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Proxy-equation paradigm: A strategy for massively parallel asynchronous computations

Physical Review E 2017年第3期96卷 033304-033304页

作者： Ankita Mittal Sharath Girimaji Department of Aerospace Engineering Texas A&M University College Station Texas 77843 USA.

Massively parallel simulations of transport equation systems call for a paradigm change in algorithm development to achieve efficient scalability. Traditional approaches require time synchronization of processing elements (PEs), which severely restricts scalability. Relaxing synchronization requirement introduces error and slows down convergence. In this paper, we propose and develop a novel “proxy equation” concept for a general transport equation that (i) tolerates asynchrony with minimal added error, (ii) preserves convergence order and thus, (iii) expected to scale efficiently on massively parallel machines. The central idea is to modify a priori the transport equation at the PE boundaries to offset asynchrony errors. Proof-of-concept computations are performed using a one-dimensional advection (convection) diffusion equation. The results demonstrate the promise and advantages of the present strategy.

关键词： finite-difference time-domain method Information theory Numerical techniques

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Abnormal blocking of a guided mode propagating in a silicon optical waveguide with periodic tunnel coupling

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QUANTUM ELECTRONICS 2017年第1期47卷 58-64页

作者： Kolosovskii, E. A. Tsarev, A. V. Russian Acad Sci AV Rzhanov Inst Semicond Phys Siberian Branch Prosp Akad Lavrenteva 13 Novosibirsk 630090 Russia

This paper considers abnormal blocking of a guided mode propagating in a silicon optical waveguide with periodic tunnelling inserts. Using an independent two-dimensional analysis by the method of lines (MoL) and direct simulation by the finite-difference time-domain (FDTD) method, we have identified additional signal blocking bands, unrelated to Bragg conversion to backward guided modes of the parent silicon waveguide. These bands are due to the conversion of the incident wave energy to a leaky quasi-mode of the periodically segmented structure, which subsequently transfers the energy to the ambient medium in the form of radiation modes. A distinctive feature of this phenomenon is resonant coupling of the guided mode of the strip waveguide with its radiation modes, which is due to the weak tunnel coupling with the periodically segmented structure. This structure does not support independent guided propagation, so the energy stored in it is re-emitted to space. The abnormal blocking effect may find application in optical telecommunications elements and in the fabrication of optical sensors.

关键词： silicon photonics integrated optics optical waveguide diffraction grating segmented waveguide method of lines finite-difference time-domain method nanophotonics

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Mode Matching for Optical Antennas

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Physical Review Letters 2017年第21期119卷 217401-217401页

作者： Thorsten Feichtner Silke Christiansen Bert Hecht Helmholtz-Zentrum Berlin für Materialien und Energie GmbH Institut Nanoarchitekturen für die Energieumwandlung Hahn-Meitner-Platz 1 14109 Berlin Germany Max Planck Institute for the Science of Light Staudtstrasse 2 91058 Erlangen Germany Nano-Optics & Biophotonics Group Department of Experimental Physics 5 Röntgen Research Center for Complex Material Research (RCCM) Physics Institute University of Würzburg Am Hubland 97074 Würzburg Germany Freie Universität Berlin Arnimallee 14 14195 Berlin Germany

The emission rate of a point dipole can be strongly increased in the presence of a well-designed optical antenna. Yet, optical antenna design is largely based on radio-frequency rules, ignoring, e.g., Ohmic losses and non-negligible field penetration in metals at optical frequencies. Here, we combine reciprocity and Poynting’s theorem to derive a set of optical-frequency antenna design rules for benchmarking and optimizing the performance of optical antennas driven by single quantum emitters. Based on these findings a novel plasmonic cavity antenna design is presented exhibiting a considerably improved performance compared to a reference two-wire antenna. Our work will be useful for the design of high-performance optical antennas and nanoresonators for diverse applications ranging from quantum optics to antenna-enhanced single-emitter spectroscopy and sensing.

关键词： Electromagnetism Plasmonics Plasmons finite-difference time-domain method

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Coupled-Mode Theory for Semiconductor Nanowires

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Physical Review Applied 2017年第3期7卷 034028-034028页

作者： Robert Buschlinger Michael Lorke Ulf Peschel Institute of Condensed Matter Theory and Optics Abbe Center of Photonics Friedrich Schiller University of Jena Max-Wien-Platz 1 07743 Jena Germany Institute for Theoretical Physics University of Bremen 28359 Bremen Germany

We present a model to describe the spatiotemporal evolution of guided modes in semiconductor nanowires based on a coupled-mode formalism. Light-matter interaction is modeled based on semiconductor Bloch equations, including many-particle effects in the screened Hartree-Fock approximation. Appropriate boundary conditions are used to incorporate reflections at waveguide end facets, thus allowing for the simulation of nanowire lasing. We compute the emission characteristics and temporal dynamics of CdS and ZnO nanowire lasers and compare our results both to finite-difference time-domain simulations and to experimental data. Finally, we explore the dependence of the lasing emission on the nanowire cavity and on the material relaxation time.

