Finite-difference Time-domain Simulation Method Research Articles

Beneficial roles of Al back reflectors in optical absorption of Si nanowire array solar cells

We investigate the influence of Al back reflectors on the optical absorption spectra of Si nanowire (NW) arrays by using the finite-difference time-domain simulation method. A flat Al layer enhances the absorption in the NW array due to not only the reflection-induced optical path length enlargement but also reflection of light between NWs and localized surface plasmon induced optical field confinement. An Al underlayer with a grating structure allows grating-coupled surface plasmon polariton excitation and raise the optical absorption in the Si NWs. Interplay among all these factors on the optical absorption and expected solar cell performance of the NW arrays is discussed.

Ultra-compact polarization beam splitter based on photonic crystal waveguides

The authors design an ultra-compact all-PC-integrated polarization beam splitter which is only composed of three waveguides: one input waveguide and two output waveguides. The input waveguide can support both TM and TE modes, but one of the two output waveguides can only support TM modes while the other can only support TE modes. So an incident beam will be separated into two different polarization beams which emerge from different output waveguides. By the simulation of finite-difference time-domain method, we know that the polarization beam splitter really works the way as we predict.

Accelerated Electromagnetic Field Simulation on Graphical Processing Unit by the Finite Difference Time Domain Method

This paper details about the implementation of the Finite Difference Time Domain Method (FDTD) on a Graphical Processing Unit (GPU) and demonstrated the ability of low cost GPU’s to accelerate real life problems of interest in the Electromagnetic Field Simulation domain. Source code and the achieved results for a simple simulation problem implemented in C is presented. The same problem is then discussed by implementation using Nvidia’s software development platform CUDA. The methodology for implementation of some advanced FDTD simulation problems is also described, and the achieved results are presented using both, CUDA and MatLab R2011a. Finally, a comparative performance analysis of the FDTD implementation using CUDA for a 256x256x256 volume is described for 5000 time steps, by comparing the CPU implementation time with the GPU implementation time. This is a reasonably good example for judging the applicability of low cost GPU’s not just for Electromagnetic Simulation, but also for real time applications.

금속 슬릿 배열로 구성된 편광 선택 가능한 블레이즈드 회절 격자

본 논문에서는 금속 슬릿 배열로 구성된 편광 선택 가능한 블레이즈드 회절 격자를 제안한다. 마이크로 스케일의 블레이즈드 회절 격자를 디자인 하고 이 구조에 추가적으로 나노스케일의 금속 슬릿 구조를 적용하여 편광 선택기능을 부여하였다. 이 소자에 대하여 사례 연구로 예를 디자인하여 FDTD(Finite Difference Time Domain method)방법을 이용한 전산모사를 통해 성능을 검증해 보았다. 임의의 디자인 사례에 대해 대략 77.61% 의 회절효율과 8.99의 편광선택비가 얻어졌으며 편광선택비를 높이기 위한 또 다른 디자인에서는 64.22%의 회절효율과 81.09의 편광선택비가 얻어졌다. A polarization selective blazed grating employing metal-slit arrays was proposed. Nano-scale metal-slits were applied to the micro-scale blazed grating to give the functionality of polarization selection. Case study was carried out for the proposed structure utilizing numerical FDTD (Finite Difference Time Domain method) simulation. Diffraction efficiency of 77.61% and polarization extinction ratio of 8.99 was achieved with arbitrary parameters and diffraction efficiency of 64.22% and polarization extinction ratio of 81.09 was achieved with other parameters to enhance extinction ratio.

APPLICATION OF FDTD METHOD IN SIMULATION OF FLAT-TOP BEAM IN OPTICAL FIBER

In this work, an investigation of the propagation characteristics of a flat-top laser beam in Kerr nonlinear media using Finite-Difference-Time- Domain Method (FDTD) has been presented. First an appropriate nonlinear Schrodinger equation (NLSE) has been chosen to find the beam properties in optical fiber, then the discretization method is used to find central difference in space for FDTD simulation, which is written in C language. This simulation work has revealed the unstable beam behavior in the system.

