Finite-difference Time-domain Simulations Research Articles

Photodetector with a metalens packaging module for visible light communication based on RGBY illumination LED light source

A novel, to the best of our knowledge, photodetector with a metalens packaging module used as the visible light communication (VLC) receiver is proposed and designed. An LED consisting of red, green, blue, and yellow chips (RGBY-LED) is adopted as the transmitter for intensity modulation direct detection VLC systems. A metalens array with a numerical aperture (NA) of 0.707 used as a polarization-insensitive planar lens of the VLC system receiver is designed at wavelengths of 457, 523, 592, and 623 nm corresponding to blue, green, yellow, and red for high efficiency. Compared with a traditional Fresnel lens positive-intrinsic-negative (PIN) photodetector module as the VLC receiver, the introduction of a metalens module can decrease the form factor of the VLC receiver module and, in particular, it is much thinner. The combination of the multi-color LED transmitter and photodetector metalens packaging module receiver can increase the modulation bandwidth due to four different wavelengths used for the VLC system. Finite-difference time domain (FDTD) simulations are performed to validate the performance of the photodetector with a metalens module. It is revealed that the corresponding efficiencies of 57.5%, 55.4%, 57%, and 56.3% were achieved at wavelengths of 623, 592, 523, and 457 nm, respectively, based on a metalens array with a 0.707 NA and 2.5 µm radius of the active area of the photodetector. It is a promising technology for indoor VLC systems such as those for smart phones and other Internet of Things devices due to the need for compact packaging for the receiver.

Dual-functional Au-porous anodic alumina (PAA) sensors for enrichment and label-free detection of airborne virus with surface-enhanced Raman scattering

The development of a rapid and sensitive method for the enrichment and direct detection of airborne viruses has become urgently needed to prevent their spread. We have developed a dual-functional, label-free platform for the enrichment and optical identification of airborne viruses. This platform allows for the on-site enrichment and identification of airborne viruses through a virus enrichment component combined with surface-enhanced Raman scattering (SERS). In this work, we created an Au-porous anodic alumina (PAA) composite film, by ion sputtering Au nanoparticles (Au NPs) on a porous structure. The Au-PAA serves as a dual-functional sensor: it combines ultrafiltration through the vertical nanopores in the PAA for virus enrichment and constitutes a plasmonic substrate due to the surface plasmon near-field enhancements within the nanochannel structure. This sensor demonstrates an extraordinary enrichment efficiency (about 98 %) of obtained bioaerosols and enables simultaneous, sensitive detection at the single-virus level. Finite-difference time-domain (FDTD) simulations evidenced the electric field intensity and charge distribution of the Au-PAA. This Au-PAA composite film offers a powerful system for on-site, label-free viral particle enrichment and SERS detection. Contaminated air containing viruses can be collected in a few minutes and analyzed immediately by our platform, which is particularly beneficial for timely monitoring of viruses in the air of large public spaces. Additionally, this platform can be applied to evaluate the indoor air quality.

A four-band coherent perfect absorber of borophene metamaterial operating in the communication band

We propose a coherent perfect absorber based on a borophene metamaterial grating structure. Based on the theoretical analysis of the temporal coupled mode theory and the numerical results of the finite-difference time-domain simulations, the transmission characteristics of the system are investigated. The dynamic tunability of the absorption intensity and resonance wavelength is realized by adjusting the phase difference of the coherent incident beams and carrier density of borophene. Moreover, by optimizing the parameters, the coherent perfect absorption peak (99.99%) is obtained near the wavelength of 1550 nm, that is, the third communication window. Meanwhile, the structure can be operated in ultra-thin gratings as low as 6 nm and angle tolerances as large as 45°, which provides the possibility for applications in highly integrated and wide-angle tolerated devices.

High-Efficiency Coupling of Free Electrons to Sub-λ3 Modal Volume, High-Q Photonic Cavities.

