Finite-difference Time-domain Simulations Research Articles

Voltage-induced modulation of interfacial ionic liquids measured using surface plasmon resonant grating nanostructures.

We have used surface plasmon resonant metal gratings to induce and probe the dielectric response (i.e., electro-optic modulation) of ionic liquids (ILs) at electrode interfaces. Here, the cross-plane electric field at the electrode surface modulates the refractive index of the IL due to the Pockels effect. This is observed as a shift in the resonant angle of the grating (i.e., Δϕ), which can be related to the change in the local index of refraction of the electrolyte (i.e., Δnlocal). The reflection modulation of the IL is compared against a polar (D2O) and a non-polar solvent (benzene) to confirm the electro-optic origin of resonance shift. The electrostatic accumulation of ions from the IL induces local index changes to the gratings over the extent of electrical double layer (EDL) thickness. Finite difference time domain simulations are used to relate the observed shifts in the plasmon resonance and change in reflection to the change in the local index of refraction of the electrolyte and the thickness of the EDL. Simultaneously using the wavelength and intensity shift of the resonance enables us to determine both the effective thickness and Δn of the double layer. We believe that this technique can be used more broadly, allowing the dynamics associated with the potential-induced ordering and rearrangement of ionic species in electrode-solution interfaces.

Porous GaN Nanopyramids: Advancing Beyond Conventional Nanostructures for High‐Brightness InGaN/GaN Quantum Wells Emission

AbstractGaN nanostructures hold significant promise in advancing nanoscale light‐emitting devices. However, significant progress remains elusive, possibly due to the absence of innovative approaches. Beyond the smaller size of GaN nanostructures, the main interest lies in growing InGaN active layers on semi‐ and non‐polar orientations. Nevertheless, it is evident that while leveraging such a method holds initial promise, this approach may eventually reach its limits, prompting the need for fresh perspectives. In this manuscript, electrochemical etching is utilized to fabricate porous GaN nanopyramids. Given the absence of prior studies on electrochemical etching of GaN nanostructures, its etching behavior is investigated, revealing its selectivity concerning doping concentration and geometry. Furthermore, how the choice of electrolyte can differently impact the optical response of the etched template is explored. Subsequently, an InGaN/GaN quantum wells shell is grown as the active region. The significance of the approach is evidenced by a tenfold enhancement in photoluminescence emission. Porous nanopyramids demonstrate a higher internal quantum efficiency of 44.7% and faster radiative recombination time. Additionally, finite difference time domain simulation reveals a twofold increase in light extraction efficiency. This research will lead the way toward the development of a new class of nanostructures offering potential advancements in optoelectronic devices.

Promoting Photon-to-Chemical Conversion through a Dielectric Antenna-Hybrid Bilayered Reactor Configuration.

The application of scattered light via an antenna-reactor configuration is promising for converting thermocatalysts into photocatalysts. However, the efficiency of dielectric antennas in photon-to-chemical conversion remains suboptimal. Herein, we present an effective approach to promote light utilization efficiency by designing dielectric antenna-hybrid bilayered reactors. Experimental studies and finite-difference time-domain simulations demonstrate that the engineered SiO2-carbon/metal dielectric antenna-hybrid bilayered reactors exhibit a synergy of absorption superposition and electric field confinement between carbon and metals, leading to the improved absorption of scattered light, upgraded charge carriers density, and ultimately promoted photoactivity in hydrogenating chlorobenzene with an average benzene formation rate of 18 258 μmol g-1 h-1, outperforming the reported results. Notably, the carbon interlayer proves to be more effective than the commonly explored dielectric TiO2 interlayer in boosting the benzene formation rate by over 3 times. This research paves the way for promoting near-field scattered photon-to-chemical conversion through a dielectric antenna-hybrid reactor configuration.