Towards FDTD modeling of spectroscopic ellipsometry data at large angles of incidence

Precise modeling of the spectroscopic ellipsometry (SE) response of arbitrary near-planar structures by finite-difference time-domain method (FDTD) is desirable for developing a general method for advanced quantitative optical characterization of complex samples. Such a synergetic SE–FDTD approach would be of great interest for fundamental applications such as, the understanding and characterization of nanostructure and plasmonic materials, and industrial applications providing additional tools for optical metrology. For both cases it is of great interest to achieve sub-monolayer modeling precision at large angles of incidence (AoI) where SE measurements are usually more sensitive. To advance this goal, we report the correlation between mean square error (MSE) and AoI studied for prototypical thin film structures. We show that an observed increase in FDTD simulation errors at large AoI can be correlated to the development of spurious resonances, resulting from the use of periodic boundary conditions. A solution to this problem is proposed resulting in ∼10× improved MSE values. It is believed that the combined FDTD–SE approach presented in this contribution could enable the SE description of increasingly complex structures thus paving the way for a powerful synergetic and complimentary SE–FDTD approach to study the optical properties at the sub-wavelength scale of complex nanostructures.

Detection of recombinant growth hormone by evanescent cascaded waveguide coupler on silica‐on‐silicon

An evanescent wave based biosensor is developed on the silica-on-silicon (SOS) with a cascaded waveguide coupler for the detection of recombinant growth hormone. So far, U -bends and tapered waveguides are demonstrated for increasing the penetration depth and enhancing sensitivity of the evanescent wave sensor. In this work, a monolithically integrated sensor platform containing a cascaded waveguide coupler with optical power splitters and combiners designed with S -bends and tapper waveguides is demonstrated for an enhanced detection of recombinant growth hormone. In the cascaded waveguide coupler, a large surface area to bind the antibody with increased penetration depth of evanescent wave to excite the tagged-rbST is obtained by splitting the waveguide into multiple paths using Y splitters designed with s -bends and subsequently combining them back to a single waveguide through tapered waveguide and combiners. Hence a highly sensitive fluoroimmunoassay sensor is realized. Using the 2D FDTD (Finite-difference time-domain method) simulation of waveguide with a point source in Rsoft FullWAVE, the fluorescence coupling efficiency of straight and bend section of waveguide is analyzed. The sensor is demonstrated for the detection of fluorescently-tagged recombinant growth hormone with the detection limit as low as 25 ng/ml.

Dynamical modulating the directional excitation of surface plasmons sources

We demonstrate two methods to dynamically modulate the directional excitation of surface plasmons (SPs) in subwavelength slits structure by embedding nonlinear media or introducing oblique incidence. Finite-difference time-domain method simulation shows that the electromagnetic field intensity, patterned by the interference of two SPs waves along the exit interface, can be arbitrarily tailored by adjusting the phase difference between the two slits. The modulation of SPs has the potential application for all-optical SPs switch.

Direct observation of surface plasmon far field for regular surface ripple formation by femtosecond laser pulse irradiation of gold nanostructures on silicon substrates

We have directly observed the interference ripple pattern between surface plasmon far field by gold nanosphere and the incident laser on silicon substrate. We explained the ripple formation using three-dimensional finite-difference time-domain simulation method. Nanosphere is an origin for regular ripple formation due to Mie scattering. We present a new method to control the plasmonic far-field pattern using an arbitrary gold nanostructure on the silicon substrate. Previously, the formed ripples were not regular but wavy because they were formed incoherently through the self organization process originating from the random surface roughness. The ripple structure was well controlled coherently.

Design and fabrication of high efficiency power coupler between different photonic crystal waveguides

Based on inspiration from an inverse optimization strategy and theoretical finite-difference time-domain method simulations, an ultralow loss power coupler between two different photonic crystal waveguides was designed, fabricated and characterized. The experimental results showed that the loss was less than 1 dB for transverse electric polarized light at a wavelength of 1550 nm, which is consistent with expectations from numerical modeling. High efficiency optical couplers are critical for development of integrated optical circuit functionality.

