Time-domain Theory Research Articles

Effects of dielectric core and embedding medium on plasmonic coupling of gold nanoshell arrays

The effects of the dielectric core and the dielectric embedding medium separately on transmission spectra and plasmon resonance properties of gold nanoshell arrays were investigated by using the finite-difference time-domain (FDTD) theory. It is found that when the hollow nanoshell arrays are placed in air, the wide photonic band gap becomes narrower as the core dielectric constant increases. On the contrary, when the nanoshell arrays with dielectric core are placed in the dielectric medium, the photonic band gap becomes wider. Furthermore, increasing core or medium dielectric constant leads to a redshift of the transmission spectra due to the polarization of the dielectric. Based on the electric field distributions, we also clearly show that the plasmon properties of the nanoshell arrays are strongly influenced by the presence of the dielectric.

Coupled-resonator-induced transparency in photonic crystal waveguide resonator systems

We present an optical coupling system, which consists of waveguide, cavity and waveguide resonator, to investigate coupled-resonator-induced transparency effect. The transmission properties are analyzed theoretically by using coupled-mode theory in time domain. We also numerically demonstrate the effect by simulating the propagation of electromagnetic waves in photonic crystals by finite-difference time-domain method.

OPTICAL PROPERTIES AND PLASMON RESONANCE OF COUPLED GOLD NANOSHELL ARRAYS

The optical properties and plasmon resonances coupling of ordered gold nanoshell arrays are investigated theoretically by means of finite-difference time-domain (FDTD) theory. We showed that the thickness, size and inter-shell distance of the nanoshells can tune the optical transmission of the system and the highly geometry-dependent plasmon response can be seen as an interaction between the essentially fixed-frequency plasmon response of a nanosphere and that of a nanocavity for the nanoshells. We also revealed the two different resonance modes by analyzing the spatial distributions of electric field component Ez. We proposed that the peaks of the lower energy mainly originate from the sphere plasmons coupling and the peaks of the higher energy are mainly attributed to the coupling between the sphere plasmons and the cavity plasmons.

Seakeeping computations using double-body basis flows

Three-dimensional, time-domain, ship–wave interactions are studied in this paper for problems with forward speed. Free surface boundary conditions are derived based on a double-body linearization and the mixed Euler–Lagrange time stepping technique. The boundary integral equations are solved at each time step by distributing desingularized sources above the calm water surface and employing constant-strength panels on the body surface. Radiation, diffraction, and free motion results for a Wigley hull and a Series 60 hull are presented and systematically compared with the experiments and other numerical solutions using the Neumann–Kelvin approach with simplified m -terms, linearized free surface boundary conditions with double-body m -terms, and the time-domain body-exact strip theory. By comparing the present results to experiments and other numerical solutions, it is found that the present computational model using double-body linearization gives improved results. It is also demonstrated that the m -terms are very important to obtain accurate hydrodynamic coefficients, while the leading-order terms included in the free surface boundary conditions of the present model can also improve the computational accuracy of the cross-coupling radiation damping.

Transient Response of Straight Thin Wires Located at Different Heights Above a Ground Plane Using Antenna Theory and Transmission Line Approach

Transient electromagnetic field coupling to straight thin wires parallel to each other and located at different heights above a perfectly conducting or dielectric ground plane is analyzed using wire antenna theory and a transmission line method. The time-domain antenna theory formulation is based on a set of the space-time Hallen integral equations. The transmission line approximation is based on the corresponding time-domain Telegrapher's equations. The space-time integral equations arising from the wire antenna theory are handled by the time-domain Galerkin-Bubnov scheme of the indirect boundary element method. The time-domain Telegrapher's equations are solved using the finite-difference time-domain method. Time-domain numerical results obtained with both approaches are compared to the results computed via NEC 2 code combined with an inverse Fourier transform procedure. Some illustrative comparisons of results obtained by means of antenna theory and transmission line approach are presented throughout the paper.

