Finite-difference Time-domain Analysis Research Articles

Electromagnetic Wave Absorption in the Human Head: A Virtual Sensor Based on a Deep-Learning Model.

Determining the amount of electromagnetic wave energy absorbed by the human body is an important issue in the analysis of wireless systems. Typically, numerical methods based on Maxwell's equations and numerical models of the body are used for this purpose. This approach is time-consuming, especially in the case of high frequencies, for which a fine discretization of the model should be used. In this paper, the surrogate model of electromagnetic wave absorption in human body, utilizing Deep-Learning, is proposed. In particular, a family of data from finite-difference time-domain analyses makes it possible to train a Convolutional Neural Network (CNN), in view of recovering the average and maximum power density in the cross-section region of the human head at the frequency of 3.5 GHz. The developed method allows for quick determination of the average and maximum power density for the area of the entire head and eyeball areas. The results obtained in this way are similar to those obtained by the method based on Maxwell's equations.

Collective lattice and plasmonic resonances in the enhancement of circular dichroism in disk–rod metasurface

Compact planar photonic elements serving for efficient control over the polarization of light are of paramount importance in photonics. Here, we propose a design of a chiral periodic metasurface based on plasmonic nanodisks and nanorods arranged asymmetrically in a unit cell. Using the finite-difference time-domain analysis, we show that the collective lattice resonance harnessed by the diffraction coupling of the plasmonic unit cells is the heart of the revealed resonant 38% circular dichroism effect. The circular dichroism enhancement of the considered structure is improved using the deep-learning-assisted optimization of the metasurface design.

Extremely long nanojet formation from a ballpoint photonic pen

Certain microelements can facilitate light manipulation in the form of an intense electromagnetic beam known as a “photonic nanojet” (PNJ), which is an emerging candidate in the field of spectroscopy, nanolithography, and bio/chemical sensing. However, inherent limitations of the PNJs such as short jet length, focal point, and working length minimize their application areas. Herein, by considering readily available materials and technologies, we report a simple and physically realizable “photonic pen” (PP)-like microstructure to yield extremely long PNJs under plane-wave illumination. Essentially, by semi-immersing a sphere of certain diameter (acting as pen tip) in a barrel material with the width equal to and height 5 times the sphere diameter, a “ballpoint pen”-like microstructure is formed where illuminated light acts as the ink. Finite-difference time-domain analysis shows the use of low refractive index (RI) spherical tips ( n = 1.48 ) with 10 µm diameter, and a high RI SU-8 barrel ( n = 1.62 ) creating a net index difference of Δ n = 0.14 to effectively form PNJs with extremely long jet length, working distance, and full width at half-maximum of over 330 λ (121.6 µm), 319.45 λ (116.6 µm), and 4.22 λ (1.54 µm) respectively, and a figure of merit of ∼ 155 in water environment. S i O 2 / SU- 8 PPs show over ∼ 3 times longer jet length than previous studies. We show that such SU- 8 / SiO 2 PPs with extremely long PNJs can act as epidermal photonic imagers to scan biological tissues such as trilayer human skin to detect melanoma (skin cancer), both in contact and in noncontact mode with efficiency enhancement reaching ∼ 45 % compared to plain microspheres without barrels.

Predicting strongly localized resonant modes of light in disordered arrays of dielectric scatterers: a machine learning approach.

In this work, we predict the most strongly confined resonant mode of light in strongly disordered systems of dielectric scatterers employing the data-driven approach of machine learning. For training, validation, and test purposes of the proposed regression architecture-based deep neural network (DNN), a dataset containing resonant characteristics of light in 8,400 random arrays of dielectric scatterers is generated employing finite difference time domain (FDTD) analysis technique. To enhance the convergence and accuracy of the overall model, an auto-encoder is utilized as the weight initializer of the regression model, which contains three convolutional layers and three fully connected layers. Given the refractive index profile of the disordered system, the trained model can instantaneously predict the Anderson localized resonant wavelength of light with a minimum error of 0.0037%. A correlation coefficient of 0.95 or higher is obtained between the FDTD simulation results and DNN predictions. Such a high level of accuracy is maintained in inhomogeneous disordered media containing Gaussian distribution of diameter of the scattering particles. Moreover, the prediction scheme is found to be robust against any combination of diameters and fill factors of the disordered medium. The proposed model thereby leverages the benefits of machine learning for predicting the complex behavior of light in strongly disordered systems.