High-sensitive and compact plasmonic temperature sensor based on square-shaped ring resonators

In this study, a plasmonic structure based on metal-insulator-metal waveguides and square ring resonators is designed and simulated for temperature sensing. The sensing operation is carried out using an enhancement in the interference of sub-wavelength waves. The effect of various factors, such as the number and geometric dimensions of the resonators, on the sensor's reflection spectrum is thoroughly investigated. The results obtained from the finite-difference time-domain simulations indicate that the sensitivity, figure of merit, and temperature sensitivity values for a square ring resonator are 1281 nm/RIU, 73 RIU−1, and 0.505 nm/°C, respectively. Additionally, by using three square ring resonators, the figure of merit increases to 154 RIU−1. The size of the sensors is smaller than 1 µm2. Due to their highly compact size and desirable sensing characteristics, the proposed sensors have great potential for application in various industrial and medical applications.

Advancing COVID-19 Diagnosis: Enhancement in SERS-PCR with 30-nm Au Nanoparticle-Internalized Nanodimpled Substrates.

Real-time polymerase chain reaction (RT-PCR) with fluorescence detection is the gold standard for diagnosing coronavirus disease 2019 (COVID-19) However, the fluorescence detection in RT-PCR requires multiple amplification steps when the initial deoxyribonucleic acid (DNA) concentration is low. Therefore, this study has developed a highly sensitive surface-enhanced Raman scattering-based PCR (SERS-PCR) assay platform using the gold nanoparticle (AuNP)-internalized gold nanodimpled substrate (AuNDS) plasmonic platform. By comparing different sizes of AuNPs, it is observed that using 30nm AuNPs improves the detection limit by approximately ten times compared to 70nm AuNPs. Finite-difference time-domain (FDTD) simulations show that multiple hotspots are formed between AuNPs and the cavity surface and between AuNPs when 30nm AuNPs are internalized in the cavity, generating a strong electric field. With this 30nm AuNPs-AuNDS SERS platform, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) ribonucleic acid (RNA)-dependent RNA polymerase (RdRp) can be detected in only six amplification cycles, significantly improving over the 25 cycles required for RT-PCR. These findings pave the way for an amplification-free molecular diagnostic system based on SERS.

High-efficiency ultra-thin GaAs solar cells based on ITO/Ag/ITO transparent electrodes and photonic crystals

Ultrathin photovoltaic technology has great potential to improve efficiency and reduce costs. However, achieving optical thickness requires an effective light capture strategy. In this study, for GaAs thin film solar cells with an active layer thickness of 500 nm, we adopt a combination strategy of front-end ITO/Ag/ITO transparent electrodes and back-end photonic crystals (PC). By employing the finite difference time domain (FDTD) simulation, we verify that ITO/Ag/ITO transparent electrodes and AlGaAs PC can effectively enhance the light-harvesting capacity of cells. The proposed cell structure has a spectral absorption rate (SAR) of 0.9781 in the visible wavelength range. To further optimize the photovoltaic performance parameters, we optimize the doping concentration of the pn junction in the cell. Finally, the short circuit current density Jsc, open circuit voltage Voc, fill factor (FF) and photoelectric conversion efficiency (PCE) of GaAs thin film solar cells are successfully increased to 31.10mAcm−2, 1.28V, 88.18% and 35.12%, respectively. This provides crucial experimental validation and theoretical guidance for advancing ultra-thin photovoltaic technology.

Novel application of ground penetrating radar for damage detection in thick FRP composites

This paper presents the first successful application of ground penetrating radar (GPR) to the inspection of thick (≥100 mm) fiber-reinforced composites. These thick composites are found in wind/tidal turbine blades and composite-hulled ships, where sufficient non-destructive testing (NDT) remains challenging. Polyester-glass specimens, ranging in thickness from 100 to 120 mm, were created with delamination-mimicking damage. Specimen thickness, damage depth location, antenna orientation and damage dryness were the test variables. Finite-difference time-domain simulations indicated the method’s feasibility, and experimental results confirmed these findings. GPR effectively detected and precisely located dry, in-plane damage, with increased detectability for water-filled damage due to the enhanced contrast of electrical properties that creates the damage response in the signal. This capability is particularly advantageous for marine composites, where extensive damage may lead to water ingress. In a comparison with an ultrasonic inspection, GPR proved superior for the thicker composites (≥100 mm). As the first successful application of GPR to composite structures, these findings significantly advance the field of NDT of these materials.