Theoretical investigation of waveguide power splitters with parallel output ports in two-dimensional square-lattice photonic crystals

We theoretically investigate waveguide power splitters with parallel output ports by the time-domain coupled-mode theory. Conditions for perfect transmission and zero reflection are obtained that are different from the idea of a structure with a perfect T-type branch and two perfect bends. Using theoretical analysis, waveguide power splitters in two-dimensional square-lattice photonic crystals are modeled and optimized. Their transmission properties are simulated by using the finite-difference time-domain method, and an excellent agreement has been found with the present analysis.

Advances in the three-dimensional hydroelasticity of ships

The study of hydroelasticity of ships first gained momentum in the late 1970s with the work of Bishop and Price, who established the two-dimensional (2D) hydroelasticity theory of ships. The concept and basic principle presented in their work to embody the structure and the surrounding fluid as a coupled entirety was further employed and extended in the creation of the general linear three-dimensional (3D) theory of hydroelasticity for an arbitrary shaped flexible marine structure travelling with a forward speed in a seaway in the middle of 1980s (Wu, 1984; Price and Wu, 1985a; Bishop et al., 1986). Since then, great progress has been achieved in the development and application of 3D hydroelasticity theories. These include the more rigorous methods of frequency-domain linear analysis accounting for the forward speed effect and the steady flow effect, the time-domain linear 3D theory, the non-linear 3D theory and the numerical methods for a floating structure travelling in rough seas with large motions, experimental techniques of 3D flexible ship models, the hydroelasticity-based design and safety assessment, etc. This paper presents an overview of these developments and achievements of linear and non-linear 3D hydroelasticity theories of ships, and the corresponding numerical and experimental techniques.

Temperature dependence of hot-carrier relaxation in PbSe nanocrystals: Anab initiostudy

Temperature-dependent dynamics of phonon-assisted relaxation of hot carriers, both electrons and holes, is studied in a PbSe nanocrystal using ab initio time-domain density-functional theory. The electronic structure is first calculated, showing that the hole states are denser than the electron states. Fourier transforms of the time-resolved energy levels show that the hot carriers couple to both acoustic and optical phonons. At higher temperature, more phonon modes in the high-frequency range participate in the relaxation process due to their increased occupation number. The phonon-assisted hot-carrier relaxation time is predicted using nonadiabatic molecular dynamics, and the results clearly show a temperature-activation behavior. The complex temperature dependence is attributed to the combined effects of the phonon occupation number and the thermal expansion. Comparing the simulation results with experiments, we suggest that the multiphonon relaxation channel is efficient at high temperature, while the Auger-type process may dominate the relaxation at low temperature. This combined mechanism can explain the weak temperature dependence at low temperature and stronger temperature dependence at higher temperature.

Orthogonal Hybrid Waveguides: An Approach to Low Crosstalk and Wideband Photonic Crystal Intersections Design

A low crosstalk and wideband photonic crystal (PC) waveguide intersection design based on two orthogonal hybrid waveguides in a crossbar configuration is proposed. The finite-difference time-domain (FDTD) and coupled-mode theory (CMT) methods are used to simulate the hybrid waveguides of square lattice. The bandwidth (BW) and crosstalk of the intersection are investigated for various radii of the coupled cavities. It is shown that simultaneous crossing of the lightwave signals through the intersection with negligible interference is possible. The transmission of a 200-fs pulse at 1550 nm is simulated by using the FDTD method, and the transmitted pulse shows negligible crosstalk and very little distortion.

TIME-DOMAIN THEORY OF METAL CAVITY RESONATOR

This paper presents a thorough study of the time-domain theory of metal cavity resonators. The completeness of the vector modal functions of a perfectly conducting metal cavity is first proved by symmetric operator theory, and analytic solution for the field distribution inside the cavity excited by an arbitrary source is then obtained in terms of the vector modal functions. The main focus of the present paper is the time-domain theory of a waveguide cavity, for which the excitation problem may be reduced to the solution of a number of modified Klein-Gordon equations. These modified KleinGordon equation are then solved by the method of retarded Green’s function in order that the causality condition is satisfied. Numerical examples are also presented to demonstrate the time-domain theory. The analysis indicates that the time-domain theory is capable of providing an exact picture for the physical process inside a closed cavity and can overcome some serious problems that may arise in traditional time-harmonic theory due to the lack of causality.

