3D Finite-difference Time-domain Research Articles

Rounded square ring resonator based add drop filter for WDM applications using two dimensional photonic crystals

In this paper, Two Dimensional Photonic Crystals based Add Drop filter (ADF) is designed for DWDM applications. The existing work concentrates in Coarse Wavelength Division Multiplexing and few filters reported for DWDM with non-uniform channel spacing, and low quality factor. The proposed ADF enhances the quality factor using Rounded Square Ring Resonator (RSRR) for ensuring efficient bandwidth in supporting WDM systems. The design consists of bus waveguide, drop waveguide and RSRR with inner quasi-square ring in the square lattice. The dielectric constant of the Si rod is 11.68 which are hosted in the air. The line defect is created by removing 4 rods (Quasi ring) in the center of the inner square ring. The line defect in the ring resonator is used to reduce the radiation field components surrounded by the resonator. The performance parameters of ADF are investigated using 2D Finite Difference Time Domain algorithm. The proposed ADF drops a channel at 1636.2 nm, with the bandwidth of 0.7 nm, high quality factor of 2337, and the dropping efficiency of about 100%. The size of the device is 412.76 µm2. It is highly sufficient to support WDM systems for future Photonic Integrated Circuits (PIC). Further, the impact of functional parameters such as transmission efficiency, quality factor, bandwidth are investigated by varying the structural parameters, namely, adjacent rod radius, scatterer rod radius, coupling rod radius, lattice constant, inner rod radius and rod radius.

High-Contrast and Compact Integrated Wavelength Diplexer Based on Subwavelength Grating Anisotropic Metamaterial for 1550/2000 nm

A high-contrast and compact wavelength diplexer is presented for conventional 1550 nm and emerging 2000 nm based on a subwavelength-grating (SWG) coupler. The SWG silicon waveguide thoroughly blocks the light propagation around 1550 nm but fully supports 2000 nm (extinction ratio: 43.11 dB). This grating type anisotropic metamaterial not only efficiently reduces the coupling length but also expands the operational bandwidth. Simulated by 3D finite-difference time-domain method, the proposed diplexer possesses a remarkably low insertion loss and a high contrast of 25.24 (31.7) dB at 1550 (2000) nm. The operational bandwidth of 200 (108) nm is achieved with contrast over 15 dB and insertion loss below 0.22 dB. The footprint of diplexer is only 5.27 μm × 11.85 μm. Moreover, such design has the scalability by simply tuning the geometrical parameters of SWG.

Finite-Difference Time-Domain Modeling of Periodic Structures: A review of constant wave vector techniques

This article reviews state-of-the-art periodic boundary conditions (PBCs) in finite difference time-domain (FDTD) simulations that operate by enforcing constant wave vector components. The mathematical principles and 3D FDTD implementation details are systematically outlined. Techniques for extracting scattering parameters, Brillouin diagrams, and attenuation constants are presented along with the array scanning method (ASM) used to model the interaction of nonperiodic sources with periodic structures. Through these techniques, the robustness, utility, and efficiency of PBCs are demonstrated and a unified view of the various approaches to the FDTD implementation of PBCs is presented.

Gyroelectric guided modes with transverse optical spin.

The transverse nature of light leads to longitudinal optical spin. Here, the unprecedented transverse optical spin of propagating waves and guided modes in a gyroelectric medium is clarified. We identify the propagation modes in a bulk gyroelectric medium and their polarization in terms of optical spin. The anisotropic permittivity of a gyroelectric medium results in two propagation modes, slow and fast, in which the optical spin varies according to the propagation direction. When the magnetization direction of the gyroelectric medium and the propagation direction of the light are not parallel, these modes possess both the longitudinal and transverse components of optical spin. We also confirm that a gyroelectric slab waveguide induces transverse optical spin in the guided light. We investigate the transport behavior of transverse optical spin in a gyroelectric slab using numerical calculations with a modified 3D finite difference time domain method. These new gyroelectric guided modes offer a novel approach to the manipulation of optical spin on a nanoscale.