Nano-pixel polarization rotator for a photonic integrated breath sensor

In response to the global aging population, a photonic-integrated-circuit sensor is investigated for the detection of disease markers within human breath content. The device relies on cavity-ring-down spectroscopy with an amplifying medium and loop feedback to secure sufficient sensitivity down to ppm-order concentration detection. This configuration, however, might cause unwanted oscillation, and the polarization rotation method has been proposed to prevent this issue. We have researched a waveguide-based polarization rotator using nano-pixels. The device consists of two regions: (1) From TE00 mode TE10 modes conversion and (2) TE10 to TM00 modes conversion. As the intermediary TE10 mode quality is key to realizing polarization rotation performance, the purpose of this study is to realize high-quality TE10 by employing the mean-squared-error criterion for waveguide design optimization. A finite-difference time-domain simulation with this method reveals a TE10 mode with 1% accuracy that results in a polarization extinction ratio improved from 4.3 to 8.6 dB.

Impact of nanopores in porous GaN on LED emission based on FDTD simulations

We simulated the light extraction efficiency (LEE) of porous GaN-based InGaN/GaN micrometer-sized light-emitting diodes (μLEDs) emitting within the visible wavelength range using the finite-difference time-domain (FDTD) method. The simulations show that the embedding of a porous GaN layer with 40% porosity reduces the bottom LEE, while the top side LEE of the μLEDs is increased. In addition, it also exhibits complex scattering properties that affect the microcavity structure of these devices. The LEE and the degree of microcavity structure disruption are related to nanopore size and location. This association weakens with increasing wavelength. Also, a decrease in nanopore size corresponds to a diminished impact on μLED optical properties. Since the porous GaN layer contributes to the deposition of high-quality InGaN, controlling pore size of the porous GaN layer will aid the development of GaN-based red μLEDs and full-color displays.

Impact of nanopores in porous GaN on LED emission based on FDTD simulations

We simulated the light extraction efficiency (LEE) of porous GaN-based InGaN/GaN micrometer-sized light-emitting diodes (μLEDs) emitting within the visible wavelength range using the finite-difference time-domain (FDTD) method. The simulations show that the embedding of a porous GaN layer with 40% porosity reduces the bottom LEE, while the top side LEE of the μLEDs is increased. In addition, it also exhibits complex scattering properties that affect the microcavity structure of these devices. The LEE and the degree of microcavity structure disruption are related to nanopore size and location. This association weakens with increasing wavelength. Also, a decrease in nanopore size corresponds to a diminished impact on μLED optical properties. Since the porous GaN layer contributes to the deposition of high-quality InGaN, controlling pore size of the porous GaN layer will aid the development of GaN-based red μLEDs and full-color displays.

Low loss ridge waveguides on the hybrid single crystalline silicon and lithium niobate thin films

The hybridization of silicon (Si) thin film and lithium niobate (LN) thin film with remarkable properties, including the excellent optical properties of LN, excellent electrical properties and mature processing advantages of Si, can provide an important material platform for integrated photonic applications. In this work, a 4-inch single crystalline Si thin film wafer was prepared using wafer bonding, grinding and polishing techniques on LN thin film substrate. The films were characterized by using high-resolution transmission electron microscopy (HRTEM). The interplanar spacings and strain of Si film were calculated by analysis of high-resolution X-ray diffraction (HRXRD). Si ridge straight waveguides were fabricated on the hybrid thin films, and the transmission loss was 1.2dB/cm. The near-field mode distribution of the waveguide was investigated by combining FDTD (Finite-Difference Time-Domain) simulation and end coupling measurement.