Highly Sensitive Micro-Opto-Electromechanical Systems Accelerometer Based on MIM Waveguide Wavelength Modulation

In this article, a novel micro-opto-electro- mechanical system (MOEMS) accelerometer sensor based on metal–insulator–metal (MIM) waveguide wavelength modulation is proposed. The device senses the vibration signal of external acceleration by the mechanical sensing system and uses the MIM waveguide in the optical sensing system to sense the displacement of the mechanical vibration component to cause the change of the light wavelength. The characteristics of the optical sensing system and the mechanical sensing system of the MOEMS accelerometer are analyzed by the finite-difference time-domain (FDTD) and finite-element analysis (FEA) methods, respectively. The simulation results show that the proposed accelerometer sensor has an optical system sensitivity of up to 7.827 in the wavelength modulation range of 1– <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$12~\mu \text{m}$ </tex-math></inline-formula> , a mechanical sensitivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.292963~\mu \text{m}$ </tex-math></inline-formula> /g, a natural frequency of 939.28 Hz, and a linear measurement range of ±4.4 g. Surprisingly, we obtained accelerometer sensitivity up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.293~\mu \text{m}$ </tex-math></inline-formula> /g and resolution up to 436.1 ng ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\delta \lambda ={1}$ </tex-math></inline-formula> pm). We also show the performance advantages of this accelerometer with high sensitivity and high resolution over the measurement range by comparing this MOEMS accelerometer sensor with previously reported work. Based on these performance characteristics, we expect this work to be widely used in high-precision measurements for inertial navigation.

Design of Wavelength Selectively Metamaterial Antenna for Thermal Camouflage

Phase transition materials offer exciting opportunities to dynamically control the optical properties of photonic devices. As a special phase change material (PCM), vanadium dioxide (VO2) is an excellent alternative for thermal camouflage technology research. Due to the unique property of VO2, coupled with the metallic structure can enable the design of thermally reconfigurable absorbers that allow resonance tuning. We propose a very simple, multi-layered with metamaterial excellent thermal emitter nanoantenna. The size of our design is only 1365 nm for the infrared wavelength range from 0.8 to 12 µm. Concept silver (Ag), VO2, dielectric separator and silver (Ag) configuration are used for finite difference time domain (FDTD) analysis. The absorption band can be further expanded with more deposition layers with various metals. In this paper, a thermal camouflage mid-infrared adaptive metamaterial antenna based on a PCM metamaterial is designed to fit perfectly into atmospheric windows. The spectral properties of the structure were calculated using FDTD method. The distinctive feature of the proposed structure is that adaptations can be made by placing an ultra-thin VO2 interlayer between the metallic structures. We believe that the proposed design is very promising in terms of simple fabrication and modifiable aspects for wide-area broadband unity absorption.

Gas sensing of organophosphorous compounds with III–V semiconductor plasmonics

Chemical warfare agents including nerve agents, e.g., sarin, are part of the organophosphorous compounds group known for their extreme lethal potency towards humans. With increasing risks of exposure to these molecules, there is a need for sensitive and selective detection. Infrared absorption spectroscopy is a powerful technique for the identification of molecules by probing their vibro-rotational molecular resonances. However, infrared light wavelength and molecule absorption cross-section dimensions mismatch, significantly hampering light-matter interaction especially at low concentrations, therefore lowering the sensitivity. To overcome this challenge, we propose a rapid chemistry-free III–V semiconductors InAsSb plasmonic sensor working in the mid-infrared with ribbon-shape nano-antennas associated to a confined enhanced electric field at the nanoscale. Exploiting well-known Salisbury perfect absorber structure coupled to an epsilon-near-zero layer benefit both spectral and spatial overlap, two required conditions for surface-enhanced infrared spectroscopy. Finite difference time domain and rigorous coupled-wave analysis coupled to Drude formalism were performed to design the sensor. The plasmonic sensor was exposed to hundreds of ppm-level concentration vapors of dimethyl methylphosphonate, known as DMMP, a commonly used sarin simulant. We determined that DMMP bound to the sensor native oxide to form a 5 Å estimated monolayer. The sensor response results from the coupling between surface plasmons and DMMP molecular vibrations upon adsorption at resonances frequency. Further simulations confirmed experimental results. This work demonstrates the application of III–V semiconductor plasmonics for gas sensing of complex molecules exploiting surface-enhanced infrared absorption.

