3D Finite-difference Time-domain Research Articles

Tunable modulators based on single and double graphene-based resonator systems in the mid-infrared spectrum

Tunable modulators based on single and double graphene-based resonator systems are studied numerically in the mid-infrared spectrum. Employing 3D-FDTD (3D Finite-Difference Time-Domain) simulations, multiple factors are discussed, including the geometrical parameters on the resonance wavelengths of single resonator systems, modulation depth calculations, and especially negative, positive and linear amplitude modulation are discussed for first time in current research. Furthermore, plasmon-induced transparency (PIT) windows are revealed for double resonator systems, and modulation depth as well as the negative and positive amplitude modulation are examined for two different wavelengths. The maximum value of modulation depth is 74 %, and linear modulation depth is 65 %. Moreover, for double resonator systems, a slow light capability is realized. The maximum tg equals 0.2 ps which coincides with 60 µm of light traveling in a vacuum.

Design and simulation of type-I graphene/Si quantum dot superlattice for intermediate-band solar cell applications

Recent experiments suggest graphene-based materials as candidates for use in future electronic and optoelectronic devices. In this study, we propose a new multilayer quantum dot (QD) superlattice (SL) structure with graphene as the core and silicon (Si) as the shell of QD. The Slater–Koster tight-binding method based on Bloch theory is exploited to investigate the band structure and energy states of the graphene/Si QD. Results reveal that the graphene/Si QD is a type-I QD and the ground state is 0.6 eV above the valance band. The results also suggest that the graphene/Si QD can be potentially used to create a sub-bandgap in all Si-based intermediate-band solar cells (IBSC). The energy level hybridization in a SL of graphene/Si QDs is investigated and it is observed that the mini-band formation is under the influence of inter-dot spacing among QDs. To evaluate the impact of the graphene/Si QD SL on the performance of Si-based solar cells, we design an IBSC based on the graphene/Si QD (QDIBSC) and calculate its short-circuit current density (Jsc) and carrier generation rate (G) using the 2D finite difference time domain (FDTD) method. In comparison with the standard Si-based solar cell which records Jsc = 16.9067 mA/cm2 and G = 1.48943 × 1028 m−3⋅s−1, the graphene/Si QD IBSC with 2 layers of QDs presents Jsc = 36.4193 mA/cm2 and G = 7.94192 × 1028 m−3⋅s−1, offering considerable improvement. Finally, the effects of the number of QD layers (L) and the height of QD (H) on the performance of the graphene/Si QD IBSC are discussed.Graphical abstract

Evolutionary optimization of the short-circuit current enhancement in organic solar cells by nanostructured electrodes

Using a particle swarm optimization algorithm (a population-based stochastic optimization technique) combined with 3D finite-difference time-domain simulations, we inverse design periodic arrays of metallic nanoparticles on indium-tin-oxide electrodes and nanoholes in metallic thin films working as electrodes in P3HT (Poly(3-hexylthiophene-2,5-diyl)):PCBM ([6,6]-Phenyl C61 butyric acid methyl ester) organic solar cells to achieve the maximum short-circuit currents (Jsc). Nanohole-array electrodes have large optical losses, leading to a net reduction of Jsc compared to a reference solar cell. On the other hand, nanoparticle arrays can lead to a significant enhancement of Jsc of up to 20%. Detailed simulations show that this enhancement is caused by the grating coupling of the incident light to surface plasmon polaritons at the interface of the metal electrode and the hole transport layer, leading to the enhancement of the electromagnetic field in the organic blend.

Light extraction enhancement of light-emitting diodes by nanopillar hierarchical structures

Patterning of p-GaN and/or indium-tin-oxide (ITO) surfaces is investigated to improve light extraction efficiency for GaN-based light-emitting diodes (LEDs). A nanopillar-patterned p-GaN layer, a nanopillar-patterned ITO contact layer, and dual nanopillar-patterned p-GaN and ITO layers are fabricated. When compared with conventional LEDs, LEDs with the nanopillar-patterned p-GaN, nanopillar-patterned ITO, and dual nanopillar-patterned p-GaN and ITO layers showed light output enhancement by 50%, 90%, and 120%, respectively. The light extraction enhancement mechanism of these nanopillar-patterned nanostructures is investigated via the 3D finite-difference time-domain method. The results show that the LED with the dual nanopillar-patterned p-GaN and ITO layers offers the best light extraction efficiency, which could stimulate further efforts to increase the escape light cone effectively and overcome total internal reflection. This methodology for synchronous nanopatterning of two light-emitting interfaces is believed to provide a major light extraction enhancement.

