode position and its localization.Comment: 12 pages, 7 figure

Cognitive plasticity in normal and pathological aging

The main goal of the present study is to examine to what extent age and cognitive impairment contribute to learning performance (cognitive plasticity, cognitive modifiability, or learning potential). To address this question, participants coming from four studies (Longitudinal Study of Active Aging, age range, 55–75 years, N = 458; Longitudinal Study in the very old [90+], age range, 90–102, N = 188, and Cognitive Plasticity within the Course of Cognitive Impairment, 97 “Normal”, 57 mild cognitive impairment [MCI], and 98 Alzheimer’s disease [AD] patients) were examined through a measure of verbal learning (developed from Rey). The results show that all age, MCI, and AD groups learned across the five learning trials of that test, but significant differences were found due to age, pathology, and education. The effects of pathology (MCI and AD) can be expressed in a metric of “years of normal decline by age”; specifically, being MCI means suffering an impairment in performance that is equivalent to the decline of a normal individual during 15 years, whereas the impact of AD is equivalent to 22.7 years. Likewise, the improvement associated with about 5 years of education is equivalent to about 1 year less of normal aging. Also, the two pathological groups significantly differed from “normal” groups in the delayed trial of the test. The most dramatic difference is that between the “normal” group and the AD patients, which shows relatively poorer performance for the AD group in the delayed trial than in the first learning trial. The potential role of this unique effect for quick detection purposes of AD is assessed (in the 75–89 years age range, sensitivity and specificity equal 0.813 and 0.917, respectively).This study has been granted by the Research General Direction, MICINN: Project SEJ-2006-14438/PSIC; IMSERSO I+D+I Projects: 15-05 and 35-06

Assessment of LTCC-Based Dielectric Flat Lens Antennas and Switched-Beam Arrays for Future 5G Millimeter-Wave Communication Systems

[EN] This paper presents the design, low-temperature co-fired ceramics (LTCC) fabrication, and full experimental verification of novel dielectric flat lens antennas for future high data rate 5G wireless communication systems in the 60 GHz band. We introduce and practically completely evaluate and compare the performance of three different inhomogeneous gradient-index dielectric lenses with the effective parameters circularly and cylindrically distributed. These lenses, despite their planar profile antenna configuration, allow full 2-D beam scanning of high-gain radiation beams. A time-domain spectroscopy system is used to practically evaluate the permittivity profile achieved with the LTCC manufacturing process, obtaining very good results to confirm the viability of fabricating inhomogeneous flat lenses in a mass production technology. Then, the lenses performance is evaluated in terms of radiation pattern parameters, maximum gain, beam scanning, bandwidth performance, efficiencies, and impedance matching in the whole frequency band of interest. Finally, the performance of the three lenses is also experimentally evaluated and compared to a single omni-directional antenna and to a ten-element uniform linear array of omni-directional antennas in real 60 GHz wireless personal area network indoor line-of-sight (LOS) and obstructed-LOS environments, obtaining interesting and promising remarkable results in terms of measured received power and root-mean-square delay spread. At the end of this paper, an innovative switched-beam antenna array concept based on the presented cylindrically distributed effective parameters lens is also introduced and completely evaluated, confirming the potential applicability of the proposed antenna solution for future 5G wireless millimeter-wave communication system.This work was supported in part by the Spanish Inter-Ministerial Commission on Science and Technology (CICYT) under Projects TEC2013-47360-C3-1-P, TEC2013-47360-C3-2-P, and TEC2016-78028-C3-1-P, by FEDER and the Unidad de Excelencia Maria de Maeztu MDM-2016-0600, which is financed by the Agenda Estatal de Investigacion, Spain, and by the "Ministerio de Economia" through the FPI fellowship program.Imbert, M.; Romeu, J.; Baquero Escudero, M.; Martinez-Ingles, M.; Molina-García-Pardo, JM.; Jofre, L. (2017). Assessment of LTCC-Based Dielectric Flat Lens Antennas and Switched-Beam Arrays for Future 5G Millimeter-Wave Communication Systems. IEEE Transactions on Antennas and Propagation. 65(12):6453-6473. https://doi.org/10.1109/TAP.2017.2767821S64536473651