Analysis of BCB and SU 8 photonic waveguide in MZI architecture for point-of-care devices

The proposed refractive index-based biosensor made of cost-effective polymer material in Mach-Zehnder configuration with novel horizontal slot waveguide structure offers high sensitivity. Due to the recent demand for low-cost point-of-care biosensor, silicon wafer-based polymer material has the potential for developing new hybrid waveguides for sensors. Hence we designed BenzoCycloButene (BCB), SU8 material as the core, and polymethyl methacrylate (PMMA) clad on a silicon wafer. The novelty of proposed BCB and SU8 horizontal slot waveguide structure of 2.5 μ m wide and 1.8μ m thick with a slot height of 400 ​nm, which handles the analyte effectively without any external sample holder. In Mach-Zehnder Interferometer architecture, reference arm 1 ​cm and sensing arm at a length of 1.1 ​cm with analyte refractive index of cancer cell (RI ​= ​1.401) and Influenza A type virus (RI ​= ​1.48) are used. Effective mode index of BCB core and SU8 are investigated through Finite-difference time domain (FDTD) analysis, and sensitivity results are calculated. Detection of disease are plotted in the transmission spectrum with reference to normal human serum (RI ​= ​1.35). This horizontal slot waveguide of BCB core achieves a sensitivity of 19280 nm/RIU and SU8 horizontal slot core waveguide provides a sensitivity of 16500 nm/RIU, which is higher in this polymer based refractive index biosensing.

Scalar diffraction analysis of the dispersion effect on the focal length of low-index thin flat lenses

We analyze the dispersion effect of the focal length of low-index thin lenses by using scalar diffraction and finite difference time domain (FDTD) methods. We compare the dispersion results obtained by using these methods with reported experimental results, and the well-known analytical formula for focal length (f) of diffractive lenses as a function of wavelength ( λ ) , f ( λ ) = f 0 λ 0 λ where f0 is the designed focal length for wavelength λ0. We show that when the analytical formula is applied to thin flat lenses with low-refractive index, the results are accurate for small numerical aperture (NA) up to 0.2. For larger NA, the error between the analytical approximation and the FDTD analysis remains around 8% over a wide range of NA.

Architectural improvements to increase reverberation and reduce flutter echo in two music rehearsal rooms

New music rehearsal rooms at a small college suffered from flutter echo and very short reverberation times. The architecture of these rooms included visually elegant designs which the college wanted to retain if feasible. We coordinated with the college, the architect, and a wood worker to develop concept ideas for improvement which would be cost effective and retain the elegant architectural. By using finite difference time domain (FDTD) simulation and mock-up listening tests, we evaluated the acoustic properties of these concept options. After completion of these tests and simulations, a design was implemented in each room based on acoustic performance and visual impact. We will present measurement results pre- and post-renovation, along with our methodology used for the FDTD analysis and mockup testing.

Disorder induced absorption enhancement of light in GaAs nanowire array

Light absorption characteristics of vertically aligned GaAs nanowire (NW) arrays with disordered diameters and heights are investigated in this work using finite difference time domain (FDTD) analysis technique. By varying the random height ranges, an optimum variation range of 1000–2000 nm is obtained that provides the maximum average absorbance at different fill-factors of the arrays. An array having random heights of the NWs within the optimized range is found to have better absorbance for both normal and oblique incidence of light compared to the uniform height structure. Even for 45° incidence angle of light, average absorbance is obtained to be 2% higher for the random-height array, compared to the case of absorbance obtained for the uniform height structure. The proposed arrays having random diameters provide up to 12.8% improvement in short circuit current density whereas the random-height structure enhances the short circuit current density by 1.1% compared to the arrays having uniform diameter and height. The present work also provides an effective medium theory based theoretical model taking into account the random height variation of the arrays. The theoretically calculated values are found to be in good agreement with FDTD simulation results, thereby providing further guidelines for designing random array based high performance photonic devices.

A novel design of printed ridge gap waveguide‐based 0‐dB backward‐wave coupler

This article presents the design and implementation of a 0-dB coupler based on Gap Waveguide (GW) technology. The proposed coupler is considered as the first backward-wave 0-dB realized using this technology. It operates at a central frequency of 30.5 GHz and maintains good performance regarding isolation and directivity all over the band. A parametric sweep is carried out to study the effect of the coupling section dimensions on the coupler performance to achieve nearly 0-dB coupler. The design is based on finite difference time domain (FDTD) analysis using CST microwave studio and Finite Element Method (FEM) using Ansoft HFSS for validation. The novelty of this article is that the achievable coupling section structure is considered as a genuine design for most 0-dB couplers for different operating frequency bands with its corresponding unit cell design based on GW technology. To validate this concept, the design steps are applied to four different frequency bands without changing the shape of the coupling section. The corresponding GW feeding networks are implemented on different materials for different couplers: Rogers RT5880, Rogers RO3003, and Rogers RO4350B. Furthermore, one of the couplers is fabricated and tested. The measured and simulated results show an acceptable agreement.