Triple-channel glasses-shape nanoplasmonic demultiplexer based on multi nanodisk resonators in MIM waveguide

In this paper a metal-insulator-metal (MIM) plasmonic waveguide is proposed. Based on the proposed MIM waeguide and nanodisk resonators a novel 1*3 wavelength demultiplexer (WDM) structure is designed. The proposed WDM structure is based on three plasmonic filters. Each of this plasmonic filters consists of two waveguides (input and drop), and a middle cavity that are coupled with nanodisk resonators horizontally. Using these proposed plasmonic filters a three channel plasmonic demultiplexer also designed and simulated in this paper. By seting suitable values for the designed structural parameters (refractive index and geometric parameters), the structure output can be modified and adjusted in a controlled. According to the simulation results of the plasmonic demultiplexer structure, the transmission efficiency of one of the channels reaches about 80 % and The effect of cross talk is less than -23 dB. There are three different wavelengths for each channel in this proposed WDM structure. It is also notable that, 2D Finite-Difference Time-Domain (FDTD) numerical method is imployed in this paper. Large-scale photonic integration, the ultra-compact demultiplexer devices, ultra-compact demultiplexer devices, the development of optical integrated circuits and integrated plasmonic devices are among of the application that the proposed structure can be used in them.

A 3D FDTD approach for calculation of geomagnetically induced currents considering complex multilayer ground structure

A rigorous finite difference time domain (FDTD) method is proposed for accurate and fast calculation of geomagnetically induced currents (GICs). The proposed method takes into account complex 3D ground structures. The effect of multilayer stratified ground, shield wire and different excitation sources on GICs are evaluated. It is shown that the ground stratification can significantly affect GICs. The increasing effect of shield wires on GICs are also presented. It is also found that the sheet-current electrojet yields GICs with lower amplitudes compared to the line-current electrojet.

Compact resonant 2 × 2 crossbar switch using three coupled waveguides with a central nanobeam.

This theoretical simulation paper presents designs and projected performance of ∼1550-nm silicon-on-insulator (SOI) and ∼2000-nm Ge-on-Si-on-nitride and Ge-on-nitride 2×2 optical crossbar switches based upon a three-waveguide coupler in which the central waveguide is a nanobeam actuated by the thermo-optical (TO) effect. A TO heater stripe is located atop the central nanobeam. To implement accurate and realistic designs, the 3D finite difference time domain approach was employed. The metrics of crossbar switching, insertion loss (IL) and crosstalk (CT) were evaluated for choices of 3-waveguide structure parameters and TO-induced index changes. The predicted ILs and CTs were excellent, enabling the designed devices to be considered as fundamental building blocks in wavelength-division-multiplexed cross-connect (WXC) applications. Proposed here are compact, nonblocking space-and-wavelength routing switches to be constructed in a monolithic, industry-standard SOI chip (and in Ge-on-SON and GON chips). Specifics are given for realizing 16 × 16 × Mλ WXCs as well as reconfigurable, multi-resonant, programmable hexagonal and diamond meshes.

Design of Polarization Independent SERS Substrate with Raman Gain Evaluated Using Purcell Factor

Surface-enhanced Raman scattering (SERS) is a very promising detection/diagnostic technique at trace levels as the molecules exhibit a significant increase in their Raman signals when they are attached or are in proximity to plasmonic structures. In this study, a numerical design of SERS substrate as a probe has been demonstrated for detection and diagnosis of blood, water and urea samples. The proposed nanospiral design is polarization independent, and it offers the enhancement of the electric field strength ~ 109. The substrate design is based on 3D finite difference time domain simulations and is robust, versatile and sensitive even at low concentrations of the analyte. It works equally well when used in the reflection mode. In this study, the cavity quantum electrodynamics (CQED) Purcell factor has also been transposed to plasmonics. The Purcell factor in corroboration with CQED has been used to achieve efficient light–matter interaction at nanoscale by providing a more realistic result. It takes into account the randomness of incident wave polarizations and arbitrary orientations of interacting molecules. This gives a deeper insight into electromagnetic Raman gain in SERS and can be used to design novel SERS substrates.

