Frequency-independent scattering for the large sphere

The high frequency scattering of a scalar plane wave from an impenetrable sphere with the diameter of one thousand wavelength is treated by the saddle-point technique and the numerical steepest descent method. The far-field solution for the sphere is computed in the observation angle range of 0 to 180 degree. In particular, a novel numerical steepest descent method is proposed to overcome the breakdown of the traditional saddle-point technique in the forward region. Numerical results show that the CPU time for the far-field calculation is frequency-independent with controllable error. This work can be used to benchmark future works in frequency-independent methods. ©2009 IEEE.published_or_final_versionThe 2009 Asia Pacific Microwave Conference (APMC 2009), Singapore, 7-10 December 2009. In Proceedings of the Asia Pacific Microwave Conference, 2009, p. 854-85

Multiphysics Modeling of Plasmonic Organic Solar Cells with a Unified Finite-Difference Method

Invited Paper 13A multiphysics study carries out on plasmonic organic solar cells (OSCs) by solving Maxwell's equations and semiconductor (Poisson, drift-diffusion, and continuity) equations simultaneously with unified finite-difference framework. Regarding the Maxwell's equations, the perfectly matched layer and periodic boundary conditions are imposed at the vertical and lateral directions of OSCs to simulate the infinite air region and metallic grating electrode, respectively. In view of the semiconductor equations, the Scharfetter-Gummel scheme and semi-implicit strategy are adopted respectively in the space and time domains. To model the bulk heterojunction OSCs, the Langevin bimolecular recombination and Onsager-Braun exciton dissociation models are fully taken into account. The exciton generation rate depending on the optical absorption of the organic active material can be obtained by solving the Maxwell's equations and will be inserted into the semiconductor equations. Through the multiphysics model, we observed the increased shortcircuit current and dropped fill factor when OSCs incorporate a metallic grating anode supporting surface plasmon resonances. This work provides fundamental multiphysics modeling and understanding for plasmonic organic photovoltaics.published_or_final_versio

The roles of metallic rectangular-grating and planar anodes in the photocarrier generation and transport of organic solar cells

A multiphysics study carries out on organic solar cells (OSCs) by solving Maxwells and semiconductor equations simultaneously. By introducing a metallic rectangular-grating as the anode, surface plasmons are excited resulting in nonuniform exciton generation. Meanwhile, the internal E-field of plasmonic OSCs is modified with the modulated anode boundary. The plasmonic OSC improves 13 of short-circuit current but reduces 7 of fill factor (FF) compared to the standard one with a planar anode. The uneven photocarrier generation and transport by the grating anode are physical origins of the dropped FF. This work provides fundamental multiphysics modeling and understanding for plasmonic OSCs. © 2012 American Institute of Physics.published_or_final_versio

Angular response of thin-film organic solar cells with periodic metal back nanostrips

We theoretically study the angular response of thin-film organic solar cells with periodic Au back nanostrips. In particular, the equation of the generalized Lambert's cosine law for arbitrary periodic nanostructure is formulated. We show that the periodic strip structure achieves wide-angle absorption enhancement compared with the planar nonstrip structure for both the s-and p-polarized light, which is mainly attributed to the resonant Wood's anomalies and surface plasmon resonances, respectively. The work is important for designing and optimizing high-efficiency photovoltaic cells. © 2011 Optical Society of America.published_or_final_versio

Systematic study of spontaneous emission in a two-dimensional arbitrary inhomogeneous environment

The spontaneous emission (SE) of the excited atoms in a two-dimensional (2D) arbitrary inhomogeneous environment has been systematically studied. The local density of states, which determines the radiation dynamics of a point source (for 3D) or a line source (for 2D), in particular, the SE rate, is represented by the electric dyadic Green's function. The numerical solution of the electric Green's tensor has been accurately obtained with the finite-difference frequency-domain method with the proper approximations of the monopole and dipole sources. The SE of atoms in photonic crystal and plasmonic metal plates has been comprehensively and comparatively investigated. For both the photonic crystal and plasmonic plates systems, the SEs depend on their respective dispersion relations and could be modified by the finite-structure or finite-size effects. This work is important for SE engineering and the optimized design of optoelectronic devices. © 2011 American Physical Society.published_or_final_versio