Compact asymmetric directional-coupler-based two-mode optical switch utilizing low loss Sb2Se3 phase-change material

Mode division multiplexing (MDM) has provided a new trend in high capacity optical transmission systems. Some of the most important devices in MDM systems are multimode switches that can route signals simultaneously into different paths. In this work, a two-mode 1 × 2 optical switch based on an asymmetric directional coupler and using Sb2Se3 phase-change material is proposed with a compact size of 45 µ m × 6 µ m . This switch can route the fundamental and first transverse magnetic modes in a wide bandwidth in the third telecommunication window from 1535 to 1565 nm. According to the 3D finite-difference time-domain simulation results, this switch has a low insertion loss of 0.83 dB and cross talk of − 7.93 d B across the bandwidth in the “on” state. In addition, when the switch is off, it has suitable specifications in terms of insertion loss and cross talk with values lower than 0.29 dB and − 11.98 d B across the bandwidth, respectively.

2D- PhC based all optical AND, OR and EX-OR logic gates with high contrast ratio operating at C band

In this article, photonic crystal (PhC) based all-optical logic gates, namely AND, OR and EX-OR have been explored and their characteristics are reported. Two-dimensional (2D) PhC based logic gates are proposed using a hexagonal lattice with T-shaped structure by incorporating line defects. The total size of the device is 84 µm2 and it operates at a wavelength of 1550 nm (C band). The designed optical logic gates are investigated using 2D finite difference time domain simulators by means of the plane wave expansion method. Parameters, such as response time and contrast ratio of the designed all-optic logic gates, are analyzed. The proposed AND, OR and EX-OR logic gates offer a contrast ratio of 14.48 dB, 14.57 dB and 12.9 dB respectively with, in order, response times of 0.159 ps, 0.168 ps and 0.1672 ps. The designed logic gates work in the third optical window as they are operating at the 1550 nm wavelength. To specify the performance of the proposed logic gates, their field distribution has been determined and is exhibited. The designed logic gates are suitable for use in photonic integrated circuits, all optical computing and optical sensing applications.

Breast Tumor Detection and Classification Based on Microwave Imaging

Limitations caused by traditional breast cancer detection and screening techniques have encouraged researchers to investigate alternative solutions. This study examines the use of a microwave-based approach for tumor detection in breast tissue and related tumor type classification using matched-filtering. Radar-like confocal microwave imaging (CMI) method constructs the foundation of such tumor detection approach. In particular, a microwave pulse is first transmitted, then back-scattered pulses are collected. All major reflective sites in the breast tissue are detected by repeating this procedure on a microwave pulse transmission-reception grid, aligning captured signals in-time to focus on a particular region in the breast tissue and superimposing such time-shifted signals to improve signal-to-clutter level. In the observed signals, clutter is originated by the heterogeneity of the breast tissue while signal is originated by a tumor site as a function of its water content.
 All calculations, in the study, were performed computationally in terms of a 3D Finite-Difference Time-Domain (FDTD) simulation models. For the antenna system, two cross-polarized resistively loaded bow-ties antennas were used in the computational model, and the tumor site was modeled using five different size and morphologies. Matched-filtering, on the other hand, was performed matching such obtained observations with that of a homogenous breast tissue, namely clutter-free model. Performance of the proposed approach was tested for two different antenna array resolutions, and it was observed that this parameter is important for successful detection and classification of a tumor-site in a realistic heterogenous breast tissue model.

Enhanced confinement of infrared surface plasmon polaritons in borophene waveguides

In this paper, a sub-wavelength terahertz plasmonic waveguide based on a borophene–insulator–metal (BIM) structure is investigated by using the 2D finite-difference time-domain method. It is proved there is a plasmonic mode of high confinement and low propagation loss in an optimized wavelength range. A dynamically adjustable filter, formed by introducing a borophene nanoribbon into the BIM waveguide, is computationally and theoretically explored, which can achieve tailoring terahertz surface plasmon polaritons in deep-subwavelength scale. These results can further provide a broader path for dynamic regulation of light transmission in the borophene-based waveguide and the development of adjustable borophene-based optoelectronic filtering.