Design and simulation of waveguide-integrated Ge/SiGe quantum-confined Stark effect optical modulator based on adiabatic coupling with SiGe waveguide

We report on the design and simulation of a waveguide-integrated Ge/SiGe quantum-confined Stark effect (QCSE) optical modulator based on the use of a Ge-rich SiGe relaxed buffer on a graded buffer as an optical waveguide. Despite the promising potential of this waveguide platform, efficient and wideband optical integration with a Ge-based active device has not been properly addressed so far. In this paper, via 3D finite-difference time domain simulation, we demonstrate that a simple 2D taper is sufficient to enable adiabatic optical coupling from the fundamental mode of the input SiGe waveguide to the fundamental mode of the Ge/SiGe multiple quantum well (MQW) modulator without the excitation of higher-order modes in Ge/SiGe MQWs. The 2D taper shows good fabrication tolerance considering critical variations in its dimensions. Significantly, wideband optical modulation performance in terms of extinction ratio and insertion loss is presented over the whole low-loss spectral range of the Ge/SiGe MQWs at different electrical bias values, device lengths, and numbers of quantum wells in order to comprehensively report its potential for Si-based optical modulators.

Diverse Substrate-Mediated Local Electric Field Enhancement of Metal Nanoparticles for Nanogap-Enhanced Raman Scattering.

The localized surface plasmon resonance of plasmonic nanoparticles (NPs) can be coupled with a noble metal substrate (S) to induce a localized augmented electric field (E-field) concentrated at the NP-S gap. Herein, we analyzed the fundamental near-field properties of metal NPs on diverse substrates numerically (using the 3D finite-difference time-domain method) and experimentally [using surface-enhanced Raman scattering (SERS)]. We systematically examined the effects of plasmonic NPs on noble metals (Ag and Au), non-noble metals (Al, Ti, Cu, Fe, and Ni), semiconductors (Si and Ge), and dielectrics (TiO2, ZnO, and SiO2) as substrates. For the AgNPs, the Al (11,664 times) and Si (3969 times) substrates produced considerable E-field enhancements, with Al in particular generating a tremendous E-field enhancement comparable in intensity to that induced by a Ag (28,224 times) substrate. Notably, we found that a superior metallic character of the substrate gave rise to easier induction of image charges within the metal substrate, resulting in a greater E-field at the NP-S gap; on the other hand, the larger the permittivity of the nonmetal substrate, the greater the ability of the substrate to store an image charge distribution, resulting in stronger coupling to the charges of localized surface plasmon resonance oscillation on the metal NP. Furthermore, we measured the SERS spectra of rhodamine 6G (a commonly used Raman spectral probe), histamine (a biogenic amine used as a food freshness indicator), creatinine (a kidney health indicator), and tert-butylbenzene [an extreme ultraviolet (EUV) lithography contaminant] on AgNP-immobilized Al and Si substrates to demonstrate the wide range of potential applications. Finally, the NP-S gap hotspots appear to be widely applicable as an ultrasensitive SERS platform (∼single-molecule level), especially when used as a powerful analytical tool for the detection of residual contaminants on versatile substrates.

Highly efficient SiO2 trapezoidal grating-based thin-film solar cell

A crystalline silicon thin-film solar cell (c-Si TFSC) with a trapezoidal grating is newly introduced and analyzed. The three-dimensional (3D) finite-difference time-domain (FDTD) method is employed to optimize the geometrical parameters of the trapezoidal grating and hence maximize the light absorption. The proposed trapezoidal grating TFSC offers an optical ultimate efficiency ( η ) of 32.3%, with an enhancement of 81% relative to the conventional TFSC. The light absorption enhancement within wavelength range (300–1100 nm) is based on the diffraction grating, which supports Bloch modes through the suggested solar cell. The electrical characteristics of the proposed design are also studied using the finite-element method. The cell doping concentration, junction thickness, and recombination process are also investigated to further enhance the power conversion efficiency (PCE). The reported design offers a short circuit current density J s c of 24.8 m A / c m 2 and PCE of 12.5% with an improvement of 83% over the conventional TFSC.

Design optimization for bright electrically-driven quantum dot single-photon sources emitting in telecom O-band.