关键词： Lasers Light-matter interaction Nonlinear waveguides Optoelectronics Photonics Plasmonics Bloch wave theory finite-difference time-domain method

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A Consideration on Appropriate Parameters for Identifying Electron Density Profile in the Lower Ionosphere by using VLF Wave Propagation in the Earth-Ionosphere Waveguide

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ELECTRONICS AND COMMUNICATIONS IN JAPAN 2017年第8期100卷 16-23页

作者： Ando, Yoshiaki Ito, Hitoshi Univ Electrocommun Grad Sch Informat & Engn Chofu Tokyo Japan Japan Radio Co Ltd Tokyo Japan

We consider a problem identifying electron density profile in the lower ionosphere by observing electric fields at the Earth's surface in very-low-frequency (VLF) band. The identification problem is one of nonlinear inverse problems, and we solve the problem numerically by using the genetic algorithm in this study. The focus of this study is on appropriate parameters to solve the identification problem by using the VLF (22.2 kHz) wave propagating in the earth-ionosphere waveguide. Numerical results show that the range of the electron density which can be identified is from 107 m(-3) to 108 m(-3), appropriate discretization of the solution space along the altitude is 10km, and a sufficient number of observation points are necessary to mitigate the effect of noise.

关键词： very-low-frequency band earth-ionosphere waveguide finite-difference time-domain method

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Propagating Polaritons in III-Nitride Slab Waveguides

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Physical Review Applied 2017年第3期7卷 034019-034019页

作者： J. Ciers J. G. Roch J.-F. Carlin G. Jacopin R. Butté N. Grandjean Institute of Physics École Polytechnique Fédérale de Lausanne (EPFL) CH-1015 Lausanne Switzerland

We report on III-nitride waveguides with c-plane GaN/(Al,Ga)N quantum wells in the strong light-matter coupling regime supporting propagating polaritons. They feature a normal-mode splitting as large as 60 meV at low temperatures thanks to the large overlap between the optical mode and the active region, a polariton decay length up to 100 μm for photonlike polaritons and a lifetime of 1 to 2 ps, with the latter values being essentially limited by residual absorption occurring in the waveguide. The fully lattice-matched nature of the structure allows for very low disorder and high in-plane homogeneity, which is an important asset for the realization of polaritonic integrated circuits that could support nonlinear polariton wave packets up to room temperature thanks to the large exciton binding energy of 40 meV.

关键词： Exciton polariton Photonics Polaritons Waveguides III-V semiconductors Quantum wells finite-difference time-domain method time-resolved photoluminescence

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Fluorescent Nanowire Ring Illumination for Wide-Field Far-Field Subdiffraction Imaging

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Physical Review Letters 2017年第7期118卷 076101-076101页

作者： Xiaowei Liu Cuifang Kuang Xiang Hao Chenlei Pang Pengfei Xu Haifeng Li Ying Liu Chao Yu Yingke Xu Di Nan Weidong Shen Yue Fang Lenian He Xu Liu Qing Yang State Key Laboratory of Modern Optical Instrumentation College of Optical Science and Engineering Zhejiang University Hangzhou 310027 China School of Materials Science and Engineering Georgia Institute of Technology Atlanta Georgia 30332-0245 USA Key Laboratory for Biomedical Engineering of the Ministry of Education Department of Biomedical Engineering Zhejiang University Hangzhou 310027 China Institute of VLSI Design Zhejiang University Hangzhou 310027 China

Here we demonstrate an active method which pioneers in utilizing a combination of a spatial frequency shift and a Stokes frequency shift to enable wide-field far-field subdiffraction imaging. A fluorescent nanowire ring acts as a localized source and is combined with a film waveguide to produce omnidirectional illuminating evanescent waves. Benefitting from the high wave vector of illumination, the high spatial frequencies of an object can be shifted to the passband of a conventional imaging system, contributing subwavelength spatial information to the far-field image. A structure featuring 70-nm-wide slots spaced 70 nm apart has been resolved at a wavelength of 520 nm with a 0.85 numerical aperture standard objective based on this method. The versatility of this approach has been demonstrated by imaging integrated chips, Blu-ray DVDs, biological cells, and various subwavelength 2D patterns, with a viewing area of up to 1000 μm2, which is one order of magnitude larger than the previous far-field and full-field nanoscopy methods. This new resolving technique is label-free, is conveniently integrated with conventional microscopes, and can potentially become an important tool in cellular biology, the on-chip industry, as well as other fields requiring wide-field nanoscale visualization.