Large emergent optoelectronic enhancement in molecularly cross-linked gold nanoparticle nanosheets

A central goal in molecular electronics and optoelectronics is to translate tailorable molecular properties to larger materials and to the device level. Here, we present a method to fabricate molecularly cross-linked, self-assembled 2D nanoparticle sheets (X-NS). Our method extends a Langmuir approach of self-assembling gold nanoparticle (NP) arrays at an air-water interface by replacing the liquid sub-phase to an organic solvent to enable cross-linking with organic molecules, and then draining the sub-phase to deposit films. Remarkably, X-NS comprising conjugated oligophenylene dithiol cross-linkers (HS-(C6H4)n-SH, 1 ≤ n ≤ 3) exhibit increasing conductance with molecule length, ~6 orders of magnitude enhancement in UV-Vis extinction coefficients, and photoconductivity with molecule vs. NP contributions varying depending on the excitation wavelength. Finite difference time domain (FDTD) analyses and control measurements indicate that these effects can be modeled provided the local complex dielectric constant is strongly modified upon cross-linking. This suggests quantum hybridization at a molecule–band (q-MB) level. Given the vast number of molecules and nano-building blocks available, X-NS have potential to significantly increase the range of available 2D nanosheets and associated quantum properties.

Theoretical simulation of DNA hybridization based on cascaded plasmonic V-shaped channel biosensor

A compact biosensor based on specially designed Cascaded V-shaped Channels (CVSChs) for the analysis and detection of Deoxyribonucleic Acid (DNA) hybridization is presented in this paper. The proposed biosensor employs a Gold (Au) nanowire in then CVSChs sensing region and provides more flexibility as far as tailoring the detection range and sensitivity as compared to the single V-shaped channel (VSC). It has been numerically demonstrated that the detection sensitivity of DNA hybridization is significantly dependent on the number of the V shaped channels. A two – dimensional Finite Difference Time Domain (FDTD) analysis with Perfectly Matched Layers (PMLs) is employed to evaluate the sensor performance with different structural parameters. Simulation results indicate that the proposed biosensor can achieve a high sensitivity of 2541 nm / RIU by optimizing the structural parameters. We demonstrate a proof-of-concept hybrid biosensor by monitoring the resonance shift due to the hybridization event. The proposed biosensor will have great impact on early stage detection of infected cells.

Finite-difference time-domain analysis of ELF radio wave propagation in the spherical Earth–ionosphere waveguide and its validation based on analytical solutions

Abstract. The finite-difference time-domain (FDTD) model of electromagnetic wave propagation in the Earth–ionosphere cavity was developed under assumption of an axisymmetric system, solving the reduced Maxwell equations in a 2D spherical coordinate system. The model was validated on different conductivity profiles for the electric and magnetic field components for various locations on Earth along the meridian. The characteristic electric and magnetic altitudes, phase velocity, and attenuation rate were calculated. We compared the results of numerical and analytical calculations and found good agreement between them. The undertaken FDTD modeling enables us to analyze the Schumann resonances and the propagation of individual lightning discharges occurring at various distances from the receiver. The developed model is particularly useful when analyzing ELF measurements.

Tunable and anisotropic perfect absorber using graphene-black phosphorus nanoblock.

Two-dimensional (2D) materials, which have attracted attention due to intriguing optical properties, form a promising building block in optical and photonic devices. This paper numerically investigates a tunable and anisotropic perfect absorber in a graphene-black phosphorus (BP) nanoblock array structure. The suggested structure exhibits polarization-dependent anisotropic absorption in the mid-infrared, with maximum absorption of 99.73% for x-polarization and 53.47% for y-polarization, as determined by finite-difference time-domain FDTD analysis. Moreover, geometrical parameters and graphene and BP doping amounts are possibly employed to tailor the absorption spectra of the structures. Hence, our results have the potential in the design of polarization-selective and tunable high-performance devices in the mid-infrared, such as polarizers, modulators, and photodetectors.