Novel polarization beam splitter with a tolerance to large random disorder

We propose a design for a simple broad-angle polarization beam splitter (PBS) consisting of two rows of dielectric cylinders with different space periods. The finite-difference time-domain method simulations show that TM polarized light is reflected totally by this PBS but TE polarized light passes through it freely in a broad incident angle range (from −50° to 50°). The PBS can work over a wide frequency range (from 0.22 × (c/a) to 0.28 × (c/a)) with a high efficiency. Moreover, the PBS structure has a novel capability of tolerance to large random disorder, which is very advantageous for practical applications. Even when a random disorder of 15%a (a is space period) is introduced into the radius and position of each cylinder, the PBS structure can still maintain almost the same good performance and high splitting efficiency.

Modeling bidirectional reflection distribution function of microscale random rough surfaces

The radiative properties of three different materials surfaces with one-dimensional microscale random roughness were obtained with the finite difference time domain method (FDTD) and near-to-far-field transformation. The surface height conforms to the Gaussian probability density function distribution. Various computational modeling issues that affect the accuracy of the predicted properties were discussed. The results show that, for perfect electric conductor (PEC) surfaces, as the surface roughness increases, the magnitude of the spike reduces and eventually the spike disappears, and also as the ratio of root mean square roughness to the surface correlation distance increases, the retroreflection becomes evident. The predicted values of FDTD solutions are in good agreement with the ray tracing and integral equation solutions. The overall trend of bidirectional reflection distribution function (BRDF) of PEC surfaces and silicon surfaces is the same, but the silicon’s is much less than the former’s. The BRDF difference from two polarization modes for the gold surfaces is little for smaller wavelength, but it is much larger for the longer wavelength and the FDTD simulation results agree well with the measured data. In terms of PEC surfaces, as the incident angle increases, the reflectivity becomes more specular.

Modal Characteristics in a Single-Nanowire Cavity with a Triangular Cross Section

In this study, the modal characteristics of a single-GaN nanowire cavity with a triangular cross section surrounded by air or located on a silicon dioxide substrate have been analyzed. Two transverse resonant modes, transverse electric-like and transverse magnetic-like modes, are dominantly excited for nanowire cavities that have a small cross-sectional size of <300 nm and length of 10 microm. Using the three-dimensional finite-difference time-domain simulation method, quality factors, confinement factors, single-mode conditions, and far-field emission patterns are investigated for a nanowire cavity as a function of one length of the triangular cross section. The results of these simulations provide information that will be vital for the design and development of efficient nanowire lasers and light sources in ultracompact nanophotonic integrated circuits.

Limitation of the electromagnetic cloak with dispersive material

Based on causality, the limitation of the electromagnetic cloak with dispersive material is investigated in this letter. The results show that perfect invisibility cannot be achieved because of the dilemma that either the group velocity Vg diverges or a strong absorption is imposed on the cloaking material. It is an intrinsic conflict which originates from the demand of causality. However, the total cross section can really be reduced through the approach of coordinate transformation. A simulation of finite-difference time-domain method is performed to validate our analysis.

Characterization of a subwavelength-scale 3D void structure using the FDTD-based confocal laser scanning microscopic image mapping technique

In this paper, a simple confocal laser scanning microscopic (CLSM) image mapping technique based on the finite-difference time domain (FDTD) calculation has been proposed and evaluated for characterization of a subwavelength-scale three-dimensional (3D) void structure fabricated inside polymer matrix. The FDTD simulation method adopts a focused Gaussian beam incident wave, Berenger's perfectly matched layer absorbing boundary condition, and the angular spectrum analysis method. Through the well matched simulation and experimental results of the xz-scanned 3D void structure, we first characterize the exact position and the topological shape factor of the subwavelength-scale void structure, which was fabricated by a tightly focused ultrashort pulse laser. The proposed CLSM image mapping technique based on the FDTD can be widely applied from the 3D near-field microscopic imaging, optical trapping, and evanescent wave phenomenon to the state-of-the-art bio- and nanophotonics.