We report on the design, realization, and experimental investigation by spatially resolved monochromated electron energy loss spectroscopy (EELS) of high-quality-factor cavities with modal volumes smaller than λ3, with λ being the free-space wavelength of light. The cavities are based on a slot defect in a 2D photonic crystal slab made up of silicon. They are optimized for high coupling of electrons accelerated to 100 kV to quasi-transverse electrical modes polarized along the slot direction. We studied the cavities in two geometries and took advantage of the deep sub-optical wavelength spatial resolution of the electron microscope and high spectral resolution of the monochromator to comprehensively describe the optical excitations of the slab. The first geometry, for which the cavities have been designed, corresponds to an electron beam traveling along the slot direction. The second consists of the electron beam traveling perpendicular to the slab. In both cases, a large series of modes is identified. The dielectric slot mode energies are measured to be in the 0.8-0.85 eV range, as per design, and surrounded by two bands of dielectric and air modes of the photonic structure. The dielectric even slot modes, to which the cavity mode belongs, are highly coupled to the electrons with up to 3.2% probability of creating a slot photon per incident electron. Although the experimental spectral resolution (around 30 meV) alone does not allow to disentangle cavity photons from other slot photons, the excellent agreement between the experiments and finite-difference time-domain simulations allows us to deduce that among the photons created in the slot, around 30% are stored in the cavity mode. A systematic study of the energy and coupling strength as a function of the photonic band gap parameters permits us to foresee an increase of coupling strength by fine-tuning phase-matching. Our work demonstrates free electron coupling to high-quality-factor cavities with low mode densities and sub-λ3 modal volumes, making it an excellent candidate for applications such as quantum nano-optics with free electrons.

Ultrafast wavelength sweeping of surface plasmon lasing in planar metal grating with semiconductor gain

Surface plasmon (SP) lasing, characterized by strong light–matter interaction at a metal interface, can derive from SP cavities coupled with a semiconductor gain material. With this setup, we demonstrate ultrafast wavelength sweeping of SP lasing in a metal grating. When the device was optically pumped by ultrashort pulses with sufficient power, a wavelength sweep over 10 nm within 50 ps was obtained in the telecom band of 1550 nm. Based on the pump-power-dependent lasing wavelength change in combination with finite-difference time-domain simulation, we attribute the wavelength sweep to a modulation of the SP cavity that induces changes in the refractive index from changes in the carrier density. Such a refractive index change is a critical factor in determining SP cavity resonance.

On the utilization of the adjoint method in microwave tomography.

In microwave imaging, the adjoint method is widely used for the efficient calculation of the update direction, which is then used to update the unknown model parameter. However, the utilization and the formulation of the adjoint method differ significantly depending on the imaging scenario and the applied optimization algorithm. Because of the problem-specific nature of the adjoint formulations, the dissimilarities between the adjoint calculations may be overlooked. Here, we have classified the adjoint method formulations into two groups: the direct and indirect methods. The direct method involves calculating the derivative of the cost function, whereas, in the indirect method, the derivative of the predicted data is calculated. In this review, the direct and indirect adjoint methods are presented, compared, and discussed. The formulations are explicitly derived using the two-dimensional wave equation in frequency and time domains. Finite-difference time-domain simulations are conducted to show the different uses of the adjoint methods for both single source-multiple receiver, and multiple transceiver scenarios. This study demonstrated that an appropriate adjoint method selection is significant to achieve improved computational efficiency for the applied optimization algorithm.

High-Efficiency Metamaterial-Engineered Grating Couplers for Silicon Nitride Photonics.

Silicon nitride (Si3N4) is an ideal candidate for the development of low-loss photonic integrated circuits. However, efficient light coupling between standard optical fibers and Si3N4 chips remains a significant challenge. For vertical grating couplers, the lower index contrast yields a weak grating strength, which translates to long diffractive structures, limiting the coupling performance. In response to the rise of hybrid photonic platforms, the adoption of multi-layer grating arrangements has emerged as a promising strategy to enhance the performance of Si3N4 couplers. In this work, we present the design of high-efficiency surface grating couplers for the Si3N4 platform with an amorphous silicon (α-Si) overlay. The surface grating, fully formed in an α-Si waveguide layer, utilizes subwavelength grating (SWG)-engineered metamaterials, enabling simple realization through single-step patterning. This not only provides an extra degree of freedom for controlling the fiber-chip coupling but also facilitates portability to existing foundry fabrication processes. Using rigorous three-dimensional (3D) finite-difference time-domain (FDTD) simulations, a metamaterial-engineered grating coupler is designed with a coupling efficiency of -1.7 dB at an operating wavelength of 1.31 µm, with a 1 dB bandwidth of 31 nm. Our proposed design presents a novel approach to developing high-efficiency fiber-chip interfaces for the silicon nitride integration platform for a wide range of applications, including datacom and quantum photonics.

A Structured Mesh Generation Based on Improved Ray-Tracing Method for Finite Difference Time Domain Simulation

A structured mesh generation method based on an improved ray-tracing technique is proposed for finite time difference domain (FDTD) simulation in this paper. This method converts the triangular faces provided by the STL file into the structured mesh required by the FDTD method. The size of the generated structured mesh is determined by the size of the triangular element, so it is non-uniform and can adapt to the shape of the target. In addition, this method proposes an improved ray-tracing technique to avoid the singularities of the object, so it has much better accuracy than the conventional method. Two representative electromagnetic objects, an Su-30 aircraft and a complex bow-tie antenna, representing a typical electrically large model and fine structure, respectively, are used to validate the accuracy of the mesh generation method. The results show that the presented method has high accuracy, regardless of whether it is used for large targets or fine structures such as a small hole, strip, thin layer, etc.