A combination of advanced light engineering concepts enables a substantial improvement in photon extraction efficiency of micro-cavity-based single-photon sources in the telecom O-band at ∼1.3 µm. We employ a broadband bottom distributed Bragg reflector (DBR) and a top DBR formed in a dielectric micropillar with an additional circular Bragg grating in the lateral plane. This device design includes a doped layer in pin-configuration to allow for electric carrier injection. It provides broadband (∼8-10 nm) emission enhancement with an overall photon-extraction efficiency of ∼83% into the upper hemisphere and photon-extraction efficiency of ∼79% within numerical aperture NA=0.7. The efficiency of photon coupling to a single-mode fiber reaches 11% for SMF28 fiber (with NA=0.12), exceeds 22% for 980HP fiber (with NA=0.2) and reaches ∼40% for HNA fiber (with NA=0.42) as demonstrated by 3D finite-difference time-domain modeling.

Correlation between mechanical and optical properties of ZnO nanowire

Understanding mechanical and optical responses is significantly important for the development and application of nanowire devices. Hence, special structures are required to provide a level of performance that is adequate for the given application. With this breakthrough, in this work, a model is proposed by combining finite element modeling (FEM) software package ABAQUS and three-dimensional (3D) finite-difference time-domain (FDTD) techniques to measure the strain dependence on the optical behaviors of ZnO nanowire with and without embedding ZnS nanowire and also by the effects of various diameter sizes for the first time. The effects that bending strain have on the optical properties of the material such as electric field intensity and magnetic field intensity have been investigated. The obtained results indicate that the optical characteristics are strongly dependent on the applied strain. In addition, the novel method introduced, is able to furnish a powerful tool for quick comparison between theoretical models and experimental setups for the different properties of nanomaterial, and also should be useful to future predictions for other complex structures. Furthermore, the insights acquired in this study may provide valuable information on the development of robust ZnO nanowire based nanodevices.

Study on surface enhanced fluorescence based on Ag@razor clam substrate

In this paper, we prepared a low cost, environmentally friendly and stable surface enhanced fluorescence (SEF) substrate, namely, Ag@razor clam (Ag@RC), for fluorescence enhancement via magnetron sputtering technique. When the magnetron sputtering time was 10 min (Ag-10), we obtained optimal fluorescence enhancement effect for this substrate. The maximum enhancement factor (EF) for rhodamine 6G (R6G) was 1574 with small relative standard deviation (RSD). Above results indicate that this SEF substrate possesses strong EF, excellent stability and repeatability. In order to better understand the experimental results, the three-dimensional (3D) finite-difference time-domain (FDTD) method is used to simulate the electric field distribution of Ag-10 substrate and further study the enhancement effect of fluorescence. Finally, we find that Ag-10 substrate is applied not only to enhance fluorescence intensity of R6G molecules, but also to improve that of a variety of fluorescence molecules, which provides a new inspiration on the multiple molecules detection.

Doubly Photopolymerized Holographic Sensors.

Holographic sensors are two-dimensional (2D) photonic crystals that diffract narrow-band light in the visible spectrum to quantify analytes in aqueous solutions. Here, a holographic fabrication setup was developed to produce holographic sensors through a doubly polymerization system of a poly-2-hydroxyethyl methacrylate hydrogel film using a pulsed Nd:YAG laser (λ = 355 nm, 5 ns, 100 mJ). Wavelength shifts of holographic Bragg peak in response to alcohol species (0-100 vol %) were characterized. Diffraction spectra showed that the holographic sensors could be used for short-chain alcohols at concentrations up to 60 vol %. The reversibility of the sensor was demonstrated, exhibiting a response time of 7.5 min for signal saturation. After 30 cycles, the Bragg peak and color remained the same in both 20 and 60 vol %. The fabrication parameters were simulated in MATLAB using a 2D finite-difference time-domain algorithm to model the interference pattern and energy flux profile of laser beam recording in the hydrogel medium. This work demonstrates a particle-free holographic sensor that offers continuous, reversible, and rapid colorimetric readouts for the real-time quantification of alcohols.