Tunable and Sensitive Biophotonic Waveguides Based on Photonic-Bandgap Microcavities

This paper presents a theoretical study of the single-mode and birefriengence condition of the silicon photonic wires using the imaginary distance beam propagation method. The photonic wires are suitable for integration with active one-dimensional silicon photonic bandgap waveguides. This inherently will reduce the propagation loss caused by the scattering factors within the photonic bandgap structure itself. To the best of our knowledge, we provide for the first time, a systematic study of the various physical parameters that can affect the Q-factor and transmission properties in such waveguides. In order to make this technology viable, the waveguides must be tunable, have low attenuation, possess high Q-factor, and can be switched. Can these be achieved simultaneously without changing the device width and height dimensions? Furthermore, can we meet these aims without placing unrealistic demands in fabrication? The electrical switching of this device is implemented using a p-i-n optical diode. The diode is predicted to require an on state power of 81 nW with rise and fall times of 0.2 and 0.043 ns, respectively. The length of the microcavity and the diameter of the air holes are finely tuned with reference to the Q-factor and transmission. It will be shown that for certain desired resonant wavelength, the Q-factor and transmission properties can be optimized by tuning the length of the cavity and the diameter of the two inner most air holes. This method allows ease of fabrication by not having to vary the waveguide width and height to obtain the tuning effects. Optical simulation was performed using the three-dimensional finite-difference time-domain simulation method

Novel FDTD Simulation Method Using Multiple-Analysis-Space for Electromagnetic Far Field

Wide-band undesired electromagnetic noise near electronic systems, which includes small noise source like the printed circuit board (PCB), is a current problem in the field of electromagnetic interference. However, the estimation method for the electromagnetic noise near a system under test has not been established. This paper proposes a newly developed estimation method of the electromagnetic noise for a wide area, from near to far field, using the finite difference time-domain (FDTD) method. The proposed FDTD simulation method is an estimation technique for near to far field with multiple analysis spaces (MAS). The MAS has an internal analysis space (IAS) and an external analysis space (EAS). The analysis near a radiation source can be calculated in the IAS. The EAS is the outside space from IAS, which is for calculation of the far field. It is expected that the proposed FDTD method by MAS (FDTD-MAS) decrease in the calculation cost in terms of computational time and memory costs, especially for estimation of radiation from PCB. The principle procedure of the FDTD-MAS method is described in the first part of this paper. As example of advantages of the calculation and confirmation of the calculation accuracy, the electric field distributions radiated from a 1-GHz half-wavelength dipole antenna in an IAS of 0.3/spl times/0.3 m/sup 2/ area and an EAS of 7/spl times/7 m/sup 2/ area are used as examples. When the cell size ratio of IAS to EAS is changed from 6 to 20, the FDTD and theoretical values show good agreement. It is indicated that the FDTD-MAS simulation method is one of the most powerful tools for the estimation of electromagnetic noise from near field to far field from small and thin source.

A Nonunitary Transfer Matrix Method for Practical Analysis of Racetrack Microresonator Waveguide

We report that a nonunitary transfer matrix (NU-TM) method that we have developed is highly accurate and effective in analyzing and designing racetrack microresonator devices. Relatively poor output efficiency of a fabricated racetrack microresonator, unexplained by the conventional unitary transfer matrix (U-TM) method, are analyzed successfully by the NU-TM method. The track-length dependence, which is not obvious in the U-TM analysis, is clearly shown in the NU-TM analysis and is found to stem from the nonorthogonal phase difference between two branching waves in the directional coupling process. The results show excellent agreement with the results obtained by the finite-difference time-domain simulation method, thus confirming the validity and superior capability of the NU-TM method.

A Different Method Determining Dielectric Constant of Soil and Its FDTD Simulation

In this study, a different method determining the dielectric constant of soil via probes is presented. The method is based on the principle of measuring pulse delay in a given matter. The experimental study, which was carried out basically using an HP8753A vector network analyser, was repeated for various soil mixtures having different values of wetness. The results obtained from the measurements have clearly shown that the dielectric constant of the soil was increasing almost proportionally with that of the moisture content in the soil. The suitability of the measuring method was also checked with a number of simulation results obtained directly using the finite difference time domain method (FDTD).

Photonic-crystal 180° power splitter based on coupled-cavity waveguides

We propose a structure that allows the splitting of electromagnetic waves with a phase shift of 180° between output signals based on photonic crystals. The structure consists of two parallel coupled-cavity waveguides placed in proximity. The performance of the splitting structure is theoretically discussed, evaluated by means of finite-difference time-domain method simulations and experimentally demonstrated at microwave frequencies. As both output paths have the same physical length, the two output signals are synchronized, which is very attractive for splitting high-speed optical signals in photonic-crystal-based integrated circuits.