Finite-difference Time-domain Simulations Research Articles

Unique Enhancement of the Whispering Gallery Mode in Hexagonal Microdisk Resonator Array with Embedded Ge Quantum Dots on Si.

The coupling between the quantum dots (QDs) and silicon-based microdisk resonator facilitates enhancing the light-matter interaction for the novel silicon-based light source. However, the typical circular microdisks embedded with Ge QDs still have several issues, such as wide spectral bandwidth, difficult mode selection, and low waveguide coupling efficiency. Here, by a promising structural modification based on the mature nanosphere lithography (NSL), we fabricate a large area hexagonal microdisk array embedded with Ge QDs in order to enhance the near-infrared light emissions by a desired whispering gallery modes (WGMs). By comparing circular microdisks with comparable sizes, we found the unique photoluminescence enhancement effect of hexagonal microdisks for certain modes. We have confirmed the WGMs which are supported by the microdisks and the well-correlated polarized modes for each resonant peak observed in experiments through the Finite Difference Time Domain (FDTD) simulation. Furthermore, the unique enhancement of the TE5,1 mode in the hexagonal microdisk is comparatively analyzed through the simulation of optical field distribution in the cavity. The larger enhanced region of the optical field contains more effectively coupled QDs, which significantly enhances the PL intensity of Ge QDs. Our findings offer a promising strategy toward a distinctive optical cavity that enables promising mode manipulation and enhancement effects for large-scale, cost-effective photonic devices.

Influence of Core Type and Shell Thickness on Avian-Inspired Structural Colors Produced from Melanin Nanoparticle Assemblies.

Hollow melanosomes found in iridescent bird feathers, including violet-backed starlings and wild turkeys, enable the generation of diverse structural colors. It has been postulated that the high refractive index (RI) contrast between melanin (1.74) and air (1.0) results in brighter and more saturated colors. This has led to several studies that have synthesized hollow synthetic melanin nanoparticles and fabricated colloidal nanostructures to produce synthetic structural colors. However, these studies use hollow nanoparticles with thin shells (<20 nm), even though shell thicknesses as high as 100 nm have been observed in natural melanosomes. Here, we combine experimental and computational approaches to examine the influence of the varying polydopamine (PDA, synthetic melanin) shell thickness (0-100 nm) and core material on structural colors. Experimentally, a concomitant change in overall particle size and RI contrast makes it difficult to interpret the effect of a hollow or solid core on color. Thus, we utilize finite-difference time-domain (FDTD) simulations to uncover the effect of shell thickness and core on structural colors. Our FDTD results highlight that hollow particles with thin shells have substantially higher saturation than same-sized solid and core-shell particles. These results would benefit a wide range of applications including paints, coatings, and cosmetics.

Mixing rule for calculating the effective refractive index beyond the limit of small particles.

Considering light transport in disordered media, the medium is often treated as an effective medium requiring accurate evaluation of an effective refractive index. Because of its simplicity, the Maxwell-Garnett (MG) mixing rule is widely used, although its restriction to particles much smaller than the wavelength is rarely satisfied. Using 3D finite-difference time-domain simulations, we show that the MG theory indeed fails for large particles. Systematic investigation of size effects reveals that the effective refractive index can be instead approximated by a quadratic polynomial whose coefficients are given by an empirical formula. Hence, a simple mixing rule is derived which clearly outperforms established mixing rules for composite media containing large particles, a common condition in natural disordered media.

The Rise of Structurally Anisotropic Plasmonic Janus Gold Nanostars.

Nanostructures intrinsically possessing two different structural or functional features, often called Janus nanoparticles, are emerging as a potential material for sensing, catalysis, and biomedical applications. Herein, we report the synthesis of plasmonic gold Janus nanostars (NSs) possessing a smooth concave pentagonal morphology with sharp tips and edges on one side and, contrastingly, a crumbled morphology on the other. The methodology reported herein for their synthesis - a single-step growth reaction - is different from any other Janus nanoparticle preparation involving either template-assisted growth or a masking technique. Interestingly, the coexistence of lower- and higher-index facets was found in these Janus NSs. The general paradigm for synthesizing gold Janus NSs was investigated by understanding the kinetic control mechanism with the combinatorial effect of all the reagents responsible for the structure. The optical properties of the Janus NSs were realized by corelating their extinction spectra with the simulated data. The size-dependent surface-enhanced Raman scattering (SERS) activity of these Janus NSs was studied with 1,4-BDT as the model analyte. Finite-difference time-domain simulations for differently sized particles revealed the distribution of electromagnetic hot-spots over the particles resulting in enhancement of the SERS signal in a size-dependent manner.

