3D Finite-difference Time-domain Research Articles

Silver films coated inverted cone-shaped nanopore array anodic aluminum oxide membranes for SERS analysis of trace molecular orientation

Abstract Reproducible surface enhanced Raman scattering (SERS) enhanced performance, affected by molecular orientations in plasmonic nanostructures, attracts more attention than SERS detection sensitivity for satisfying requirements of practical applications. In this work, by coating silver films on the inverted cone-shaped nanopore array anodic aluminum oxide membranes, we provide a way to study the effect of molecular orientations on SERS enhancement with mapping technology. SERS enhanced performance of the fabricated substrates is assessed with trace Rhodamine 6G detection, enhancement factor calculation and 3D finite-difference time-domain simulation respectively. It is found that inverted cone-shaped nanopore and silver films both contribute to the highly SERS enhancement. And special inverted cone-shaped nanopore plasmonic structures limit molecular vibrations so as to affect Raman scattering enhancement. These findings can help us to understand the electromagnetic mechanism of molecular orientations on SERS enhancement, so as to design highly reproducible SERS substrates for various applications.

Directing Gold Nanoparticles into Free-Standing Honeycomb-Like Ordered Mesoporous Superstructures.

2D mesoporous materials fabricated via the assembly of nanoparticles (NPs) not only possess the unique properties of nanoscale building blocks but also manifest additional collective properties due to the interactions between NPs. In this work, reported is a facile and designable way to prepare free-standing 2D mesoporous gold (Au) superstructures with a honeycomb-like configuration. During the fabrication process, Au NPs with an average diameter of 5.0 nm are assembled into a superlattice film on a diethylene glycol substrate. Then, a subsequent thermal treatment at 180 °C induces NP attachment, forming the honeycomb-like ordered mesoporous Au superstructures. Each individual NP connects with three neighboring NPs in the adjacent layer to form a tetrahedron-based framework. Mesopores confined in the superstructure have a uniform size of 3.5 nm and are arranged in an ordered hexagonal array. The metallic bonding between Au NPs increases the structural stability of architected superstructures, allowing them to be easily transferred to various substrates. In addition, electron energy-loss spectroscopy experiments and 3D finite-difference time-domain simulations reveal that electric field enhancement occurs at the confined mesopores when the superstructures are excited by light, showing their potential in nano-plasmonic applications.

Compact and efficient 2D and 3D designs for photonic-to-plasmonic coupler

Two ultra-compact designs of wideband photonic to metal-dielectric-metal (MDM) couplers are introduced. The two proposed couplers show a high efficiency of 92% and 94% at unexpected very small coupling lengths, using MDM as a coupling waveguide. Emerging from the half-wavelength transformer concept, when the characteristic impedances of the coupled waveguides are equal in magnitude but have a phase difference, we report a slight shift in the coupler length of the first maximum. Relying on this concept, the lengths of the proposed couplers are 10 nm and 15 nm, respectively, to match the phase shift. The proposed couplers also enjoy a very wide bandwidth of 1785 nm for the 10 nm design and 1865 nm for the 15 nm design because of the short coupling length. Also, we introduce a 3D design for the coupler based on a quasi-MDM waveguide of a 20 nm silver layer to facilitate the fabrication process. The simulation results are produced by 2D finite element methods and verified by 3D finite difference time domain, but with incomparable computational cost.

Design of low-loss and near-zero ultraflattened dispersion PCF for broadband optical communication

A 2D finite difference time domain method is used to design a single-mode photonic crystal fiber (PCF) with engineered parameters that is studied with the aim of simultaneous achievement of low loss (below 10−14dB/km) and low chromatic dispersion over a broad band for future optical communication. The superiority of the proposed structure over recently reported structures is shown by the near-zero ultraflattened chromatic dispersion of 1.69±0.08 ps/(km nm) obtained over a broad wavelength range from 1 to 2 μm, covering the entire optical communication bands (O, E, S, C, L, and U). Also, the large effective area of more than 130 μm2 and as a result the low nonlinearity in addition to the near-zero ultraflattened chromatic dispersion and ultralow loss achieved for the proposed PCF make the structure a good candidate for high-capacity optical communication with a high bitrate over long distances for future ultradense wavelength-division multiplexing.

