A vector fast multipole algorithm for low frequency problems

Instead of the traditional factorization of the scalar Green's function by using scalar addition theorem in the lowfrequency fast multipole algorithm (LF-FMA), we adopt the vector addition theorem (VAT) for the factorization of the dyadic Green's function to realize memory savings for large scale problems. We validate this factorization and use it to develop a low-frequency vector fast multipole algorithm (LF-VFMA) for low-frequency problems. © 2010 IEEE.published_or_final_versionThe URSI International Symposium on Electromagnetic Theory (EMTS 2010), Berlin, Germany, 16-19 August 2010. In Proceedings of the URSI International Symposium on Electromagnetic Theory, 2010, p. 620-62

A memory saving vector fast multipole algorithm for solving the augmented EFIE

An augmented EFIE (A-EFIE)[9], [10] has been proposed to separate the contributions of the vector potential and the scalar potential for avoiding the imbalance at low frequencies. The corresponding low frequency fast multipole algorithm (LFFMA) [11] was also developed for solving the A-EFIE. Instead of the factorization of the scalar Green's function by using scalar addition theorem in the LF-FMA, we adopt the vector addition theorem for the factorization of the dyadic Green's function to realize memory savings. We are to develop a vector fast multipole algorithm for solving the A-EFIE. © 2010 IEEE.published_or_final_versionThe URSI International Symposium on Electromagnetic Theory (EMTS 2010), Berlin, Germany, 16-19 August 2010. In Proceedings of the URSI International Symposium on Electromagnetic Theory, 2010, p. 134-13

Directional far-field response of a spherical nanoantenna

We study the directional far-field response of a spherical nanoantenna via engineering the plasmonic nanosphere's distance, size, and material. A unified pattern synthesis approach based on the T-matrix method and the particle swarm optimization is proposed for the directional beamforming of the nanoantenna. The angular response of the directional nanoantenna is very sensitive to the material change but is immunized to the random error of the spatial position of each particle. The physical origin of the high directionality is attributed to the coherent near-field distribution with large correlation length. This work provides the fundamental theory and physics for future nanoantenna design. © 2011 Optical Society of America.published_or_final_versio

Calderon Multiplicative Preconditioned EFIE With Perturbation Method

In this paper, we address the low-frequency breakdown and inaccuracy problems in the Calderón multiplicative preconditioned electric field integral equation (CMP-EFIE) operator, and propose the perturbation method as a remedy for three-dimensional perfect electric conductor (PEC) scatterers. The electric currents at different frequency orders as a power series can be obtained accurately in a recursive manner by solving the same matrix system with updated right hand side vectors. This method does not either require a search for the loops in the loop-tree/-star based method or include charge as additional unknown in the augmented EFIE method. Numerical examples show the far-field pattern can be accurately computed at extremely low frequencies by the proposed perturbation method. © 1963-2012 IEEE.published_or_final_versio

Near-field multiple scattering effects of plasmonic nanospheres embedded into thin-film organic solar cells

We investigate near-field multiple scattering effects of plasmonic nanospheres (NSPs) embedded into organic solar cells (OSCs). When NSPs are embedded into a spacer layer, the near-field scattering from the NSPs shows strong direction-dependent features, which significantly affects the optical absorption. When NSPs are embedded into an active layer, the absorption enhancement is attributed to the interplay between longitudinal and transverse modes supported by the NSP chain. The breakdown of electrostatic scaling law is confirmed by our theoretical model and should be accounted for optical designs of OSCs. The work provides the fundamental physical understanding and design guidelines for plasmonic photovoltaics. © 2011 American Institute of Physics.published_or_final_versio

A study of Inx Ga1-x N growth by reflection high-energy electron diffraction

Epitaxial growth of Inx Ga1-x N alloys on GaN (0001) by plasma-assisted molecular-beam epitaxy is investigated using the in situ reflection high-energy electron-diffraction (RHEED) technique. Based on RHEED pattern changes over time, the transition of growth mode from two-dimensional (2D) nucleation to three-dimensional islanding is studied for various indium compositions. RHEED specular-beam intensity oscillations are recorded during the 2D wetting-layer growth, and the dependences of the oscillation period/frequency on the substrate temperature and source flux are established. By measuring the spacing between diffraction spots in RHEED, we also estimated indium composition, x, in alloys grown under different flux combinations. Incorporation coefficients of both gallium and indium are derived. Possible surface segregation of indium atoms is finally examined. © 2005 American Institute of Physics.published_or_final_versio

Generalized modal expansion of electromagnetic field in 2-D bounded and unbounded media

A generalized modal expansion theory is presented to investigate and illustrate the physics of wave-matter interaction within arbitrary two-dimensional (2-D) bounded and unbounded electromagnetic problems. We start with the bounded case where the field excited by any sources is expanded with a complete set of biorthogonal eigenmodes. In regard to non-Hermitian or nonreciprocal problems, an auxiliary system is constructed to seek for the modal-expansion solution. We arrive at the unbounded case when the boundary tends to infinity or is replaced by the perfectly matched layer (PML). Modes are approximately categorized into two types: trapped modes and radiation modes, which respond differently to environment variations. When coupled with the source, these modes contribute to the modal-expansion solution with different weights, which leads to a reduced modal representation of the excited field in some geometries. © 2002-2011 IEEE.published_or_final_versio

Coherent and dislocated three-dimensional islands of Inx Ga1-x N self-assembled on GaN(0001) during molecular-beam epitaxy

Molecular-beam epitaxy of Inx Ga1-x N alloy on GaN(0001) is investigated by scanning tunneling microscopy. The Stranski-Krastanov mode of growth of the alloy is followed, where the newly nucleated three-dimensional islands are initially coherent to the underlying GaN and the wetting layer, but then become dislocated when grown bigger than about 20 nm in the lateral dimension. Two types of islands show different shapes, where the coherent ones are cone shaped and the dislocated ones are pillar like, having flat-tops. Within a certain range of material coverage, the surface contains both coherent and dislocated islands, showing an overall bimodal island-size distribution. The continued deposition on such surfaces leads to the pronounced growth of dislocated islands, whereas the sizes of the coherent islands change very little. © 2005 The American Physical Society.published_or_final_versio

Three-band tight-binding model for monolayers of group-VIB transition metal dichalcogenides

published_or_final_versio

A pancake-shaped nano-aggregate for focusing surface plasmons

We proposed a pancake-shaped nano-aggregate that highly focuses surface plasmons. The structure is a superposition of bowtie-shaped dimers, where surface plasmons are excited, resonated with the structure, and coupled. Surface integral equation method (Poggio-Miller-Chang-Harrington-Wu-Tsai method) is used to predict the performance of the proposed structure. It is a method which can accurately calculate the near-fields of nanoparticles. Based on the numerical prediction, the proposed structure shows an electric field (E-field) enhancement of more than 400 times, which is equivalent to a Raman enhancement factor of more than 2.5 e 10 times. It is promising for single molecule detections using surface-enhanced Raman scattering. The physics of the proposed structure are revealed. It is useful to design nanostructures for high E-field enhancement. © 2012 American Institute of Physics.published_or_final_versio