Quantifying Sub-gridding Errors in Standard and Hybrid Higher Order 2D FDTD Simulations

Sub-gridding errors for a 2D Finite-Difference Time-Domain (FDTD) simulation are compared for both the standard FDTD and Hybrid higher order FDTD cases. Subgridding contrast ratios of 1:3, 1:9, 1:15, and 1:27 are considered and analyzed. A correlation is seen between the increase of contrast ratio with the increase of sub-gridding errors for both standard and hybrid cases. However, a trend of errors reduction when using hybrid formulations over standard formulations is apparent for each contrast ratio.

Three dimensional reverse time migration on common offset GPR data for void imaging

Three dimensional (3D) Reverse time migration (RTM), as an indispensable imaging technique, has been widely applied to the observed seismic data imaging. However, the RTM of Ground Penetrating radar (GPR) is usually implemented in its two-dimensional (2D) from. This paper proposes a 3D RTM algorithm on common offset GPR data for void imaging. In this algorithm, the 3D finite difference time domain (FDTD) method is applied to calculate the backward extrapolation of electromagnetic fields. And the time zero imaging condition is applied to obtain the final RTM results. Comparisons of the imaging results of 3D and 2D RTM demonstrated that the 3D RTM imaging results were superior to those of 2D RTM. And 3D spatial distribution of Void can be imaged by the proposed 3D RTM algorithm with high accuracy, which can help the subsequent interpretation of GPR profile.

Investigation of Light-Extraction Efficiency of Flip-Chip AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes Adopting AlGaN Metasurface

Herein, we investigate the influence of an AlGaN metasurface on AlGaN-based deep-ultraviolet light-emitting diodes' light-extraction efficiency by utilizing the 3D finite-difference time-domain method. As the first step in this study, we optimize an AlGaN metasurface to maximize the transmittance from LED to air. Then, six different flip-chip structures' light-extraction efficiencies are compared with each other to judge the usefulness of the metasurface's adoption on AlGaN-based LED. Considered structures are classified into two categories, with and without inclined sidewall, respectively. Each classified structure is subdivided again by three factors: flat interface, roughed cones, and metasurface on n-AlGaN. Extracted results show that the combination of inclined sidewall and the AlGaN metasurface shows synergetic results positively increasing light-extraction efficiency.

Finite-difference time-domain simulation of cathodoluminescence patterns of ZnO hexagonal microrods

The Finite-Difference Time-Domain (FDTD) numerical simulation method has been applied to interpret cathodoluminecence patterns observed for ZnO nanorods grown by a hydrothermal method. The 3D FDTD simulation reproduced the radial electromagnetic field pattern in the hexagonal resonator, corresponding to the CL emission maps of real ZnO microrods. The simulation result for the H z (TE) polarization—the intense field distribution along edges of the structure, in particular in the corners, but weak in the centre—matched the CL pattern particularly well. Since the experiment was not polarization sensitive, we suppose that polarisation sensitive transmission of electromagnetic field through the ZnO/air interface leads to such an observation. The results of the simulation show also that the lack of axial Fabry-Pérot-like resonances in the CL experiments is caused by leaking of the electromagnetic field from the ZnO resonator into the GaN substrate.

Compact, broadband, and low-loss silicon photonic arbitrary ratio power splitter using adiabatic taper.

The arbitrary ratio power splitter is widely used in photonic integrated circuits (PICs), for signal monitoring, power equalization, signal feedback, and so on. Here we designed a fabrication-tolerant, compact, broadband, and low-loss arbitrary ratio power splitter. The proposed arbitrary ratio power splitter was realized with an adiabatically tapered silicon rib waveguide with 70 nm shallow etches and an Si3N4 waveguide. The fabrication analysis confirmed that both of them are robust to fabrication errors. 3D finite-difference time-domain simulations show a very low excess loss (less than 0.02 dB for Si3N4 waveguide and 0.05 dB for Si rib waveguide), and a broadband operating wavelength range (100 nm). Good fabrication tolerance and standard critical dimensions make the arbitrary ratio power splitter compatible with the standard fabrication process of commercial silicon photonic foundries.