Underwater acoustic propagation and scattering from a dynamic rough sea surface using the Finite-Difference Time-Domain method.

Models for underwater acoustic propagation typically assume that the sea surface is smooth or rough but frozen in time. Long-duration transmissions on the order of tens of seconds are being considered for next-generation SONAR. These types of signals improve target resolution and tracking. However, they can interact with the sea surface at many different wave displacements during transmission. This violates the "frozen" boundary assumption and causes additional transmission losses and Doppler effects on the received signal. Full-wave propagation models can be used to better understand the mechanisms behind these phenomena. This understanding leads to better system design without having to perform expensive at-sea experiments. In this paper, a finite-difference time-domain (FDTD) method is implemented to model the impact of roughness and motion on the sea surface. The FDTD method is a full-wave numeric technique that allows an arbitrary function to define the boundary points. Surface motion is accomplished by modifying these boundary points at each time step. A variable subgrid sea surface boundary technique is developed to improve the accuracy of these FDTD simulations. The rough, time-evolving sea surface is modeled using a Pierson-Moskowitz frequency spectrum, which is simple to implement and fully defined by wind speed and direction.

Multi-functional metasurface as a transmissive/reflective FSS and an on-air frequency mixer

In this paper, a multi-functional metasurface is proposed, which can work as a narrowband transmissive/reflective frequency selective surface (FSS) and an on-air frequency mixer based on its switching response. The metasurface is made up of unit cells with square and circular metallic loops connected by PIN diodes controlled by a bias source. In contrast to typical wideband FSSs, the structure provides 0.55 GHz of narrow stopband (a fractional bandwidth of 22%) at 2 GHz in the OFF state bias. The bandstop response can be adjusted by varying the reverse bias voltage. The metasurface alternates between its functionalities when in forward bias by providing a passband at the operational frequency. The structure is compact and operates as a transmissive/reflective surface under two different bias conditions (ON and OFF). The design is angularly stable and polarization-insensitive for both TE and TM polarisation. A prototype of the designed structure is developed and the measured results correlate well with the simulated responses from the finite-difference time-domain (FDTD) method-based simulation of the circuit model. On-air frequency mixing for a wave propagating through the metasurface is demonstrated and the effects of different parameters affecting the mixing are parametrically studied through FDTD simulations and experiments.

Multifunctional terahertz device based on plasmon-induced transparency

Enhancing light-matter interaction is crucial in optics for boosting nanophotonic device performance, which can be achieved via plasmon-induced transparency (PIT). In this study, a polarization-insensitive PIT effect at terahertz frequencies is achieved using a novel metasurface composed of a cross-shaped graphene structure surrounded by four graphene strips. The high symmetry of this metasurface ensures its insensitivity to changes in the polarization angle of incident light. The PIT effect, stemming from the coupling of graphene bright modes, was explored through finite difference time domain (FDTD) simulations and coupled mode theory (CMT) analysis. By tuning the Fermi level in graphene, we effectively modulated the PIT transparent window, achieving high-performance optical switching with a modulation depth (88.9% < MD < 98.0%) and insertion losses (0.17 dB < IL < 0.51 dB) at a carrier mobility of 2 m2/(V·s). Furthermore, the impact of graphene carrier mobility on the slow-light effect was examined, revealing that increasing the carrier mobility from 0.5 m2/(V·s) to 3 m2/(V·s) boosts the group index from 126 to 781. These findings highlight the potential for developing versatile terahertz devices, such as optical switches and slow-light apparatus.

