Finite-difference Time-domain Solution Research Articles

Wide-angle, polarization-insensitive, and broadband metamaterial absorber based on multilayered metal-dielectric structures.

A wide-angle, polarization-insensitive, and broadband metamaterial absorber (MA) based on multilayered metal-dielectric structures was designed and investigated using the finite-difference time-domain solution. The device exhibited polarization-insensitive absorption in the 475-592 nm range, with the absorption higher than 95%. At the same time, the broadband absorption was nearly unaffected for incident angles below 50°. To understand this absorption mechanism of this broadband MA, we investigated the magnetic field distributions at the resonance wavelength. Different metallic layers yielded different resonant wavelengths, leading to the observed high absorption in the broadband spectrum. For broadband MAs, the absorption spectrum can be expanded by adjusting structural parameters.

Topology Optimisation of Wideband Coaxial-to-Waveguide Transitions

To maximize the matching between a coaxial cable and rectangular waveguides, we present a computational topology optimisation approach that decides for each point in a given domain whether to hold a good conductor or a good dielectric. The conductivity is determined by a gradient-based optimisation method that relies on finite-difference time-domain solutions to the 3D Maxwell’s equations. Unlike previously reported results in the literature for this kind of problems, our design algorithm can efficiently handle tens of thousands of design variables that can allow novel conceptual waveguide designs. We demonstrate the effectiveness of the approach by presenting optimised transitions with reflection coefficients lower than −15 dB over more than a 60% bandwidth, both for right-angle and end-launcher configurations. The performance of the proposed transitions is cross-verified with a commercial software, and one design case is validated experimentally.

Electromagnetic Simulation and Alignment of Dual-Polarized Array Antennas in Multi-Mission Phased Array Radars

Electromagnetic (EM) simulation of dual-polarized antennas is necessary for precise initial alignments, calibration and performance predictions of multi-function phased array radar systems. To achieve the required flexibility and scalability, a novel Finite-Difference Time-Domain (FDTD) solution is developed for rectangular, cylindrical and non-orthogonal coordinate systems to simulate various types of array antenna manifolds. Scalable array pattern predictions and beam generations are obtained by combining the FDTD simulation solutions with the Near-Field (NF) chamber measurements. The effectiveness and accuracy of this approach are validated by comparing different simulations and comparing simulations with measurements.

Thin-slot formalism for the split-field FDTD analysis of periodic structure

An expression of the thin-slot formalism is presented to alleviate the gridding of the split-field finite-difference time-domain (FDTD) solution for periodic structure. The varying auxiliary-field ( , ) and split-field ( , ) distributions near the slots are analytically derived from the varying field ( , ). The update equations for the split-field FDTD are obtained by incorporating those varying field distributions into the split-field equations in integral form. A frequency selective surface (FSS) structure is applied to verify the proposed method. The results indicate that the computational efficiency is improved.

Electromagnetic Compatibility in Air Insulation Substation

In this work, transient coupling between the bus bars of the Air Insulation Substation (AIS) and the control cable located at its proximity is analyzed. The present study accounts for the real complex geometry of the bus bars and the finite conductivity of the soil. A time domain formulation of the problem is proposed in this work. The direct time domain formulation based on the use of Transmission Line (TL) theory and the Finite Difference Time Domain (FDTD) solution method. The excitations are given in the form of the localized current or voltage generator, respectively, and related electrical field radiated by bus bars. To ensure the electromagnetic compatibility (EMC) requirements the current induced in the transmission cable is analyzed by means of the Pencil Method that will allow one to identify the dominant frequencies in the interference signal. Various illustrative examples are given in the paper.

Independently tunable dual-band plasmonically induced transparency based on hybrid metal-graphene metamaterials at mid-infrared frequencies.

A tunable dual-band plasmonically induced transparency (PIT) device based on hybrid metal-graphene nanostructures is proposed theoretically and numerically at mid-infrared frequencies, which is composed of two kinds of gold dolmen-like structures with different sizes placed on separate graphene interdigitated finger sets respectively. The coupled Lorentz oscillator model is used to explain the physical mechanism of the PIT effect at multiple frequency domains. The finite-difference time-domain (FDTD) solutions are employed to simulate the characteristics of the hybrid metal-graphene dual-band PIT device. The simulated spectral locations of multiple transparency peaks are separately and dynamically modulated by varying the Fermi energy of corresponding graphene finger set, which is in good accordance with the theoretical analysis. Distinguished from the conventional metallic PIT devices, multiple PIT resonances in the hybrid metal-graphene PIT device are independently modulated by electrostatically changing bias voltages applied on corresponding graphene fingers, which can be widely applied in optical information processing as tunable sensors, switches, and filters.