Ultra low loss broadband 1 × 2 optical power splitters with various splitting ratios

We designed Si-based all-dielectric 1 × 2 TE and TM power splitters with various splitting ratios by combining the use of the inverse design of adjoint and numerical 3D finite-difference time-domain methods. The structure of the designed Si-based power splitters contains two Si waveguide branches on a SiO2 substrate that is compatible with CMOS fabrication technology. The proposed devices exhibit ultra-high transmission efficiency above 98 and 99%, and excess losses below 0.1 and 0.035 dB, for TE and TM splitters, respectively. The merits of these devices include a minor footprint of 2.2 × 2.2 µm2 and a flat-broad operating bandwidth of 200 nm with a center wavelength of λ = 1.55 µm. Also, the other advantage of these optical power splitters is the very short optimization time of 2 h for each device. Because of the aforementioned merits, the optimized devices can be crucial candidates for optical integrated circuits.

Annular hole array design as a high efficiency absorber for photovoltaic applications

In order to benefit from thin-film solar cells cost advantage, increasingly effort is deployed to maximize their harvesting capability. In this work, we focus on improving an a-Si solar thin-film by engraving it with an Annular Hole Array (AHA). The design parameters are fashioned to maximize the absorption across the visible spectral range exploiting the 3D Finite-Difference Time-Domain method (3D−FDTD). When comparing the AHA device to an unpatterned a-Si solar cell, an important absorption enhancement of 64.2% was obtained along with a broad spectral absorption bandwidth with peaks reaching 97.7% at normal incidence over the AM1.5 solar spectrum. It turns out that a proper design optimization highlighted promising results like polarization and almost incidence insensitivity. In fact, an integrated absorption of 90.5% over the AM1.5 solar spectrum was achieved and maintained over 65% for any angle of incidence for the spectral field λ∈[300nm;1000nm]. These results are beneficial for solar energy harvesting and allow this structure to be an advantageous candidate for photovoltaic applications.

Deconvoluting the Optical Response of Biocompatible Photonic Pigments.

To unlock the widespread use of block copolymers as photonic pigments, there is an urgent need to consider their environmental impact (cf. microplastic pollution). Here we show how an inverse photonic glass architecture can enable the use of biocompatible bottlebrush block copolymers (BBCPs), which otherwise lack the refractive index contrast needed for a strong photonic response. A library of photonic pigments is produced from poly(norbornene‐graft‐polycaprolactone)‐block‐poly(norbornene‐graft‐polyethylene glycol), with the color tuned via either the BBCP molecular weight or the processing temperature upon microparticle fabrication. The structure–optic relationship between the 3D porous morphology of the microparticles and their complex optical response is revealed by both an analytical scattering model and 3D finite‐difference time domain (FDTD) simulations. Combined, this allows for strategies to enhance the color purity to be proposed and realized with our biocompatible BBCP system.

Deconvoluting the Optical Response of Biocompatible Photonic Pigments.

To unlock the widespread use of block copolymers as photonic pigments, there is an urgent need to consider their environmental impact (cf. microplastic pollution). Here we show how an inverse photonic glass architecture can enable the use of biocompatible bottlebrush block copolymers (BBCPs), which otherwise lack the refractive index contrast needed for a strong photonic response. A library of photonic pigments is produced from poly(norbornene-graft-polycaprolactone)-block-poly(norbornene-graft-polyethylene glycol), with the color tuned via either the BBCP molecular weight or the processing temperature upon microparticle fabrication. The structure-optic relationship between the 3D porous morphology of the microparticles and their complex optical response is revealed by both an analytical scattering model and 3D finite-difference time domain (FDTD) simulations. Combined, this allows for strategies to enhance the color purity to be proposed and realized with our biocompatible BBCP system.

