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Three-dimensional splitting dynamics of giant vortices in Bose-Einstein condensates

Physical Review A 2018年第2期98卷 023624-023624页

作者： Jukka Räbinä Pekko Kuopanportti Markus I. Kivioja Mikko Möttönen Tuomo Rossi Faculty of Information Technology University of Jyvaskyla P.O. Box 35 40014 Jyvaskyla Finland Department of Physics University of Helsinki P.O. Box 43 00014 Helsinki Finland QCD Labs QTF Centre of Excellence Department of Applied Physics Aalto University P.O. Box 13500 00076 Aalto Finland

We study the splitting dynamics of giant vortices in dilute Bose-Einstein condensates by numerically integrating the three-dimensional Gross-Pitaevskii equation in time. By taking advantage of tetrahedral tiling in the spatial discretization, we decrease the error and increase the reliability of the numerical method. An extensive survey of vortex splitting patterns is presented for different aspect ratios of the harmonic trapping potential. The discrete rotational symmetries of the splitting patterns that emerge in the time evolution are in good agreement with predictions obtained by solving the prevailing dynamical instabilities from the Bogoliubov equations. Furthermore, we observe intertwining of the split vortices in prolate condensates and a split-and-revival phenomenon in a spherical condensate.

关键词： Bose-Einstein condensates Vortices in superfluids finite-difference time-domain method

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Broadband angle-independent antireflection coatings on nanostructured light trapping solar cells

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Physical Review Materials 2018年第3期2卷 035201-035201页

作者： Abraham Vázquez-Guardado Javaneh Boroumand Daniel Franklin Debashis Chanda CREOL College of Optics and Photonics University of Central Florida Orlando Florida 32816 USA NanoScience Technology Center University of Central Florida Orlando Florida 32826 USA Department of Physics University of Central Florida Orlando Florida 32816 USA

Backscattering from nanostructured surfaces greatly diminishes the efficacy of light trapping solar cells. While the analytical design of broadband, angle-independent antireflection coatings on nanostructured surfaces proved inefficient, numerical optimization proves a viable alternative. Here, we numerically design and experimentally verify the performance of single and bilayer antireflection coatings on a 2D hexagonal diffractive light trapping pattern on crystalline silicon substrates. Three well-known antireflection coatings, aluminum oxide, silicon nitride, and silicon oxide, which also double as high-quality surface passivation materials, are studied in the 400–1000 nm band. By varying thickness and conformity, the optimal parameters that minimize the broadband total reflectance (specular and scattering) from the nanostructured surface are obtained. The design results in a single-layer antireflection coating with normal-angle wavelength-integrated reflectance below 4% and a bilayer antireflection coating demonstrating reflection down to 1.5%. We show experimentally an angle-averaged reflectance of ∼5.2% up to 60° incident angle from the optimized bilayer antireflection-coated nanostructured surface, paving the path toward practical implementation of the light trapping solar cells.

关键词： Light propagation, transmission & absorption Nanostructures finite-difference time-domain method Lithography Optical spectroscopy

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Impact of Ag@SiO₂ core-shell nanoparticles on the photoelectric current of plasmonic inverted organic solar cells

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SYNTHETIC METALS 2018年 239卷 22-28页

作者： N'Konou, Kekeli Chalh, Malika Monnier, Virginie Blanchard, Nicholas P. Chevolot, Yann Lucas, Bruno Vedraine, Sylvain Torchio, Philippe Aix Marseille Univ IM2NP CNRS Serv 231UMR 7334 Domaine Univ St Jerome F-13397 Marseille 20 France Univ Limoges XLIM CNRS 7252 123 Ave Albert Thomas F-87060 Limoges France Univ Lyon Inst Nanotechnol Lyon UMR CNRS 5270 Ecole Cent Lyon 36 Ave Guy de Colongue F-69134 Ecully France Univ Claude Bernard Lyon 1 Univ Lyon CNRS Inst Lumiere Matiere F-69622 Villeurbanne France