Design and Optimization of Silver Nanostructured Arrays in Plasmonic Metamaterials for Sensitive Imaging Applications

This paper delves into the strategic design and optimization of silver (Ag) nanostructured arrays within plasmonic metamaterials, targeting the enhancement of imaging sensitivity. Leveraging Finite-Difference Time-Domain (FDTD) simulations, our research rigorously compares various Ag nanostructured geometries, including nanospheres, nanocones, nanodisks, and nanocubes. The aim is to pinpoint configurations that significantly enhance electric field localization on the surfaces of nanostructures, a pivotal factor. The nanocube array exhibits superior field enhancement, particularly in narrow nanogaps, suggesting its suitability for high-sensitivity applications. Further exploration into nanocube arrays reveals the crucial role of nanogap size and spacer layer thickness in tuning the optical properties through the manipulation of Fabry–Pérot and mirror image modes in metal–insulator–metal (MIM) structures. By presenting a thorough analysis of these nanostructured arrays, the study not only contributes to our understanding of the fundamental principles governing plasmonic metamaterials but also provides a solid foundation for future innovation in highly sensitive imaging applications. It underscores the importance of nanostructure design and optimization in achieving significant improvements in the performance of plasmonic devices, marking a pivotal step forward in the field of nanophotonics and its application to sensitive imaging technologies.

Phase reconstruction and singularity recovery of submicron particles in far-field phase space data using deep learning networks

Phase images often contain more comprehensive information compared to amplitude images. The precise visualization of phase images generated from light scattering by submicron particles is paramount for investigating the underlying scattering mechanisms and exploring applications centered around these submicron particles. This study showcases the efficacy of neural networks in acquiring the capacity to conduct phase reconstruction and recover singularities through appropriate training. Our deep learning-based approach introduces an entirely novel framework for phase recovery, accomplished by learning the distribution of scattered light fields from submicron particles, as obtained through a polarized indirect microscopic imaging (PIMI) system, within a custom Poincaré sphere (phase space). We validate this method by successfully reconstructing phase images of polystyrene (PS) balls with varying radii, and comparing diverse network structures and data flows in the process. Furthermore, we substantiate our experimental results by corroborating them with finite difference time domain (FDTD) simulations. These findings underscore the remarkable regularity within the distribution of phase space data, opening up new avenues for advancing the study of nanostructures.

Interplay between gain and loss in arrays of nonlinear plasmonic nanoparticles: toward parametric downconversion and amplification.

With the help of a theoretical model and finite-difference time-domain (FDTD) simulations based on the hydrodynamic-Maxwell model, we examine the effect of difference-frequency generation (DFG) in an array of L-shaped metal nanoparticles (MNPs) characterized by intrinsic plasmonic nonlinearity. The outcomes of the calculations reveal the spectral interplay between gain and loss in the vicinity of the fundamental frequency of the localized surface plasmon resonances. Subsequently, we identify different array thicknesses and pumping regimes facilitating parametric amplification and spontaneous parametric downconversion. Our results suggest that the parametric amplification regime becomes feasible on a scale of hundreds of nanometers and spontaneous parametric downconversion on the scale of tens of nanometers, opening up new exciting opportunities for developing building blocks of photonic metasurfaces.

Alignment-free coupling to arrays of diamond microdisk cavities with fabrication tolerant spin-photon interfaces.

We propose a design for an efficient spin-photon interface to a color center in a diamond microdisk. The design consists of a silicon oxynitride triangular lattice overlaid on a diamond microdisk without any aligmnent between the layers. This enables vertical emission from the microdisk into low-numerical aperture modes, with quantum efficiencies as high as 46% for a tin vacancy (SnV) center. Our design is robust to manufacturing errors, potentially enabling large scale fabrication of quantum emitters coupled to optical collection modes. We also introduce a novel approach for optimizing the free space performance of our device using a dipole model, achieving comparable results to full-wave finite difference time domain simulations with 7 · 106 reduction in computational time.