Portable organic gas detection sensor based on the effect of guided-mode resonance

A novel organic gas detection sensor based on the effect of guided-mode resonance is proposed in this paper. The sensor is designed to operate in the visible light band. It contains four main sections: a light source, a miniature gas chamber composed of a guided-mode resonant filter, a diffraction grating, and a CCD image sensor. When bunched visible light is irradiated vertically to the gas chamber, it passes through the gas chamber and diffraction grating, and is then received by the CCD sensor. The optical signal received by the CCD sensor is then reduced to the spectrum using a specific algorithm. When organic gases are injected into the gas chamber, there is a shift in the wavelength of resonant reflection, and the magnitude of this shift is proportional to the refractive index of the gas. The large variation in the refractive indexes of industrially important organic gases means that their characteristic peak wavelengths can be easily identified. As a result, this system can quickly detect organic gases. To verify the feasibility of this technique, we use finite difference time domain solutions to simulate the results. The sensitivity of this type of sensor can reach wavelength differences of 0.001 nm, which means that the sensor has high potential for application in portable, high-precision detection systems.

Monolithic integration of a InP AWG and InGaAs photodiodes on InP platform

We demonstrate a monolithic integration of a photodiode array and a 13 channels arrayed waveguide grating (AWG) grown on InP substrate with a shallow trench structure between the AWG top cladding layer and the photodiode p-doped layer. A smooth epitaxial structure interface is obtained by nonselective regrowth to make the two epitaxial structure compatible and fabrication easy. Three-dimensional finite-difference time-domain(FDTD) solutions are used in the optical simulations. The highest simulation quantum efficiency can achieve 82%. The fabricated PD with a trench structure presents a responsivity of 0.68A/W. The integrated device can achieve a total capacity of more than 200Gb/s.

On the Analytical Solution of the FDTD Method

The finite-difference time-domain (FDTD) method is an effective and widely used time-domain method for solving electromagnetic problems. Conventionally, its solutions are obtained numerically in a march-on-in-time manner. In this paper, based on the eigenmatrix theory, we derive the analytical expression for the FDTD solution. The FDTD solution is analytically expressed in terms of spatial modes modulated by discrete time sequences related to the eigenvalues of the FDTD system equation. Based on our results, we propose an alternative approach to the FDTD solutions and it could open a new horizon for applying advanced signal processing techniques as well as offering the possibility of storing the FDTD results in an analytical form for a structure after computation.

High-responsivity 40 Gbit/s InGaAs/InP PIN photodetectors integrated on silicon-on-insulator waveguide circuits**Project supported by the High-Tech Research and Development Program of China (Nos. 2015AA016904, 2015AA012302), the National Basic Research Program of China (Nos. 2012CB933503, 2013CB932904), and the National Natural Foundation of China (Nos. 61274069, 61176053, 61021003, 61435002).

This paper presents a high-responsivity and high-speed InGaAs/InP PIN photodetector integrated onto the silicon waveguide substrate utilizing the divinyltetramethyldisiloxane-benzocyclobutene (DVS-BCB) adhesive bonding method. A grating coupler is adopted to couple light from the fiber to the silicon waveguide. Light in the silicon photonic waveguide is evanescently coupled into the photodetector. The integrated photodetector structure is first simulated using the FDTD (finite difference time domain) solutions software and the simulation results show a detection efficiency of 95%. According to the simulation result, the integrated photodetector is fabricated. The measured responsivity of the fabricated integrated photodetector with a detection length of 30 μm is 0.89 A/W excluding the coupling loss between the fiber and the grating coupler and the silicon propagation loss at the wavelength of 1550 nm with a reverse bias voltage of 3 V. Measured 3-dB bandwidth is 27 GHz using the Lightwave Component Analyzer (LCA). The eye diagram signal test results indicate that the photodetector can operate at a high speed of 40 Gbit/s. The integrated photodetector is of great significance in the silicon-based optoelectronic integrated chip which can be applied to the optical communication and the super node data transmission chip of the high-performance computer.

Study and Optimization of Metal Nanoparticles for the Enhanced Efficiency Thin Film Solar Cells

Thin film silicon solar cells have the potential to considerably decrease the cost of photovoltaic. To increase the conversion efficiency of thin film solar cells, nano-sized structures, such as nanoparticle deposition at the front end, were proposed. In the present study, spherical metal nanoparticles such as gold (Au) and silver (Ag) were deployed at the front of the silicon solar cell. The effect of metal nanoparticles on the absorption enhancement factor of the thin film solar cells was investigated using Lumerical Finite Difference Time Domain (FDTD) solutions. Also the influence of geometrical parameters of spherical nanoparticles on absorption enhancement factor was examined. The maximum absorption enhancement factor was achieved by optimizing the geometrical parameters of nanoparticles. The structure with Ag nanoparticles at the front end of the silicon solar cell exhibits higher absorption enhancement factor than the structure with Au nanoparticles.