关键词： Imaging & optical processing Light-matter interaction Luminescence Nanowires Semiconductors Crystal growth finite-difference time-domain method Photoluminescence

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Geometry dependence of surface lattice resonances in plasmonic nanoparticle arrays

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Physical Review B 2017年第15期95卷 155423-155423页

作者： R. Guo T. K. Hakala P. Törmä COMP Centre of Excellence Department of Applied Physics Aalto University P.O. Box 15100 FI-00076 Aalto Finland

Plasmonic nanoarrays which support collective surface lattice resonances (SLRs) have become an exciting frontier in plasmonics. Compared with the localized surface-plasmon resonance in individual particles, these collective modes have appealing advantages such as angle-dependent dispersions and much narrower linewidths. Here, we investigate systematically how the geometry of the lattice affects the SLRs supported by metallic nanoparticles. We present a general theoretical framework from which the various SLR modes of a given geometry can be straightforwardly obtained by a simple comparison of the diffractive order vectors and orientation of the nanoparticle dipole given by the polarization of the incident field. Our experimental measurements show that while square, rectangular, hexagonal, honeycomb, and Lieb lattice arrays have similar spectra near the Γ point (k=0), they have remarkably different SLR dispersions. Furthermore, their dispersions are highly dependent on the polarization. Numerical simulations are performed to elucidate the field profiles of the different modes. Our findings extend the diversity of SLRs in plasmonic nanoparticle arrays, and the theoretical framework provides a simple model for interpreting the SLRs features, and vice versa, for designing the geometrical patterns.

关键词： Plasmonics Nanostructures finite-difference time-domain method Optical spectroscopy

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Tunable hyperbolic dispersion and negative refraction in natural electride materials

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Physical Review B 2017年第16期95卷 165436-165436页

作者： Shan Guan Shao Ying Huang Yugui Yao Shengyuan A. Yang Beijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic Systems School of Physics Beijing Institute of Technology Beijing 100081 China Research Laboratory for Quantum Materials Singapore University of Technology and Design Singapore 487372 Singapore Engineering Product Development Singapore University of Technology and Design Singapore 487372 Singapore

Hyperbolic (or indefinite) materials have attracted significant attention due to their unique capabilities for engineering electromagnetic space and controlling light propagation. A current challenge is to find a hyperbolic material with wide working frequency window, low energy loss, and easy controllability. Here, we propose that naturally existing electride materials could serve as high-performance hyperbolic medium. Taking the electride Ca2N as a concrete example and using first-principles calculations, we show that the material is hyperbolic over a wide frequency window from short-wavelength infrared to near infrared (from about 3.3μm to 880 nm). More importantly, it is almost lossless in the window. We clarify the physical origin of these remarkable properties and show its all-angle negative refraction effect. Moreover, we find that the optical properties can be effectively tuned by strain. With moderate strain, the material can even be switched between elliptic and hyperbolic for a particular frequency. Our result points out a new route toward high-performance natural hyperbolic materials, and it offers realistic materials and novel methods to achieve controllable hyperbolic dispersion with great potential for applications.

关键词： Dielectric properties Electronic structure Negative refraction Density functional theory finite-difference time-domain method

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Tunable narrow-band near-field thermal emitters based on resonant metamaterials

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Physical Review B 2017年第7期96卷 075413-075413页

作者： Jiayu Li Baoan Liu Sheng Shen Department of Mechanical Engineering Carnegie Mellon University Pittsburgh 15213 PA United States

In the near field, Planck's law of blackbody radiation breaks down, and radiative heat transfer can be enhanced by orders of magnitude when surface polaritons are supported by interacting materials. However, such thermal radiation enhancement is strongly material dependent, thus difficult to control. Here, we propose a metamaterial-based structure consisting of patterned doped silicon nanorods that exhibits tunable narrow-band thermal emission. Direct numerical simulation based on the Wiener chaos expansion (WCE) method is performed to accurately investigate the heat transfer mechanism of metamaterials in the near field. The fundamental principle of the WCE method is elucidated, and an algorithm for symmetric and periodic structures is discussed. Implementation of the WCE method with the finite-difference time-domain method using the discrete dipole approximation (DDA) is also addressed in this paper.

关键词： Metamaterials Thermal properties Nanostructures Dipole approximation Electromagnetic field calculations finite-difference time-domain method Stochastic differential equations

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