Flat optics of nonuniform phase gradient metasurfaces

Flat optics of uniform phase gradient metasurfaces based on the generalized Snell’s law has been extensively studied. The optics of nonuniform phase gradient metasurfaces (NPGMs) is less clear. Here based on Huygens’ principle and finite-difference time-domain (FDTD) simulation, we explore the optical properties of NPGMs made of nanorods (meta-atoms), which can be tuned by modulation of propagation phase and geometric phase. It is found that the nonuniformity of phase gradient may lead to multichannel anomalous reflection/refraction. The multiple beam splitters with arbitrary intensity ratio can be achieved by using the amplitude/phase modulation. Based on our design principle, we can obtain different anomalous reflection patterns (with channels ±1, ±2, both ±1 and ±2, or no reflection at anomalous angle) by different arrangement of just two types of meta-atoms. In addition, we are able to achieve chiral anomalous reflections for designed NPGMs made of nanorods and L-shaped nanoparticles. Our formulism provides general design guidance for NPGMs and can be used to realize the beam splitting function with adjustable angle and intensity ratio.

TiO2-BN/CNTs coating with radiative cooling and reduced friction

To date, the demand for electronic packaging materials with combined highly efficient heat dissipation and low friction attracts intensive attention. There have been hardly any reports of coatings that provide simultaneous radiative cooling, thermal conduction, and friction reduction properties. In the present work, a novel biomimetic nanocomposite coating is gown on the titanium alloy substrate. The coating consists of a TiO2 ceramic layer adorned with hemispherical protrusions and is enriched with hexagonal boron nitride (h-BN) flakes and CNTs on the surface. The optimal microstructures are determined through Finite Difference Time Domain (FDTD) simulation calculations and then prepared using plasma electrolytic oxidation (PEO). The results show that the hierarchical micro/nanostructures enable the coating to achieve an infrared emissivity of ∼ 0.9 within the 3–14 wavelength range, whilst h-BN and CNTs contribute to the thermal conduction of the TiO2-BN/CNTs ceramic coating. The advantage of the biomimetic nanocomposite coating over the bare titanium alloy is that the coating facilitates a reduction in the equilibrium temperature of a 5 W LED by around 21.5 °C. In addition, the coating exhibits outstanding friction reduction performance with a low friction coefficient (COF) of 0.15 over a sliding distance of 94.2 m, benefitted from the lubrication effect of h-BN and CNT. This work introduces a straightforward and environmentally friendly approach to fabricating bifunctional thermal regulation materials.

Modeling full PCSELs and VCSELs using modified rigorous coupled-wave analysis

An integrated rigorous coupled-wave analysis (RCWA) algorithm is presented in this paper, which can simulate full vertical-cavity surface-emitting laser (VCSEL) and photonic crystal surface-emitting laser (PCSEL) structures. A classic RCWA can only analyze a structure when the light source is incident from the top, bottom, or both sides of the device. However, for VCSEL applications, the light source is generated in the middle and propagates in both directions. A bidirectional scattering matrix method and doubling algorithm are implemented in RCWA. The resonant wavelength and Q factor of a VCSEL can then be found in the output spectrum. The accuracy and execution speed are compared with those of the Lumerical finite-difference time-domain (FDTD) method for several VCSEL and PCSEL designs. The results show that the maximum discrepancy between RCWA and FDTD is less than 3 nm, and the difference in the far-field divergence angle is less than 0.5°. The speed of RCWA also outperforms FDTD simulation significantly.

Super-Resolution Imaging of Sub-diffraction-Limited Pattern with Superlens Based on Deep Learning