Acoustic wave propagation in tilted transversely isotropic media: Incorporating topography

Simulating two-way acoustic wavefield propagation directly from a free-surface boundary in the presence of topography remains a computational challenge for applications of reverse time migration (RTM) or full-waveform inversion (FWI). For land-seismic settings involving heavily reworked geology (e.g., fold and thrust belts), two-way wavefield propagation operators should also handle commonly observed complex anisotropy including tilted transversely isotropic (TTI) media. To address these issues, I have extended a system of coupled partial differential equations used to model 3D acoustic TTI wave propagation in Cartesian coordinates to more generalized 3D geometries, including a deformed computational mesh with a domain boundary conformal to free-surface topography. A generalized curvilinear transformation is used to specify a system of equations governing 3D acoustic TTI wave propagation in the “topographic” coordinate system. The developed finite-difference time-domain numerical solution adapts existing Cartesian TTI operators to this more generalized geometry with little additional computational overhead. Numerical evaluations illustrate that 2D and 3D impulse responses are well-matched to those simulated on Cartesian meshes and analytic traveltimes for homogeneous elliptical TTI media. Accordingly, these generalized acoustic TTI propagators and their numerical adjoints are useful for undertaking most RTM or FWI applications using computational domains conforming to free-surface topography.

Rayleigh waves, surface disorder, and phonon localization in nanostructures

We introduce a technique to calculate thermal conductivity in disordered nanostructures: a finite-difference time-domain (FDTD) solution of the elastic wave equation combined with the Green-Kubo formula. The technique captures phonon wave behavior and scales well to nanostructures that are too large or too surface disordered to simulate with many other techniques. We investigate the role of Rayleigh waves and surface disorder on thermal transport by studying graphenelike nanoribbons with free edges (allowing Rayleigh waves) and fixed edges (prohibiting Rayleigh waves). We find that free edges result in a significantly lower thermal conductivity than fixed ones. Free edges both introduce Rayleigh waves and cause all low-frequency modes (bulk and surface) to become more localized. Increasing surface disorder on free edges draws energy away from the center of the ribbon and toward the disordered edges, where it gets trapped in localized surface modes. These effects are not seen in ribbons with fixed boundary conditions and illustrate the importance of phonon surface modes in nanostructures.

Simulation and Experimental Study on Anti-reflection Characteristics of Nano-patterned Si Structures for Si Quantum Dot-Based Light-Emitting Devices.

Surface-textured structure is currently an interesting topic since it can efficiently reduce the optical losses in advanced optoelectronic devices via light management. In this work, we built a model in finite-difference time-domain (FDTD) solutions by setting the simulation parameters based on the morphology of the Si nanostructures and compared with the experimental results in order to study the anti-reflection behaviors of the present nano-patterned structures. It is found that the reflectance is gradually reduced by increasing the depth of Si nanostructures which is in well agreement with the experimental observations. The reflectance can be lower than 10 % in the light range from 400 to 850 nm for Si nano-patterned structures with a depth of 150 nm despite the quite low aspect ratio, which can be understood as the formation of gradually changed index layer and the scattering effect of Si nano-patterned structures. By depositing the Si quantum dots/SiO2 multilayers on nano-patterned Si substrate, the reflectance can be further suppressed and the luminescence intensity centered at 820 nm from Si quantum dots is enhanced by 6.6-fold compared with that of flat one, which can be attributed to the improved light extraction efficiency. However, the further etch time causes the reduction of luminescence intensity from Si quantum dots which may ascribe to the serious surface recombination of carriers.

A POD reduced-order finite difference time-domain extrapolating scheme for the 2D Maxwell equations in a lossy medium

This paper is concerned with establishing a reduced-order finite difference time-domain (FDTD) extrapolating scheme based on proper orthogonal decomposition (POD) method for the two-dimensional (2D) Maxwell equations in a lossy medium. For this purpose, we first review the classical FDTD scheme and associated results for the 2D Maxwell equations in a lossy medium. And then, we formulate three sets of POD bases by means of the solutions obtained from the classical FDTD scheme on the very short time interval, establish the POD reduced-order FDTD extrapolating scheme, and furnish the error estimates among the POD reduced-order FDTD solutions and the classical FDTD solutions as well as the accuracy solution for the 2D Maxwell equations in a lossy medium. Finally, we enumerate two numerical examples to confirm that the POD reduced-order FDTD extrapolating scheme is feasible and efficient for simulating the phenomena for the electric and magnetic fields in a lossy medium.