We report on the influence of silver-silica core-shell nanoparticles (Ag@SiO(2)NPs) and bare silver nanoparticles (Ag NPs) on the photoelectric current of plasmonic inverted organic solar cells. The synthesized Ag@SiO(2)NPs are deposited on a zinc oxide layer. The optimized plasmonic devices concern the Ag@SiO(2)NPs with 5 nm shell thickness and present the highest short-circuit current density of 13.44 mA/cm(2), enhanced by 12% compared to the reference device. Such enhancements can mainly be attributed to the localized surface plasmon resonance and to the light scattering effect originating from Ag NPs and Ag@SiO(2)NPs. finite-difference time-domain simulation reveals that a thin shell thickness facilitates the extension of a strong localized enhanced electromagnetic field in the active layer, as confirmed by measured and simulated optical absorption and photoluminescence measurements. Furthermore, AFM images present a well-distributed Ag@SiO(2)NPs array on the ZnO films, which contributes to such an improvement.

关键词： Ag@SiO2 core-shell nanospheres Plasmonic solar cells Absorption enhancement finite-difference time-domain method

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Macroscopic Magnetization Control by Symmetry Breaking of Photoinduced Spin Reorientation with Intense Terahertz Magnetic Near Field

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Physical Review Letters 2018年第10期120卷 107202-107202页

作者： Takayuki Kurihara Hiroshi Watanabe Makoto Nakajima Shutaro Karube Kenichi Oto YoshiChika Otani Tohru Suemoto Institute for Solid State Physics The University of Tokyo 5-1-5 Kashiwanoha Kashiwa Chiba 277-8581 Japan Institute of Laser Engineering Osaka University 2-6 Yamadaoka Suita Osaka 565-0871 Japan Center for Emergent Matter Science RIKEN 2-1 Hirosawa Wako Saitama 351-0198 Japan Department of Physics Chiba University 1-33 Yayoicho Inage Ward Chiba-shi Chiba 263-8522 Japan

We exploit an intense terahertz magnetic near field combined with femtosecond laser excitation to break the symmetry of photoinduced spin reorientation paths in ErFeO3. We succeed in aligning macroscopic magnetization reaching up to 80% of total magnetization in the sample to selectable orientations by adjusting the time delay between terahertz and optical pump pulses. The spin dynamics are well reproduced by equations of motion, including time-dependent magnetic potential. We show that the direction of the generated magnetization is determined by the transient direction of spin tilting and the magnetic field at the moment of photoexcitation.

关键词： Antiferromagnetism Faraday effect Ferromagnetism Magnetic anisotropy Magnetic phase transitions Magnetization switching Magneto-optical effect Magnons Optical & microwave phenomena Antiferromagnets Ferromagnets Coherent control Femtosecond laser spectroscopy Ferromagnetic resonance finite-difference time-domain method Landau-Lifschitz-Gilbert equation Optically detected magnetic resonance time-resolved infrared spectroscopy Ultrafast pump-probe spectroscopy

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An unconditionally stable technique for uncertainty assessment in random media based on the ADI scheme

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JOURNAL OF ELECTROMAGNETIC WAVES AND APPLICATIONS 2018年第6期32卷 671-684页

作者： Salis, Christos I. Zygiridis, Theodoros T. Univ Western Macedonia Dept Informat & Telecommun Engn Kozani Greece

Stochastic media problems present a severe impact on the properties of the electromagnetic fields that usually deterministic methods tend to overlook. In this paper, we present a finite-difference time-domain (FDTD) technique, which is free from time step restrictions and manages to efficiently calculate the statistics of the involved fields in one realization. The proposed algorithm is a variation of the Stochastic FDTD method and exhibits satisfactory outcomes in high density grids. Numerical results verify the validity of our scheme compared to other methods, such as the Stochastic FDTD and the Monte Carlo techniques.