Silicon eccentric shell nanoparticles fabricated by template-assisted deposition for Mie magnetic resonances enhanced light confinement

We report a structure of silicon eccentric shell particles array, fabricated by the SiO2 particles monolayer array assisted deposition of amorphous Si, for high-efficiency light confinement. The SiO2 particles monolayer array is tailored to regulate its interparticle distance, followed by silicon film deposition to obtain silicon eccentric shell arrays with positive and negative off-center distance e. We studied the Mie resonances of silicon solid sphere, concentric shell, eccentric shell and observed that the eccentric shell with positive off-center e supports superior light confinement because of the enhanced Mie magnetic resonances. Spectroscopic measurements and finite difference time domain simulations were conducted to examine the optical performance of the eccentric shell particles array. Results show that the Mie magnetic resonance wavelength can be easily regulated by the size of the inner void of the silicon shell to realize tunable enhanced light confinement. It was found silicon shell with D = 460/520 nm offered high enhanced light absorption efficiency at wavelength of λ = 830 nm, almost beyond the bandgap of the amorphous silicon.

Ionic Liquid-Assisted Thermal Evaporation of Bimetallic Ag-Au Nanoparticle Films as a Highly Reproducible SERS Substrate for Sensitive Nanoplastic Detection in Complex Environments.

Nanoplastic particles are emerging as an important class of environmental pollutants in the atmosphere that have adverse effects on our ecosystems and human health. While many methods have been developed to quantitatively detect nanoplastics; however, sensitive detection at low concentrations in a complex environment remains elusive. Herein, we demonstrate a greener method to fabricate a surface-enhanced Raman spectroscopy (SERS) substrate consisting of self-assembled plasmonic Ag-Au bimetallic nanoparticle (NP) films for quantitative SERS detection of nanoplastics in complex media. The self-assembly of Ag-Au bimetallic NPs was achieved through thermal evaporation onto a vapor-phase compatible ionic liquid based on deep eutectic solvent over the growth substrate. The finite-difference time-domain simulation revealed that the localized field enhancement is strong in the gaps, which generate uniform SERS "hotspots" in the obtained substrate. Benefiting from highly accessible SERS "hotspots" at the gaps, the SERS substrate exhibits excellent sensitivity for detecting crystal violet with a limit of detection (LOD) as low as 10-14 M and excellent reproducibility (RSD of 5.8%). The SERS substrate is capable of detecting PET nanoplastics with LOD as low as 1 μg/mL and about 100 μg/mL in real samples such as tap water, lake water, diluted milk, and wine. Moreover, we also validated the feasibility of the designed SERS substrate for the practical detection of PET nanoplastics collected from commercial drinking water bottles, and it showed great potential applications for sensitive detection in actual environments.

Characterizing chiroptical properties of 2D/3D structures based on an improved coupled dipole theory.

To reveal the difference/connection between two-dimensional and three- dimensional (2D and 3D) chiroptical properties and their relation with 2D/3D symmetry/breaking, we develop an improved coupled dipole theory (ICDT) based on a model system of nanorod (NR) dimer. Our analytical ICDT can overcome the shortcoming of the traditional coupled dipole theory and points out the important role of scattering circular dichroism (SCD) in characterizing 2D chirality. The ICDT, supported by finite-difference time-domain (FDTD) simulation, reveals the physical origin of 2D chiroptical response: the interaction induced asymmetric effective polarizability for two identical NRs in a symmetry broken configuration. By tuning the NR's position/inter-particle distance, we find an optimal structure of maximum SCD due to the competition between geometric symmetry breaking and interaction. In addition, the interplay between 2D in-plane mirror symmetry breaking and three-dimensional (3D) mirror symmetry breaking leads to a symmetry broken system with zero SCD. The relation between chirality and reciprocity has also been addressed.

Ultra-fast and compact optical Galois field adder based on the LPhC structure and phase shift keying.

In this study, we propose a novel, to the best of our knowledge, all-optical Galois field (AOGF) adder that utilizes logic all-optical XOR gates. The design is founded on optical beams' constructive and destructive interference phenomenon and incorporates the phase shift keying technique within a two-dimensional linear photonic crystal (2D-LPhC) structure. The suggested AOGF adder comprises eight input ports and four output ports. We employ the finite difference time domain (FDTD) procedure to obtain the electric field distribution in this structure. The FDTD simulation results of the proposed AOGF adder demonstrate that the minimum and maximum values of the normalized power at ON and OFF states (P 1,min, P 0,max) for the output ports are 95% and 1.7%, respectively. Additionally, we obtain different functional parameters, including the ON-OFF contrast ratio, rise time, fall time, and total footprint, which are measured at 17.47dB, 0.1ps, 0.05ps, and 147µm 2, respectively.