Tunable multispectral plasmon induced transparency based on graphene metamaterials

A dynamically wavelength tunable multispectral plasmon induced transparency (PIT) device based on graphene metamaterials, which is composed of periodically patterned graphene double layers separated by a dielectric layer, is proposed theoretically and numerically in the terahertz frequency range. Considering the near-field coupling of different graphene layers and the bright-dark mode coupling in the same graphene layer, the coupled Lorentz oscillator model is adapted to explain the physical mechanism of multispectral EIT-like responses. The simulated transmission based on the finite-difference time-domain (FDTD) solutions indicates that the shifting and depth of the EIT resonances in multiple PIT windows are controlled by different geometrical parameters and Fermi energies distributions. A design scheme with graphene integration is employed, which allows independent tuning of resonance frequencies by electrostatically changing the Fermi energies of graphene double layer. Active control of the multispectral EIT-like responses enables the proposed device to be widely applied in optical information processing as tunable sensors, switches, and filters.

Periodic molybdenum disc array for light trapping in amorphous silicon layer

We demonstrate the light trapping effect in amorphous silicon (a-Si:H) layer by inserting a layer of periodic molybdenum disc array (MDA) between the a-Si:H layer and the quartz substrate, which forms a three-layer structure of Si/MDA/SiO2. The MDA layer was fabricated by a new cost-effective method based on nano-imprint technology. Further light absorption enhancement was realized through altering the topography of MDA by annealing it at 700°C. The mechanism of light absorption enhancement in a-Si:H interfaced with MDA was analyzed, and the electric field distribution and light absorption curve of the different layers in the Si/MDA structure under light illumination of different wavelengths were simulated by employing numerical finite difference time domain (FDTD) solutions.

Modeling the thermal radiation properties of thermal barrier coatings based on a random generation algorithm

Thermal barrier coatings (TBCs) are porous media in which many different pores and cracks are induced by different manufacturing procedures. Many studies have been conducted to investigate the impact of microstructures of TBCs on thermal conductivity; nevertheless, the influence of microstructures on the radiative properties of TBCs has not drawn significant attention. In addition, the working condition of thermal barrier coatings is at high temperatures at which the contribution of radiative heat transfer plays a very crucial role. Therefore, it is necessary to study the radiative properties of TBCs to characterize their insulation performance. In this work, the microstructures of air-plasma- sprayed (APS) 8wt% yttria stabilized zirconia (8YSZ) thermal barrier coatings (TBCs) are constructed by the quartet structure generation set (QSGS) algorithm. A finite-difference-time-domain (FDTD) method is carried out to simulate radiative heat transfer through TBCs. Three parameters—average pore size, directional growth probability Di (especially horizontal growth probability D13) and porosity—have been investigated to study the microstructural effect on the radiative properties of TBCs. The reflectance of freestanding 50-μm-thick thermal barrier coatings is studied using Lumerical FDTD Solutions in the wavelength range from 1 to 6μm at normal incidence. The absorption and scattering coefficient as a function of wavelength are extracted using the four-flux model. The results will help us to characterize the radiative heat transfer process across the TBCs and provide us with a theoretical guide to design TBCs with a high thermal insulation property.

Nanoscale refractive index fluctuations detected via sparse spectral microscopy.

Partial Wave Spectroscopic (PWS) Microscopy has proven effective at detecting nanoscale hallmarks of carcinogenesis in histologically normal-appearing cells. The current method of data analysis requires acquisition of a three-dimensional data cube, consisting of multiple images taken at different illumination wavelengths, limiting the technique to data acquisition on ~30 individual cells per slide. To enable high throughput data acquisition and whole-slide imaging, new analysis procedures were developed that require fewer wavelengths in the same 500-700nm range for spectral analysis. The nanoscale sensitivity of the new analysis techniques was validated (i) theoretically, using finite-difference time-domain solutions of Maxwell's equations, as well as (ii) experimentally, by measuring nanostructural alterations associated with carcinogenesis in biological cells.

Reconstruction of explicit structural properties at the nanoscale via spectroscopic microscopy.

The spectrum registered by a reflected-light bright-field spectroscopic microscope (SM) can quantify the microscopically indiscernible, deeply subdiffractional length scales within samples such as biological cells and tissues. Nevertheless, quantification of biological specimens via any optical measures most often reveals ambiguous information about the specific structural properties within the studied samples. Thus, optical quantification remains nonintuitive to users from the diverse fields of technique application. In this work, we demonstrate that the SM signal can be analyzed to reconstruct explicit physical measures of internal structure within label-free, weakly scattering samples: characteristic length scale and the amplitude of spatial refractive-index (RI) fluctuations. We present and validate the reconstruction algorithm via finite-difference time-domain solutions of Maxwell's equations on an example of exponential spatial correlation of RI. We apply the validated algorithm to experimentally measure structural properties within isolated cells from two genetic variants of HT29 colon cancer cell line as well as within a prostate tissue biopsy section. The presented methodology can lead to the development of novel biophotonics techniques that create two-dimensional maps of explicit structural properties within biomaterials: the characteristic size of macromolecular complexes and the variance of local mass density.