关键词： finite-difference time-domain method ADI-FDTD technique stochastic FDTD technique stochastic ADI-FDTD scheme Monte Carlo techniques random media problems

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Full-wave analysis of traveling pulses developed in a system of transmission lines with regularly spaced resonant-tunneling diodes

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INTERNATIONAL JOURNAL OF CIRCUIT THEORY AND APPLICATIONS 2018年第3期46卷 671-682页

作者： Narahara, Koichi Maezawa, Koichi Kanagawa Inst Technol Dept Elect & Elect Engn 1030 Shimoogino Atsugi Kanagawa 2430292 Japan Univ Toyama Grad Sch Sci & Engn 3190 Gofuku Toyama Toyama 9308555 Japan

In a system of transmission lines with regularly spaced resonant-tunneling diodes (RTDs), where several straight RTD lines are connected halfway to a closed RTD line, a pulse-shaped rotary traveling wave develops on the closed line by mutual synchronization of the oscillatory edge developed in each straight RTD line. The oscillating edge on each straight line is synchronized with the traveling pulse, such that the system has the potential to generate multiphase oscillatory signals in millimeter-wave frequencies. To examine the dynamics of traveling pulses at such high frequencies, the system is modeled in the framework of the finite-difference time-domain method. It is found that a traveling pulse develops in the closed RTD line synchronized with the oscillatory edges moving in the straight lines, assuming a microstrip structure for each RTD line. We then compare the results of the finite-difference time-domain calculation with those predicted by the transmission line theory with parameter values obtained by the quasi-transverse electromagnetic estimation. In addition, the RTD line that compactly confines the electromagnetic fields is shown to have the potential to generate multiphase oscillatory signals at submillimeter-wave frequencies.

关键词： finite-difference time-domain method multiphase oscillations nonlinear transmission lines resonant-tunneling diodes synchronization

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Investigation of uncertainty in lightning-produced EM fields with a polynomial-chaos FDTD approach

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INTERNATIONAL JOURNAL OF NUMERICAL MODELLING-ELECTRONIC NETWORKS DEVICES AND FIELDS 2018年第2期31卷

作者： Zygiridis, Theodoros Kantartzis, Nikolaos Tsiboukis, Theodoros Univ Western Macedonia Dept Informat & Telecommun Engn Kozani 50131 Greece Aristotle Univ Thessaloniki Dept Elect & Comp Engn Thessaloniki 54124 Greece

The aim of this paper is to investigate the impact of the ground-material uncertainty on the properties of electromagnetic fields generated by lightning strikes. Although several analytical as well as numerical approaches have been proposed throughout previous years for the prediction of lightning-induced fields, these are mostly applied within a fully deterministic context, thus overlooking the degree of randomness that the corresponding problems may exhibit. Considering the rotational independence of setups with vertical lightning channels and flat terrains, we propose a stochastic finite-difference time-domain algorithm that takes advantage of the generalized polynomial-chaos theory. The latter enables the representation of random processes via expansions with respect to specific orthogonal polynomials and facilitates capturing the fundamental statistical properties of the produced fields. The resulting scheme preserves the structure of the original Yee algorithm and, although computationally heavier than the latter, is clearly more efficient than conducting brute-force Monte Carlo studies. The potential of the proposed stochastic method is assessed via various numerical results, calculation of statistical metrics, and comparisons with reference solutions.

关键词： finite-difference time-domain method lightning polynomial chaos uncertainty

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Interaction between confined phonons and photons in periodic silicon resonators

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Physical Review B 2018年第9期97卷 094308-094308页

作者： A. Iskandar A. Gwiazda J. Younes M. Kazan A. Bruyant M. Tabbal G. Lerondel Department of Physics American University of Beirut P.O. Box 11-0236 Riad El-Solh Beirut 1107-2020 Lebanon Laboratoire de Nanotechnologie et d’Instrumentation Optique ICD CNRS UMR 6281 Université de Technologie de Troyes 10010 Troyes France

In this paper, we demonstrate that phonons and photons of different momenta can be confined and interact with each other within the same nanostructure. The interaction between confined phonons and confined photons in silicon resonator arrays is observed by means of Raman scattering. The Raman spectra from large arrays of dielectric silicon resonators exhibited Raman enhancement accompanied with a downshift and broadening. The analysis of the Raman intensity and line shape using finite-difference time-domain simulations and a spatial correlation model demonstrated an interaction between photons confined in the resonators and phonons confined in highly defective regions prompted by the structuring process. It was shown that the Raman enhancement is due to collective lattice resonance inducing field confinement in the resonators, while the spectra downshift and broadening are signatures of the relaxation of the phonon wave vector due to phonon confinement in defective regions located in the surface layer of the Si resonators. We found that as the resonators increase in height and their shape becomes cylindrical, the amplitude of their coherent oscillation increases and hence their ability to confine the incoming electric field increases.