Using different methods to measure ultrasonic attenuation in cortical bone: A comparison

The assessment of bone mineral density (BMD) and bone microstructure is important in screening for various bone diseases. Dual x-ray absorptiometry is the most commonly used technique for evaluating BMD; however, alternatives are necessary due to the use of ionizing radiation and low resolution, the latter of which prevents obtaining information on bone microstructure. Quantitative ultrasound presents a potentially attractive alternative. The microstructure of porous media, such as cortical bone, influences the attenuation of ultrasound; as such, ultrasound attenuation can be used to obtain information about the microstructure of cortical bone. Different models and measurement methods may be used to estimate ultrasonic attenuation in a cortical bone. In this study, three different methods of measuring ultrasonic attenuation were used. Finite-difference time-domain simulations were conducted in maps mimicking cortical bone. Pore densities and pore size distributions in the bone matrix, as well as absorptive properties of the bone and porous matrices, were specified for all simulations. Attenuation was measured within the pulse bandwidth as a function of frequency using the independent scattering approximation (ISA), a cortical backscatter method (CortBS), and the reflections from the two surfaces of the cortical bone map. Excellent agreement was found between these three methods.

Concentration of reflected sound in a room treated with cube corner retroreflectorsa).

A room was treated to be predominantly retroreflective in the high frequency range by introducing arrays of cube corner retroreflectors (CCRs) over most surfaces (excluding the floor). In a small room (volume 55 m3), 156 CCRs in the form of square trihedra with 350 mm edge lengths were used as wall and ceiling treatment. The horizontal plane distribution of reflected energy was measured from omnidirectional sources, and a head and torso simulator was used to measure voice support. Results show a high concentration of reflected energy returned to omnidirectional source positions in high frequency octave bands (2-8 kHz). Finite-difference time-domain (FDTD) simulations of the room yielded similar distributions to the omnidirectional measurements, showing greater sound concentration when more CCRs are introduced. By contrast, FDTD simulation of an equivalent flat-surfaced room yielded no reflected sound concentration at the source, with results close to diffuse field theory in high frequency octave bands. Measured voice support values derived from oral-binaural room impulse responses exceed diffuse theory expectations by 5 dB. Thus, the paper demonstrates that retroreflective array treatment can change room acoustical conditions, concentrating reflected energy onto an arbitrarily located source.

Multifunctional Ag-decorated core-shell heterojunction of red phosphorus/silicon carbide nanowires for highly sensitive SERS-based monitoring of ibrutinib

In this work, an innovative kind of red phosphorus (RP)-encapsulated silicon carbide (SiC) nanowires (NWs) with interface heterojunctions were developed through a conventional Muffle calcination strategy. After being decorated with Ag nanoparticles (NPs) with the help of an ion sputtering technique, a hybrid surface-enhanced Raman scattering (SERS) substrate based on SiC@RP@Ag NWs was prepared and applied in the clinical monitoring of ibrutinib. The powerful “hotspots” region was demonstrated by finite-difference time-domain (FDTD) simulation to extensively distribute on the surface Ag layer, which triggered significant electromagnetic enhancement. Moreover, the separation and transfer of charge carrier with high efficiency rendered the substrate a supplementary chemical enhancement ascribed to the appropriate energy level of the heterojunction structure. Henceforth, the lowest limit of detection for ibrutinib was found to reach 7.58 × 10−8 mg/mL. Ultimately, the real-time SERS diagnostic of ibrutinib in serum samples demonstrated relative low deviation, not exceeding 6.6%. The hybrid SERS substrate based on SiC@RP@Ag NWs provides more possibilities for clinical drug diagnostics.

Graphene metamaterials-based plasmon-induced terahertz modulator for high-performance multiband filtering and slow light applications.

We proposed multilayered graphene (Gr)-based surface plasmon resonance-induced high-performance terahertz (THz) modulators with tunable resonance frequencies. Previously reported Gr metamaterials-based THz plasmonic modulators had small group delay, low extinction ratio (ER), and difficult-to-tune resonant frequency without changing structural parameters in the THz range. A comprehensive investigation employing the finite-difference time-domain (FDTD) simulation technique revealed high group delay, broad tunability independent of structural parameters, and large ER for our proposed quadband and pentaband plasmonic modulators. We obtained tunable group delays with a maximum of 1.02 ps and 1.41 ps for our proposed quadband and pentaband plasmonic modulators, respectively, which are substantially greater compared to previously reported Gr-based metamaterial structures. The maximum ER of 22.3 dB was obtained, which was substantially high compared to previous reports. Our proposed modulators were sensitive to the polarization angle of incident light; therefore, the transmittance at resonant frequencies was increased while the polarization angle varied from 0° to 180°. These high-performance plasmonic modulators have emerging potential for the design of optical buffers, slow light devices, multistop band filters, integrated photonic circuits, and various optoelectronic systems.