Selective Raman detection and photocatalytic degradation of triphenylmethane dyes based on Ag nanoparticles anchored on the flower-like of aluminum/carbon nitride

At present, to develop an effective sensor with good photocatalytic activity for triphenylmethane (TPM) dyes in wastewater is urgently needed. Herein, Ag nanoparticles anchored on the flower like of aluminum/carbon nitride (Al/f-CN/Ag) are prepared. Importantly, the optimal Al/f-CN/Ag substrate shows not only a highly selective and sensitive SERS response but also excellent adsorption and photocatalytic performance for TPM molecules, due to the f-CN in Al/f-CN/Ag provides abundant contacting sites for AgNPs, and the highly efficient adsorption activity for TPM molecules through π-π stacking. Also, finite-difference time-domain (FDTD) analysis indicates that more vital “hot spots” are generated by AgNPs which are perfectly anchored in the petal gaps of Al/f-CN, further confirming the superiority of flower-like nanostructure. In addition, the Al/f-CN/Ag displays distinguished SERS activity with a detection limit of ~10−10 mol·L−1 for crystal violet (CV) and ~ 10−8 mol·L−1 for other TPM molecules, and the degradation rate for CV is as high as 99.9% in 140 mins. Moreover, the Raman activity of the substrate is almost unchanged in air within 95 days and after seven cycles. Thus, the constructed versatile SERS substrate will be an attractive nanomaterial for the detection and removal of TPM dyes in wastewater.

Design and Bulk Sensitivity Analysis of a Silicon Nitride Photonic Biosensor for Cancer Cell Detection

Bulk sensitivity is an important parameter to validate the efficiency of the photonic waveguide sensor. Due to recent advancements in point-of-care silicon photonic biosensing, the focus is to identify the effective way to improve sensitivity. Integrating polydimethylsiloxane (PDMS) microfluidic channel in sensor architecture decreases the sensitivity due to leakage of molecules at edges. The silicon nitride (SiN4) Mach–Zehnder interferometer utilizes the refractive index of different cancer cells (1.39–1.401) to determine the bulk sensitivity. The proposed gradient step rib-slot structure of 970 nm wide and 400 nm thickness is designed to hold the liquid sample without any PDMS material. This novel waveguide exhibits high waveguide bulk sensitivity S w , bulk and device bulk sensitivity S d compared with the gradient rib waveguide. We achieved a waveguide bulk sensitivity S w , bulk of 2.0699 RIU/RIU and device sensitivity S d of 568 nm/RIU through finite-difference time-domain (FDTD) analysis.

Design and Finite Difference Time Domain Analysis of a Dual Band Slotted Microstrip Patch Antenna for Wireless IoT Applications

Objectives: To investigate the effect of comb-like slot dimensions on the dualband microstrip antenna performance parameters. Methods: Alternate comblike slots are placed on the microstrip patch and the slot dimensions are varied to observe the effect on the antenna performance parameters. The design is simulated in the Mentor Graphics software package. The Mentor Graphics software results are verified with Finite Difference Time Domain (FDTD) analysis in MATLAB. The measured results on the Vector Network Analyzer (VNA) are in good agreement with the simulated results. Findings: A dual-band operation resonating at 2.25 GHz and 3.5 GHz is observed. The return loss of -30.23 dB and -33.3 dB at 2.25 GHz and 3.5 GHz is noticed. The gain and directivity at 2.25 GHz are 4.11 dBi and 6.2 dBi; whereas, at 3.5 GHz it is 4 dBi and 6.8 dBi respectively. It is observed that the slot width significantly influences the impedance matching. The alternate arrangement of comb-like slots on the patch increases the electrical length and improves the Return Loss (S11). Novelty: Enhancement of S11 characteristics without significant shift in the resonating frequency by felicitous placements of comb-like slots is the novelty of the proposed work. Keywords: Slotted Antenna; Dual Band; FDTD; UPML; Return Loss (S11)

Decorrelation of optical critical dimensions in mid-infrared ellipsometric spectroscopy of high aspect ratio etch profiles

Monitoring the high aspect ratio etch profiles in state-of-the-art three-dimensional NAND memory fabrication processes has pushed metrology technologies to new limits. Here, we discuss how a mid-infrared ellipsometric measurement can yield angstrom level discrimination in critical dimension changes of memory channel hole (CH) profiles across such a memory chip. Using finite-difference time-domain and rigorous coupled-wave analysis simulations, we demonstrate how dispersion mitigated mid-infrared beam penetration into these memory structures permits parameter decorrelation and the measurement of the full CH profile.