Unidirectional and wavelength-selective photonic sphere-array nanoantennas

We design a photonic sphere-array nanoantenna (NA) exhibiting both strong directionality and wavelength selectivity. Although the geometric configuration of the photonic NA resembles a plasmonic Yagi-Uda NA, it has different working principles and, most importantly, reduces the inherent metallic loss from plasmonic elements. For any selected optical wavelength, a sharp Fano resonance by the reflector is tunable to overlap spectrally with a wider dipole resonance by the sphere-chain director, leading to high directionality. This Letter provides design principles for directional and selective photonic NAs, which are particularly useful for photon detection and spontaneous emission manipulation. © 2012 Optical Society of America.published_or_final_versio

Optical and electrical study of organic solar cells with a 2D grating anode

We investigate both optical and electrical properties of organic solar cells (OSCs) incorporating 2D periodic metallic back grating as an anode. Using a unified finite-difference approach, the multiphysics modeling framework for plasmonic OSCs is established to seamlessly connect the photon absorption with carrier transport and collection by solving the Maxwell's equations and semiconductor equations (Poisson, continuity, and drift-diffusion equations). Due to the excited surface plasmon resonance, the significantly nonuniform and extremely high exciton generation rate near the metallic grating are strongly confirmed by our theoretical model. Remarkably, the nonuniform exciton generation indeed does not induce more recombination loss or smaller open-circuit voltage compared to 1D multilayer standard OSC device. The increased open-circuit voltage and reduced recombination loss by the plasmonic OSC are attributed to direct hole collections at the metallic grating anode with a short transport path. The work provides an important multiphysics understanding for plasmonic organic photovoltaics. © 2012 Optical Society of America.published_or_final_versio

The numerical steepest descent path method for calculating physical optics integrals on smooth conducting quadratic surfaces

published_or_final_versio

A comprehensive study for the plasmonic thin-film solar cell with periodic structure

A comprehensive study of the plasmonic thin-film solar cell with the periodic strip structure is presented in this paper. The finite-difference frequency-domain method is employed to discretize the inhomogeneous wave function for modeling the solar cell. In particular, the hybrid absorbing boundary condition and the one-sided difference scheme are adopted. The parameter extraction methods for the zeroth-order reflectance and the absorbed power density are also discussed, which is important for testing and optimizing the solar cell design. For the numerical results, the physics of the absorption peaks of the amorphous silicon thin-film solar cell are explained by electromagnetic theory; these peaks correspond to the waveguide mode, Floquet mode, surface plasmon resonance, and the constructively interference between adjacent metal strips. The work is therefore important for the theoretical study and optimized design of the plasmonic thin-film solar cell. © 2010 Optical Society of America.published_or_final_versio

Study on spontaneous emission in complex multilayered plasmonic system via surface integral equation approach with layered medium Green's function

A rigorous surface integral equation approach is proposed to study the spontaneous emission of a quantum emitter embedded in a multi-layered plasmonic structure with the presence of arbitrarily shaped metallic nanoscatterers. With the aid of the Fermi's golden rule, the spontaneous emission of the emitter can be calculated from the local density of states, which can be further expressed by the imaginary part of the dyadic Green's function of the whole electromagnetic system. To obtain this Green's function numerically, a surface integral equation is established taking into account the scattering from the metallic nanoscatterers. Particularly, the modeling of the planar multilayered structure is simplified by applying the layered medium Green's function to reduce the computational domain and hence the memory requirement. Regarding the evaluation of Sommerfeld integrals in the layered medium Green's function, the discrete complex image method is adopted to accelerate the evaluation process. This work offers an accurate and efficient simulation tool for analyzing complex multilayered plasmonic system, which is commonly encountered in the design of optical elements and devices. © 2012 Optical Society of America.published_or_final_versio