AbstractThe development of super-resolution imaging techniques has revolutionized our ability to study the nano-scale world, where objects are often smaller than the diffraction limit of traditional optical microscopes. Super-resolution superlenses have been proposed to solve this problem by manipulating the light wave in the near field. A superlens is a kind of metamaterial-based lens that can enhance the evanescent waves generated by nano-scale objects, utilizing the surface plasmon phenomenon. The superlens allows for the imaging of nano-scale objects that would otherwise be impossible to resolve using traditional lenses. Previous research has shown that nanostructures can be imaged using superlenses, but the exact shape of the superlens must be known in advance, and an analytical calculation is needed to reconstruct the image. Localized plasmon structured illumination microscopy is an approach to achieve super-resolution by imaging the superlens-enhanced evanescent wave with illumination shifts. This study proposes a new approach utilizing a conditional generative adversarial network to obtain super-resolution images of arbitrary nano-scale patterns. To test the efficacy of this approach, finite-difference time-domain simulation was utilized to obtain superlens imaging results. The data from the simulation were then used for deep learning to develop the model. With the help of deep learning, the inverse calculation of complex sub-diffraction-limited patterns can be achieved. The super-resolution feature of the superlens based on deep learning is investigated. The findings of this study have significant implications for the field of nano-scale imaging, where the ability to resolve arbitrary nano-scale patterns will be crucial for advances in nanotechnology and materials science.

Breaking the mirror symmetry of photonic spin-orbit interaction using a geometrically symmetric chiral resonator

The symmetries of photonic spin-orbit interaction (PSOI) at waveguide interfaces provide flexible modulation capability but limit their practical implementation due to the stringent requirements of excitation conditions. This limitation can be mitigated by intentionally breaking local symmetries, offering a novel platform for achieving directional coupling and optical isolation with PSOI-based interfaces. For example, breaking the inversion symmetry of a nanofiber PSOI interface using a nanosphere scatterer reduces the required accuracy in the size and position of excitation spots. This study introduces a novel approach to break the mirror symmetry of a PSOI-based nanofiber waveguide by coupling it with a geometrically symmetric chiral gold nanohelicoid (GNH) resonator, which relaxes the original requirement of circularly polarized excitations. Finite-difference time-domain simulations demonstrate unidirectional light coupling and propagation under both circularly and linearly polarized excitations, showcasing the versatility of this hybrid symmetry-broken system. The Fano-like features observed in directionality spectra align with the GNH’s circular dichroism spectrum, emphasizing an intricate correlation between plasmonic near-field chirality and far-field scattering dichroism. This work paves the way for enhancing the functionalities of PSOI-based waveguide interfaces through locally coupling them with nanoscale chiral resonators, thereby expanding their application in quantum photonics, information transport and plasmonic nanophotonics.

High-efficiency self-focusing metamaterial grating coupler in silicon nitride with amorphous silicon overlay

Efficient fiber-chip coupling interfaces are critically important for integrated photonics. Since surface gratings diffract optical signals vertically out of the chip, these couplers can be placed anywhere in the circuit allowing for wafer-scale testing. While state-of-the-art grating couplers have been developed for silicon-on-insulator (SOI) waveguides, the moderate index contrast of silicon nitride (SiN) presents an outstanding challenge for implementing efficient surface grating couplers on this platform. Due to the reduced grating strength, a longer structure is required to radiate the light from the chip which produces a diffracted field that is too wide to couple into the fiber. In this work, we present a novel grating coupler architecture for silicon nitride photonic integrated circuits that utilizes an amorphous silicon (α-Si) overlay. The high refractive index of the α-Si overlay breaks the coupler’s vertical symmetry which increases the directionality. We implement subwavelength metamaterial apodization to optimize the overlap of the diffracted field with the optical fiber Gaussian mode profile. Furthermore, the phase of the diffracted beam is engineered to focalize the field into an SMF-28 optical fiber placed 55 µm above the surface of the chip. The coupler was designed using rigorous three-dimensional (3D) finite-difference time-domain (FDTD) simulations supported by genetic algorithm optimization. Our grating coupler has a footprint of 26.8 × 32.7 µm2 and operates in the O-band centered at 1.31 μm. It achieves a high directionality of 85% and a field overlap of 90% with a target fiber mode size of 9.2 µm at the focal plane. Our simulations predict a peak coupling efficiency of − 1.3 dB with a 1-dB bandwidth of 31 nm. The α-Si/SiN grating architecture presented in this work enables the development of compact and efficient optical interfaces for SiN integrated photonics circuits with applications including optical communications, sensing, and quantum photonics.