Few-cycle optical field breakdown and damage of gallium oxide and gallium nitride

Both gallium oxide and gallium nitride have great potential for use as high power transparent conducting oxide materials for a wide range of optoelectronic applications. It is, therefore, important to determine the dynamic optical field breakdown for these materials. Here, we report laser damage thresholds for Ga2O3 and GaN using 9 fs few-cycle pulses with a center wavelength near 760 nm. We determine laser-induced damage thresholds (LIDTs) for both single pulse and multi-pulse exposures, with multi-pulse LIDT showing significant reduction compared to single pulse LIDT (in some cases, >50%), possibly due to laser-induced defects. We show that Ga2O3 and GaN have an ultrafast optical field breakdown of 23 and 18 GV/m when nearly polarized along their [010] and [111] planes, respectively, extending their suitability for high power applications to the ultrashort pulse regime. To accurately characterize the excited carrier density criteria in which apparent laser damage occurs, carrier excitation dynamics for the entire laser pulse interaction were simulated using a modified Keldysh ionization model. For the measured single-shot threshold fluences, the plasma critical density was surpassed by 2 orders of magnitude and 2D finite-difference time-domain simulations were employed to understand the pulse interaction near the surface.

A Clutter Suppression Method Based on LSTM Network for Ground Penetrating Radar

It is critical to estimate and eliminate the wavelets of ground penetrating radar (GPR), so as to optimally compensate the energy attenuation and phase distortion. This paper presents a new wavelet extraction method based on a two-layer Long Short-Term Memory (LSTM) network. It only uses several random A-scan echoes (i.e., single channel detection echo sequence) to accurately predict the wavelet of any scene. The layered detection scenes with objects buried in different region are set for the 3D Finite-Difference Time-Domain simulator to generate radar echoes as a dataset. Additionally, the simulation echoes of different scenes are used to test the performance of the neural network. Multiple experiments indicate that the trained network can directly predict the wavelets quickly and accurately, although the simulation environment becomes quite different. Moreover, the measured data collected by the Qingdao Radio Research Institute radar and the unmanned aerial vehicle ground penetrating radar are used for test. The predicted wavelets can perfectly offset the original data. Therefore, the presented LSTM network can effectively predict the wavelets and their tailing oscillations for different detection scenes. The LSTM network has obvious advantages compared with other wavelet extraction methods in practical engineering.

Compact and broadband silicon mode-order converter using bricked subwavelength gratings

A compact and broadband silicon mode-order converter (MOC) scheme by employing reciprocal mode evolution between asymmetric input/output taper and bricked subwavelength gratings (BSWG) is proposed. In the proposed MOC, a quasi-TE0 mode is generated in the BSWG region, which can be regarded as an effective bridge between the two TE modes to be converted. Flexible mode conversion can be realized by only choosing appropriate structure parameters for specific mode transitions between input/output modes and the quasi-TE0 mode. By combing 3D finite difference time domain (FDTD) and particle swarm optimization (PSO) method, TE0-TE1 and TE0-TE2 MOCs are optimal designed, which can efficiently convert TE0 mode to TE1 and TE2 modes with lengths of 9.39 µm and 11.27 µm, respectively. Results show that the insertion losses of <1 dB and crosstalk of <-15 dB are achieved for both TE0-TE1 and TE0-TE2 MOCs, the corresponding working bandwidth are 128 nm (1511∼1639 nm) and 126 nm (1527∼1653 nm), respectively. Additionally, the MOCs can be fabricated with only single etch step with minimum feature size of 145 nm.

P‐67: Student Poster: Design and Analysis of Deflection Structure for Light‐Field Display

The purpose of this paper is to propose the architecture of 3D display, then using commercial software RSoft® and MatLab® for 3D finite difference time domain method (FDTD) for simulation and analysis. Gaussian Beam is utilized as the light source, and the range of simulation is based on the single volume pixel (Voxel) of display defined as the simulated environmental scale.

Propagation of broadband coherent light through LIPSS-based metasurfaces in diamond

We study the propagation of coherent broadband light through laser induced periodic surface structures (LIPSS) fabricated on diamond surfaces. 3D finite-difference time-domain (FDTD) simulations were carried out for a variety of experimentally produced LIPSS morphologies, which include the specific nanometer-scale mesoscopic irregularities arising from the fabrication technique. We compare their performance with sinusoidal grating-like structures, showing that the specific features present in LIPSS nanoripples produce a considerable scattering and diffraction when compared to the ideal nanostructures. With a view on determining the scope of the potential optical and photonic applications of LIPSS, we evaluate the effect of these irregularities on the transmitted spatial beam quality and the spatial phase characteristics of the optical wavefront in a broad spectral range.