Structural instability of vortices in Bose-Einstein condensates

In this paper we study a gaseous Bose-Einstein condensate (BEC) and show that: (i) A minimum value of the interaction is needed for the existence of stable persistent currents. (ii) Vorticity is not a fundamental invariant of the system, as there exists a conservative mechanism which can destroy a vortex and change its sign. (iii) This mechanism is suppressed by strong interactions.Comment: 4 pages with 3 figures. Submitted to Phys. Rev. Let

Diphenylphenoxy-Thiophene-PDI Dimers as Acceptors for OPV Applications with Open Circuit Voltage Approaching 1 Volt

Two new perylenediimides (PDIs) have been developed for use as electron acceptors in solution-processed bulk heterojunction solar cells. The compounds were designed to exhibit maximal solubility in organic solvents, and reduced aggregation in the solid state. In order to achieve this, diphenylphenoxy groups were used to functionalize a monomeric PDI core, and two PDI dimers were bridged with either one or two thiophene units. In photovoltaic devices prepared using PDI dimers and a monomer in conjunction with PTB7, it was found that the formation of crystalline domains in either the acceptor or donor was completely suppressed. Atomic force microscopy, X-ray diffraction, charge carrier mobility measurements and recombination kinetics studies all suggest that the lack of crystallinity in the active layer induces a significant drop in electron mobility. Significant surface recombination losses associated with a lack of segregation in the material were also identified as a significant loss mechanism. Finally, the monomeric PDI was found to have sub-optimum LUMO energy matching the cathode contact, thus limiting charge carrier extraction. Despite these setbacks, all PDIs produced high open circuit voltages, reaching almost 1 V in one particular caseThis work was supported by the Spanish Ministry of Economy, Industry and Competitiveness (MEIC) (TEC2015-71324-R, CTQ2014-55798-R and TEC2015-71915-REDT (MINECO/FEDER))This work was supported by the Catalan Institution for Research and Advanced Studies (ICREA) (ICREA “Academia Award”, AGAUR 2017 SGR 017SGR1527

Soliton emission in amplifying optical lattice surfaces

We address surface solitons supported by the interface of optical lattices imprinted in saturable media with surface-localized gain. The nonlinearity saturation puts restrictions on the maximal energy flow carried by surface solitons. As a consequence, the presence of thin amplifying layer near the surface results in the controllable emission of solitons towards the lattice at angles depending on the amplification rate and on lattice depth.Comment: 14 pages, 4 figures, to appear in Optics Letter

Globally-Linked Vortex Clusters in Trapped Wave Fields

We put forward the existence of a rich variety of fully stationary vortex structures, termed H-clusters, made of an increasing number of vortices nested in paraxial wave fields confined by trapping potentials. However, we show that the constituent vortices are globally linked, rather than products of independent vortices. Also, they always feature a monopolar global wave front and exist in nonlinear systems, such as Bose-Einstein condensates. Clusters with multipolar global wave fronts are non-stationary or at best flipping.Comment: 4 pages, 5 PostScript figure

Quantum spiral bandwidth of entangled two-photon states

We put forward the concept of quantum spiral bandwidth of the spatial mode function of the two-photon entangled state in spontaneous parametric downconversion. We obtain the bandwidth using the eigenstates of the orbital angular momentum of the biphoton states, and reveal its dependence with the length of the down converting crystal and waist of the pump beam. The connection between the quantum spiral bandwidth and the entropy of entanglement of the quantum state is discussed.Comment: 10 pages, 3 figure

Bragg guiding of domain-like nonlinear modes and kink arrays in lower-index core structures

We introduce a novel class of stable nonlinear modes trapped in a lower-index film core sandwiched between two optical lattices, or in the cylindrical core of a radial lattice, imprinted in defocusing media. Such family of nonlinear modes transforms into defect lattice solitons when the core width is sufficiently small or into an array of kinks when the width is large enough. We find that higher-order modes with multiple zeros inside the guiding core can be stable in one-dimensional settings.Comment: 12 pages, 3 figures, to appear in Optics Letter