Numerical design and analysis of a 3-channel grating assisted mode-division (de)multiplexer

A 3-channel grating assisted mode-division (de)multiplexer is proposed and designed with slab waveguides using 2D Finite-Difference Time-Domain (2D-FDTD) technique. The proposed device structure consists of two identical single mode waveguides, one multimode waveguide, and two gratings with different periods. (De)multiplexing operation is achieved by contra-directionally coupling first and second order transverse electric (TE) modes of the multiomde waveguide to fundamental TE mode of the two single mode waveguides. The fundamental mode of multimode waveguide propagates in the same direction without being coupled to any other mode. Numerical analysis of the proposed device has shown insertion loss ranging from −1.27 to −0.43 dB and crosstalk between −31.20 to −15.31 dB at the operating wavelength of 1550 nm.

2D-FDTD simulation of ultra-compact multifunctional logic gates with nonlinear photonic crystal

In the present paper, we aim to report an ultra-compact AlGaAsκ(3) photonic crystal structure to realize multifunctional logic responses. The properties of the proposed model are numerically investigated with different physical parameters by the 2D-finite difference time domain (FDTD) method. For this purpose, the effect of the geometrical parameters and dielectric materials, including Si and InSb, in the near-infrared region are studied. To obtain dynamical tunability of the proposed model, the effect of the defects is then utilized. Numerical results show that the proposed devices are able to operate as multifunctional logic devices including an AND gate and a demultiplexer. Moreover, the structure has a compact footprint of 8.85 μm×8.85 μm×100 nm. We expect that this theoretical result leads to remarkable applications in photonic integrated circuits, e.g., optical memory.

An iterative inversion method using transient electromagnetic data to predict water-filled caves during the excavation of a tunnel

The correct interpretation of full-space transient electromagnetic data has always constituted a critical safety problem during tunnel excavation projects. Targeting the interpretation of water-filled caves under narrow tunnel conditions, we have developed an iterative inversion method based on 3D finite-difference time-domain (FDTD) forward calculations and a direction algorithm. In total, 125 groups of 3D FDTD forward calculation results are analyzed to identify the correlations between the response data and the geometric conditions of the cave. A direction algorithm is established based on the correlations, thereby increasing the iterative inversion convergence speed. Using the proposed iterative inversion method, the location and volume of the water-filled cave in front of the tunnel face are successfully inverted. Through an iterative program, the inversion results of simulations involving the detection of water-filled caves under tunnel conditions are accurately analyzed, and the relative error is less than 10%. The application of the iterative inversion method to the Mingyue Mountain Tunnel project suggests that this method is capable of interpreting the size of water-filled caves and it is valid for a narrow tunnel face with only a single available measurement point. The proposed iterative inversion method can be used alone or in combination with other detection techniques, thereby providing engineers with a better early warning system for detecting water-filled caves in tunnels.

An Artificial Neural Network (ANN) approach to extract micro-architectural properties of cortical bone using ultrasound attenuation: A numerical study

The goal of this study is to estimate the porosity parameters including pore diameter, pore density, and porosity of cortical bone from ultrasound attenuation measurements using an artificial neural network (ANN). Two-dimensional (2D) finite-difference time-domain simulations are conducted to calculate the frequency-dependent attenuation in the range of 1–8 MHz in mono-disperse structures (constant pore size) with a pore diameter and density ranging from 20 to 120 µm and 3–16 pore/mm2, respectively. Furthermore, poly-disperse structures (non-uniform pore distribution) are obtained from high resolution CT scans of human cortical bone and 2D numerical simulations are carried out in the same frequency range as for the mono-disperse cases. Then, a regression problem is formulated with the ultrasonic attenuation at different frequencies acting as the feature vectors and the output being set as the porosity parameters. Our dataset consists of 330 structures for the mono-disperse model and 668 structures for the poly-disperse model. ANN-based (3 hidden layers with 806 trainable weights) parameter prediction method achieves accuracies as high as 96% for pore size, 97% for porosity, and 78% for pore density for the poly-disperse model. This work demonstrates the potential of combining ultrasound methods to deep neural networks to quantify cortical bone parameters with high accuracies.

Preliminary seismic hazard assessment for the proposed Bohai Strait subsea tunnel based on scenario earthquake studies