Silicon Microtoroidal Resonators With Integrated MEMS Tunable Coupler

A single crystalline silicon microtoroidal resonator with integrated MEMS-actuated tunable optical coupler is demonstrated for the first time. It is fabricated by combining hydrogen annealing and wafer bonding processes. The device operates in all three coupling regimes: under-, critical, and over-coupling. We have also developed a comprehensive model based on time-domain coupling theory. The experimental and theoretical results agree very well. The quality factor (Q) is extracted by fitting the experimental curve with the model. The unloaded Q is as high as 110 000, and the loaded Q is continuously tunable from 110 000 to 5400. The extinction ratio of the transmittance is 22.4 dB. This device can be used as a building block of resonator-based reconfigurable photonic integrated circuits

Generalized anelastic asymptotic ray theory

This paper deals with dispersion and attenuation of body waves in a wide range of materials representing realistic rock structures. The time-domain asymptotic ray theory is applied to a new generalized coordinate-free wave equation with an arbitrary tensor relaxation function. This function describes any general viscosity that can occur in 3D anisotropic and laterally inhomogeneous media of practical interest. Perfect elasticity and existing anelastic models (linear viscoelasticity, power-law models, Biot’s fluid flow, thermoelasticity, etc.) are considered as special cases. A ray-series solution is obtained for both the principal and additional components. This solution is suitable for the analysis of anelasticity problems involving propagation of evanescent waves provided that the causality principle is respected. Accuracy of the principal component is numerically tested.

Wave propagation in a simplified modelled poroelastic continuum: fundamental solutions and a time domain boundary element formulation

AbstractIn finite element formulations for poroelastic continua a representation of Biot's theory using the unknowns solid displacement and pore pressure is preferred. Such a formulation is possible either for quasi‐static problems or for dynamic problems if the inertia effects of the fluid are neglected. Contrary to these formulations a boundary element method (BEM) for the general case of Biot's theory in time domain has been published (Wave Propagation in Viscoelastic and Poroelastic Continua: A Boundary Element Approach. Lecture Notes in Applied Mechanics. Springer: Berlin, Heidelberg, New York, 2001.). If the advantages of both methods are required it is common practice to couple both methods. However, for such a coupled FE/BE procedure a BEM for the simplified dynamic Biot theory as used in FEM must be developed.Therefore, here, the fundamental solutions as well as a BE time stepping procedure is presented for the simplified dynamic theory where the inertia effects of the fluid are neglected. Further, a semi‐analytical one‐dimensional solution is presented to check the proposed BE formulation. Finally, wave propagation problems are studied using either the complete Biot theory as well as the simplified theory. These examples show that no significant differences occur for the selected material. Copyright © 2005 John Wiley & Sons, Ltd.

Controlled transmission in the forbidden photonic bandgap via transient nonlinear states

Using three-dimensional time-domain numerical simulations of the nonlinear dispersive Maxwell equations for a defect microcavity in a photonic crystal wire, we show that the transmission through the bandgap can be all-optically modulated via the generation of transient states associated with the nonlinear splitting of the defect mode. Analytical results based on time-domain coupled-mode theory are derived as well.

Simulation of ultra‐wideband indoor propagation

AbstractA comprehensive simulation of ultra‐wideband signal propagation in indoor environments is presented. The simulation is based on time‐domain electromagnetic modeling of transmitting and receiving antennas and the analysis of wave propagation through indoor channels using the time‐domain uniform theory of diffraction. The antennas are a pair of TEM horns which are modeled as arrays of vee dipoles. The analysis of these antennas is performed directly in the time domain, without the need for transforming the solutions from the frequency domain to the time domain. The frequency dependence of materials utilized in the structure on the indoor channel is accounted for in the channel simulation. The simulation results are compared with the corresponding measured results. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 42: 103–108, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20221