Generation of few µm high optical vortex using tunable spiral plates

Optical vortices have been extensively explored, due to their widespread applications, spanning from optical trapping to laser processing. Previously, several methods for generating optical vortices had been reported. However, none of the previously reported methods demonstrated the design of a geometrically variable tunable spiral plate (SP) capable of tuning the optical vortex’s features. In this study, we present a three-dimensional tunable SP capable of generating desired vortex and focal characteristics. These SPs are 10 µm in width and 7–17 µm in height, generating few µm high vortices. We used the 3D finite difference time domain approach to model and simulate these SPs for incident plane waves with a wavelength of 632 nm. We show that the vortex profiles can be tweaked in two ways: by changing the SP’s geometrical features along the vertical axis, and by changing its refractive index.

A novel natural surface-enhanced fluorescence system based on reed leaf as substrate for crystal violet trace detection

The preparation of surface-enhanced fluorescence (SEF) substrates is often influenced by experimental strategies and factors such as the morphology and size of the nanostructures. In this study, using the natural reed leaves (RLs) without any special pretreatment as the substrate, metal silver is modified by magnetron sputtering technology to prepare a stable and efficient SEF system. The abundant “hedgehog-like” protrusions on the RL substrate surface can generate high-density “hot spots”, thus enhancement factor (EF) is enhanced up to 3345 times. The stability and reproducibility are verified in many measurements. The contribution of the intervention of silver nanostructure to the radiation attenuation process of fluorescent molecules is analyzed with the aid of Jablonski diagrams. Three-dimensional (3D) finite difference time domain (FDTD) simulates the spatial electric field and “hot spots” distribution of the substrate. The “hedgehog-like” protrusion structure generates multiple “hot spots”, which produce an excellent local surface plasmon resonance (LSPR) effect and provide higher fluorescence signal. Finally, RL/Ag-35 substrate is used to detect crystal violet (CV), and the detection limit is as low as 10−13 M. This “hedgehog-like” SEF substrate provides a new strategy for the trace detection of CV, which has a good practical application value.

Design of hybrid solar cell with GaAs1−xBix (x = 0.01) nanowire core and conformally coated P3HT/ITO shell

The optoelectronic characteristics of the vertically aligned conformally coated radial junction, core–shell nanowire (NW) hybrid solar cell (HSC) having a core active layer of GaAs0.99Bi0.01as acceptor and donor shell layer of poly(3-hexylthiophene-2,5-diyl) (P3HT), have been investigated. Since axial junction solar cells (SCs) have been experimentally showing better efficiency as compared to their radial counterpart, our objective is to improve the efficiency of radial pn junction nanowire solar cells. In order to achieve that, the geometrical parameters, including the core diameter, shell thickness comprising P3HT layer and ITO for a fixed pitch of 480 nm are optimized. A fixed thickness of 10 nm is considered for P3HT due to a diffusion length of 20 nm. 3D finite-difference time-domain (FDTD) method has been employed to examine the effect of geometrical parameter variation on carrier generation and a detailed study has been presented. The core diameter of our proposed structure is 160 nm (Rcore = 80 nm) and the thickness of the surrounding shell is considered to be 90 nm (Rshell = 90 nm) for further optoelectronic performance study. The absorption spectra, generation rate profile and electric field profiles for the wavelength (λ) range of 400–900 nm have been presented for depicting the improvement caused due to incorporation of conformal coating of P3HT on core GaAs0.99Bi0.01 NW. An extensive investigation on the variation of electrical performance parameters such as open circuit voltage (Voc), short circuit current (Jsc), fill factor (FF) and power conversion efficiency (PCE) with carrier lifetime and surface recombination velocity (SRV) is performed for a fixed core and shell doping. An efficiency of 19% is achieved for the device featuring minority carrier lifetime of 60 ns and sufficiently high SRV of 105cm/s at the contacts for lightly doped core (∼1 × 1016 cm−3) and heavily doped shell (∼1 × 1019 cm−3) and substrate (∼1 × 1019 cm−3).