In this paper we present a preliminary seismic hazard assessment for the proposed Bohai Strait subsea tunnel (BSST) based on scenario earthquakes in a strike-slip stress regime. We studied two scenarios: 1) a right-lateral, strike-slip faulting earthquake event with surface wave magnitude (Ms) 7.4 occurred on the north to northeast (NNE) oriented Tan-Lu fault zone (TLFZ) with a location about 100 km away from BSST (the far field); and 2) a left-lateral strike-slip event with magnitude Ms4.1 occurred in the direct vicinity to BSST (the near field) on the northwest to west (NWW) Zhangjiakou-Penglai fault zone (Zhang-Peng Fault Zone, or ZPFZ). Selections of these two scenario events are based on the characterization of the actual seismo-tectonic settings and representation of earthquake history in the Bohai region. We studied the impacts of these two scenario events on BSST in terms of longitudinal compression, bending, and tunnel cross-section's ovaling deformation. As a preliminary study, we excluded the analysis of formation-structure interaction and focused on the ground motion parameters expressed as the maximum displacement, the peak ground velocity (PGV) and peak ground acceleration (PGA) of the free seismic wave field surrounding the tunnel walls. This simplification is acceptable for the fact that BSST is planned to be constructed in the granite bedrock formation in which the inertial contrast between the formation and the structure is minimal. Seismic hazard assessment for tunnels is completely different from the assessment for above-ground buildings. It is also substantially different from assessment for tunnels built in soil formations, for which the formation (soil)-structure interaction evaluation is critical. We use semi-analytical approach for assessing the seismic impact of the far field, large event to BSST; whereas use the two-dimensional (2D) finite difference time domain (FDTD) numerical simulation technique for assessing the response of BSST to the seismic load of the near field, smaller event. The preliminary results suggest that 1) the impact of the low frequency, far field event has very subtle impact to BSST in terms of longitudinal compression, bending, and cross-section ovaling; and 2) the impact of the high-frequency, near field event has little impact to BSST in terms of longitudinal compression and bending; while has a measurable impact to cross-section ovaling. However, this kind of deformation is within the limit of tolerance under earthquake-resistant design. Over all, the seismic risk posed by regional large earthquakes and local small events to BSST is within allowable risk threshold and manageable with the enforcement of earthquake resistant code.

Sandwich-like Ag@Cu@CW SERS substrate with tunable nanogaps and component based on the Plasmonic nanonodule structures for sensitive detection crystal violet and 4-aminothiophenol

It is critical in nano physics nowadays to fabricate a superior surface enhanced Raman scattering (SERS) substrate which provides the high-density ‘hot spots’ for obtaining the strong local electric field. A sensitive, large-scale and low-cost sandwich-like Ag@Cu@CW SERS active substrate was successfully fabricated by decorating the Cu and Ag nanoparticles on the cicada wing (CW) with uniformly-distributed nanopillars via a simple method with the DC magnetron sputtering system. The SERS properties of the surface rough Ag@Cu@CW substrate with optimal nanogaps have been investigated in terms of the enhancement factor (EF, ~3.1369 × 105), limit of detection (10−11 M of 4-ATP), relative standard deviation (RSD, <15%) and sample-to-sample repeatability. The high-density ‘hot spots’ formed by the appropriate nanogaps and specific plasmonic nanostructures would give rise to stronger local electromagnetic field, which was also confirmed by the 3D finite-difference time-domain (3D-FDTD) simulation. For practical application, the crystal violet (CV) as the carcinogenic dyes used widely in the aquaculture were detected with the LOD of 10−10 M using our Ag@Cu@CW SERS substrate, suggested that the Ag@Cu@CW has a potential value in sensitive and quantitative detection of environment pollutant and chemical sensing.

Graphene Nanoribbon Assisted Refractometer Based Biosensor for Mid-Infrared Label-Free Analysis

In this paper, an enhanced Fabry-Perot resonator (FPR)-based refractometer sensor is proposed using graphene nanoribbon (GNR) material to enable mid-infrared application. The FPR section consists of a three vertically aligned GNR sections: a conventional basic section placed in the middle complemented with two ending tail arms for auxiliary enhancement. To enable a real-world application of sensor, three-dimensional (3D) finite difference time domain (FDTD) simulations are performed. To apply dynamic gate bias tuning, the GNR layer is isolated from the substrate by means of a 50-nm layer of silicon dioxide (SiO2), which is placed on top of a thick bulk silicon (Si). To realize sensing application for the proposed enhanced FPR, a container is placed on FPR. Refractive index sensitivity of the sensor is calculated to be in the range of 3180–4220 nm RIU−1. To compare the superior performance of the sensor, a figure of merit (FOM) is defined and extensively investigated. The FOM of proposed sensor is found to be around 2.9–3.1 RIU−1, which is a reasonable value compared to compact-sized on-chip-intended sensors. Moreover, methods of dynamic variation of resonance wavelength are discussed, which can be useful for multi-analytic applications using spectral multiplexing of the sensor. The small footprint and its single- or multi-sensing application suggest that this enhanced FPR-assisted biosensor can be used as a lab-on-a-chip sensor for a label-free analysis.