关键词： Confinement Phonons Photonics Nanostructures finite-difference time-domain method Raman spectroscopy

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Photonic engineering of highly linearly polarized quantum dot emission at telecommunication wavelengths

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Physical Review B 2018年第16期97卷 165427-165427页

作者： P. Mrowiński M. Emmerling C. Schneider J. P. Reithmaier J. Misiewicz S. Höfling G. Sęk OSN Lab Department of Experimental Physics Faculty of Fundamental Problems of Technology Wrocław University of Science Technology Poland Technische Physik & W. C. Röntgen-Center for Complex Material Systems Universität Würzburg Germany Institute of Nanostructure Technologies and Analytics (INA) CINSaT University of Kassel Heinrich-Plett-Str. 40 34132 Kassel Germany SUPA School of Physics and Astronomy University of St. Andrews North Haugh KY16 9SS St. Andrews United Kingdom

In this work, we discuss a method to control the polarization anisotropy of spontaneous emission from neutral excitons confined in quantum-dot-like nanostructures, namely single epitaxial InAs quantum dashes emitting at telecom wavelengths. The nanostructures are embedded inside lithographically defined, in-plane asymmetric photonic mesa structures, which generate polarization-dependent photonic confinement. First, we study the influence of the photonic confinement on the polarization anisotropy of the emission by photoluminescence spectroscopy, and we find evidence of different contributions to a degree of linear polarization (DOLP), i.e., from the quantum dash and the photonic mesa, in total giving rise to DOLP=0.85. Then, we perform finite-difference time-domain simulations of photonic confinement, and we calculate the DOLP in a dipole approximation showing well-matched results for the established model. Furthermore, by using numerical calculations, we demonstrate several types of photonic confinements where highly linearly polarized emission with DOLP of about 0.9 is possible by controlling the position of a quantum emitter inside the photonic structure. Then, we elaborate on anisotropic quantum emitters allowing for exceeding DOLP=0.95 in an optimized case, and we discuss the ways towards efficient linearly polarized single photon source at telecom bands.

关键词： Excitons Nanophotonics Quantum dots finite-difference time-domain method Photoluminescence

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Dirac cones in isogonal hexagonal metallic structures

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Physical Review B 2018年第12期97卷 125146-125146页

作者： Kang Wang Laboratoire de Physique des Solides UMR CNRS Université Paris-Sud Université Paris-Saclay Orsay 91405 France

A honeycomb hexagonal metallic lattice is equivalent to a triangular atomic one and cannot create Dirac cones in its electromagnetic wave spectrum. We study in this work the low-frequency electromagnetic band structures in isogonal hexagonal metallic lattices that are directly related to the honeycomb one and show that such structures can create Dirac cones. The band formation can be described by a tight-binding model that allows investigating, in terms of correlations between local resonance modes, the condition for the Dirac cones and the consequence of the third structure tile sustaining an extra resonance mode in the unit cell that induces band shifts and thus nonlinear deformation of the Dirac cones following the wave vectors departing from the Dirac points. We show further that, under structure deformation, the deformations of the Dirac cones result from two different correlation mechanisms, both reinforced by the lattice's metallic nature, which directly affects the resonance mode correlations. The isogonal structures provide new degrees of freedom for tuning the Dirac cones, allowing adjustment of the cone shape by modulating the structure tiles at the local scale without modifying the lattice periodicity and symmetry.

关键词： Photonics Honeycomb lattice Polarization Band structure methods finite-difference time-domain method

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