Time-Domain Non-Linear Strip Theory For Ship Motions

A new implementation of strip theory is proposed based on the strip theory by Salvesen, et al. [1] and early work by Westlake and Wilson [2]. Compared with traditional strip theory, the main difference is that the calculation is carried out in the time domain. This makes it possible to cope with relatively large-amplitude motions and non-constant forward speed problems. At each time step, the exact underwater sections are extracted; the velocity potential is required to satisfythis instantaneous body boundary condition, therefore the added mass, damping coefficients and exciting forces are timevariant, which produce a non-linearity. The diffraction force is added to work ofWestlake and Wilson, and the results show the effect of diffraction force is significant.

선체반류를 고려한 프로펠러 최적 스큐화

Propellers operating in a given nonuniform ship wake generate unsteady loads leading to undesirable stern vibration problems. The skew is known to be the most proper and effective geometric parameter to control or reduce the fluctuating forces on the shaft. This paper assumes the skew profile as either a quadratic or a cubic function of the radius and determines the coefficients of the polynomial function by applying the simplex method. The method uses the converted unconstrained algorithm to solve the constrained minimization problem of 6-component shaft excitation forces. The propeller excitation was computed either by applying the two-dimensional gust theory for quick estimation or by the fully three-dimensional unsteady lifting surface theory in time domain for an accurate solution. A sample result demonstrates that the shaft forces can be further reduced through optimization from the original design.

Time domain theory of open waveguide resonators: Canonical problems and a generalized matrix technique

In this paper, we consider new solutions and algorithms for initial boundary‐value problems in the electromagnetic theory of open waveguide resonators. The approaches are based on a description of the scattering properties of such resonators in terms of the transform operators for an evolutionary basis of the non‐stationary signal, that have the same significance in the course of analyses in time domain as generalized scattering matrixes in frequency domain. All suggested approaches imply using mathematically correct computational procedures at the key stages of forming the solution, in particular, the analytical regularization method—the semi‐inversion method.

Noise attenuation by a hard wedge-shaped barrier

This paper is concerned with the problem of sound screening by a wedge-like barrier. The sound source is assumed to be point like, and the receiver is located in the shadow of the source sound field, so that according to geometrical optics only the field diffracted by the edge of the barrier is considered. First, for the hard wedge in space, three models are used for calculating the amplitude of the edge-diffracted field. These are the uniform theory of diffraction (UTD), the Hadden–Pierce model, both in the frequency domain, and the Biot–Tolstoy theory of diffraction which is a time domain formulation. It is first shown that even at relatively low frequencies, the frequency domain models perform quite satisfactorily as compared to the exact time domain theory. Hence, and due to its relative simplicity the UTD is proposed as an accurate calculation scheme for solving problems with edge diffraction by hard wedges. It is also proved from theoretical calculations that the amplitude of the edge-diffracted field increases for an increasing angle of the wedge, and consequently the hard half-plane gives the lowest field amplitude in the shadow zone. Some applications are then considered for evaluating the performance of a barrier on a flat ground, either completely hard or with mixed homogeneous boundary conditions. An improvement of the scheme for calculating the sound field in the all-hard case is achieved through considering the multiple diffraction, in this case only to the second order, between the top of the wedge barrier and its base. The results show that for usually occurring situations, increasing the angle of the hard wedge barrier affects negatively its efficiency through diminishing its insertion loss. These conclusions are also supported by the results of some experimental measurements conducted at a scale-model level.

A New Approach To Optimal and Self‐Tuning Filtering, Smoothing and Prediction For Discrete‐Time Systems

ABSTRACTA new approach to optimal and self‐tuning state estimation of linear discrete time‐invariant systems is presented, using projection theory and innovation analysis method in time domain. The optimal estimators are calculated by means of spectral factorization. The filter, predictor, and smoother are given in a unified form. Comparisons are made to the previously known techniques such as the Kalman filtering and the polynomial method initiated by Kucera. When the noise covariance matrices are not available, self‐tuning estimators are obtained through the identification of an ARMA innovation model. The self‐tuning estimator asymptotically converges to the optimal estimator.