Excellent Quality Factor Ultra-Compact Optical Communication Filter on Ring-Shaped Cavity

Abstract In this paper, we propose a high-quality factor (QF) ultra-compact structure for designing all optical filter based on two dimensional (2D) photonic crystals that can operate in third communication window range (1550 nm). The simulation results calculated by using 2D finite difference time domain (FDTD) method and photonic band gap (PBG) region are calculated by plane wave expansion (PWE) method. The average transmission efficiency, average QF and optical full width at half maximum (FWHM) bandwidth more than 95 %, 4300 and 0.4 nm obtained, respectively. The value of QF for the proposed structure is a major QF for ring resonator-based filters. We studied the effect of changing the air background to liquid crystal (LC) surrounding the dielectric rods. Results showed that by increasing LC refractive index (n) peak of wavelengths red shifted. The footprint of proposed filter is 160 ${{\mu}}{{\rm{m}}^{\rm{2}}}$ which make the proposed structure promising candidate for optical integrated circuits.

RGB Magnetophotonic Crystals for High-contrast Magnetooptical Spatial Light Modulators

Magnetooptical spatial light modulators (MOSLMs) are photonic devices that encode information in photonic waveforms by changing their amplitude and phase using magnetooptical Faraday or Kerr rotation. Despite the progress on both MO materials and switching methods, significant improvements on materials engineering and SLM design are needed for demonstrating low-power, multicolor, analog and high-contrast MOSLM devices. In this study, we present design rules and example designs for a high-contrast and large figure-of-merit MOSLM using three-color magnetophotonic crystals (MPC). We demonstrate for the first time, a three-defect MPC capable of simultaneously enhancing Faraday rotation, and high-contrast modulation at three fundamental wavelengths of red, green and blue (RGB) within the same pixel. We show using 2D finite-difference time-domain simulations that bismuth-substituted yttrium iron garnet films are promising for low-loss and high Faraday rotation MOSLM device in the visible band. Faraday rotation and loss spectra as well as figure-of-merit values are calculated for different magnetophotonic crystals of the form (H/L)p/(D/L)q/(H/L)p. After an optimization of layer thicknesses and MPC configuration, Faraday rotation values were found to be between 20–55° for losses below 20 dB in an overall thickness less than 1.5 µm including three submicron garnet defect layers. The experimental demonstration of our proposed 3-color MOSLM devices can enable bistable photonic projectors, holographic displays, indoor visible light communication devices, photonic beamforming for 5 G telecommunications and beyond.

Design and Analysis of 3-Input NAND/NOR/XNOR Gate Based on 2D Photonic Crystals

Abstract In this paper, Two Dimensional (2D) Photonic Crystal (PhC) based 3-input all optical NOR, NAND and XNOR gates is proposed and designed. The proposed device is formed by the combination of line defects and square cavity. The performance of the device is analyzed using 2D Finite Difference Time Domain (FDTD) method. The band gap analysis is done by Plane Wave Expansion (PWE) method. The device has the lattice constant and refractive index of 616 nm and 3.46, respectively. The dimension of the proposed structure is about 12.5 µm*12 µm which is highly compact and suitable for photonic integrated circuits (PIC).

Plasmonic bimetallic nanodisk arrays for DNA conformation sensing.

The integration of large-scale 2D bimetallic Ag/Au nanodisk arrays with gold nanoparticles is developed for sensing DNA conformation with the assistance of 3D finite-difference time-domain simulation. The optimized system comprising Ag/Au nanodisk arrays and gold nanoparticles offers a more than 6-fold enhancement in surface plasmon resonance shift, enabling the feasibility for sensitive DNA detection with a detection limit down to 100 femtomolar. Importantly, owing to the distance-dependent nature of the surface plasmon signal, sensitive differentiation of DNA conformations can be achieved with a conventional optical measurement. This platform could provide new exciting capabilities for a reliable, reproducible, and label-free assay analysis for investigating the conformations of DNA and other biological molecules.

Fully three-dimensional analysis of a photonic crystal assisted silicon on insulator waveguide bend

A detailed numerical study of low-loss silicon on insulator (SOI) waveguide bend is presented using the fully three-dimensional (3D) finite-difference time-domain (FDTD) method. The geometrical parameters are optimized to minimize the bending loss over a range of frequencies. Transmission results for the conventional single bend and photonic crystal assisted SOI waveguide bend are compared. Calculations are performed for the transmission values of TE-like modes where the electric field is strongly transverse to the direction of propagation. The best obtained transmission is over 95% for TE-like modes.

Wavelength-Flattened Directional Coupler Based Mid-Infrared Chemical Sensor Using Bragg Wavelength in Subwavelength Grating Structure.

In this paper, we report a compact wavelength-flattened directional coupler (WFDC) based chemical sensor featuring an incorporated subwavelength grating (SWG) structure for the mid-infrared (MIR). By incorporating a SWG structure into directional coupler (DC), the dispersion in DC can be engineered to allow broadband operation which is advantageous to extract spectroscopic information for MIR sensing analysis. Meanwhile, the Bragg reflection introduced by the SWG structure produces a sharp trough at the Bragg wavelength. This sharp trough is sensitive to the surrounding refractive index (RI) change caused by the existence of analytes. Therefore, high sensitivity can be achieved in a small footprint. Around fivefold enhancement in the operation bandwidth compared to conventional DC is achieved for 100% coupling efficiency in a 40 µm long WFDC experimentally. Detection of dichloromethane (CH2Cl2) in ethanol (C2H5OH) is investigated in a SWG-based WFDC sensor 136.8 µm long. Sensing performance is studied by 3D finite-difference time domain (FDTD) simulation while sensitivity is derived by computation. Both RI sensing and absorption sensing are examined. RI sensing reveals a sensitivity of −0.47% self-normalized transmitted power change per percentage of CH2Cl2 concentration while 0.12% change in the normalized total integrated output power is realized in the absorption sensing. As the first demonstration of the DC based sensor in the MIR, our device has the potential for tertiary mixture sensing by utilizing both changes in the real and imaginary part of RI. It can also be used as a broadband building block for MIR application such as spectroscopic sensing system.

Radius estimation of subsurface cylindrical objects from ground-penetrating-radar data using full-waveform inversion

Ray-based radius estimations of subsurface cylindrical objects such as rebars and pipes from ground-penetrating-radar (GPR) measurements are not accurate because of their approximations. We have developed a novel full-waveform inversion (FWI) approach that uses a full-waveform 3D finite-difference time-domain (FDTD) forward-modeling program to estimate the radius including other object parameters. By using the full waveform of the common-offset GPR data, the shuffled complex evolution (SCE) approach is able to reliably extract the radius of the subsurface cylindrical objects. A combined optimization of radius, medium properties, and the effective source wavelet is necessary. Synthetic and experimental data inversion returns an accurate reconstruction of the cylinder properties, medium properties, and the effective source wavelet. Combining FWI of GPR data using SCE and a 3D FDTD forward model makes the approach easily adaptable for a wide range of other GPR FWI approaches.

Plasmon enhanced near-infrared downconversion luminescence of β-NaYF4:Pr3+ nanoparticles by high-ordered Ag nanopillar arrays with tunable nanogaps

Large-area high-ordered Ag nanopillar arrays with tunable nanogaps are fabricated by an electroless deposition method. Then, β-NaYF4:Pr3+ nanoparticles (NPs) are spin coated on the Ag nanopillar arrays. Plasmon enhanced near-infrared (NIR) downconversion (DC) luminescence involving the 876 nm (1D2→3F2) and 1017 nm (1D2→3F4,3), as well as the two-step sequential transitions at 915 nm (3P0→1G4) and 990 nm (1G4→3H4), are achieved by Ag nanopillar arrays under the excitation of 444 nm (3H4→3P2) of Pr3+ ions. The influence of different nanogaps between Ag nanopillar arrays on NIR DC luminescence is investigated, and the results show that the optimal nanogap size is 25 nm with a maximum enhancement factor about 3.98. Furthermore, 3D finite-difference time-domain (FDTD) simulation is performed to analyze the enhancement mechanism of NIR DC luminescence. Our study may have potential application in the field of silicon-based solar cells.

Numerical Modeling of Abnormal Blocking Effect for the Design of Novel Optical Sensor Element Constructed by Periodic Grating Strips Over Si/SiO2 Wire Waveguide

In this paper the numerical modeling of periodic structures for implementation of a novel type of photonic sensors by using the 3D finite difference time domain (FDTD) method is presented. The sensing is based on the optical phenomena occurring in the segmented grating which is placed on the thin silica buffer over the silicon wire waveguide in the silicon‐on‐insulator (SOI) structure. This design provides the effective resonance interaction of the guided wave with the virtual leaky wave supported by the segmented grating evanescently coupled with the silicon wire. The dropping wavelength of this interaction is strongly dependent on the grating environment and it provides a strong refractometricsensitivity (Sn &gt; 420 nm RIU−1) and surface sensitivity Sh &gt; 0.17, evaluated in the case of water. The modeling proves that the effect of abnormal blocking due to virtual leaky wave could be the base for the design of optical sensors with extremely high sensitivity.