Investigation of perfect narrow-band absorber in silicon nano hole array

In this paper, we proposed a triple layer structure consisting of the bottom silver layer, thin silicon oxide space layer, and ultrathin semiconductor silicon film with nano hole array achieving three absorption peaks with narrow band. The absorption spectrum can be easily controlled by adjusting the structural parameters including the radius and period of the nano hole array, and the maximal absorption can reach 99.0% and the narrowest full width of half maximum can reach about 6.5 nm in theory. We also clarified the physical mechanism of the proposed structure in details by finite-difference time-domain simulation, in which the three narrow band perfect adsorption peaks can be attributed to electric dipole resonance, magnetic dipole resonance and plasmonic resonance respectively. At the same time, we used a low-cost nanosphere lithography method to fabricate the proposed nano hole array in large area. In experiment, the absorption peak of the proposed triple layer structure can reach up to 98.3% and the narrowest full width of half maximum can reach up to about 10.1 nm. The highest quality factor Q can reach up to 98.4. This work can open a new avenue for high-quality factor narrow band perfect absorption using ultrathin semiconductor film and benefit for many fields such as infrared sensors, plasmonic filters, and hyperspectral imaging.

Ultra-Compact and Broadband Nano-Integration Optical Phased Array.

The on-chip nano-integration of large-scale optical phased arrays (OPAs) is a development trend. However, the current scale of integrated OPAs is not large because of the limitations imposed by the lateral dimensions of beam-splitting structures. Here, we propose an ultra-compact and broadband OPA beam-splitting scheme with a nano-inverse design. We employed a staged design to obtain a T-branch with a wavelength bandwidth of 500 nm (1300-1800 nm) and an insertion loss of -0.2 dB. Owing to the high scalability and width-preserving characteristics, the cascaded T-branch configuration can significantly reduce the lateral dimensions of an OPA, offering a potential solution for the on-chip integration of a large-scale OPA. Based on three-dimensional finite-difference time-domain (3D FDTD) simulations, we demonstrated a 1 × 16 OPA beam-splitter structure composed entirely of inverse-designed elements with a lateral dimension of only 27.3 μm. Additionally, based on the constructed grating couplers, we simulated the range of the diffraction angle θ for the OPA, which varied by 0.6°-41.6° within the wavelength range of 1370-1600 nm.

A durian-shaped multilayer core-shell SERS substrate for flow magnetic detection of pesticide residues on foods

A new type of durian-shaped Fe3O4@Au@Ag@Au (DFAAA) multilayer core-shell composite was prepared as an efficient surface-enhanced Raman scattering (SERS) substrate. The optimization process and SERS enhancement mechanism of the substrate were further explained with finite-difference time-domain simulation. The dense and uniform spiny array on the DFAAA surface had abundant “hot spots”, greatly improving sensitivity, uniformity and reproducibility, with a Raman enhancement factor of 3.01 × 107 and storage-life of 30 d. A “flow magnetic detection method” was proposed to realize rapid and flexible detection of pesticide residues on the surface of different foods including fish and apple. The limit of detection of malachite green and thiram on the fish and apple surfaces were 0.13 and 0.18 ng/cm2, respectively. With its high SERS performance and good magnetic, the DFAAA possessed great application prospects as a facile SERS substrate for rapid and non-destructive detection of trace pesticide residues on foods.

Investigation of the seat-dip effect using finite-difference time-domain simulations.

The seat-dip effect (SDE) occurs when low-frequency sounds propagate through the seating area of a performance space. The physical aspects governing the effect still puzzle acousticians mostly due to the large variety of seating configurations. In this study, the SDE is investigated in three parameterized hall models using the finite-difference time-domain method to simulate a large number of seat configurations in order to quantify the contribution of different geometric properties related to the seating area. The results show that the step size defining the inclination angle of the seating area and the opening underneath the seats (or underpass) are significant factors contributing to the SDE, whereas the stage height and the source position are found to be less important. The results also demonstrate that with an underpass greater than the step size, the first frequency dip occurring between 80 and 100 Hz is mitigated regardless of the hall type considered. The phenomenon is also found to be predominant in the early part of the room response. Visualizations of spatial and time-frequency evolution in the halls are also provided for the cases where the seat properties are found to visibly affect the magnitude spectrum.

The impact of variations in subject geometry, respiration and coil repositioning on the specific absorption rate in parallel transmit abdominal imaging at 7 T.

Parallel transmit MRI at 7 T has increasingly been adopted in research projects and provides increased signal-to-noise ratios and novel contrasts. However, the interactions of fields in the body need to be carefully considered to ensure safe scanning. Recent advances in physically flexible body coils have allowed for high-field abdominal imaging, but the effects of increased variability on energy deposition need further exploration. The aim of this study was to assess the impact of subject geometry, respiration phase and coil positioning on the specific absorption rate (SAR). Ten healthy subjects (body mass index[BMI] = 25 ± 5 kg m-2 ) were scanned (at 3 T) during exhale breath-hold and images used to generate body models. Seven of these subjects were also scanned during inhale. Simplifications of the coil and body models were first explored, and then finite-difference time-domain simulations were run with a typical eight-channel parallel transmit coil positioned over the abdomen. Simulations were used to generate 10 g averaged SAR (SAR10g ) maps across 100,000 phase settings, and the worst-case scenario 10 g averaged SAR (wocSAR10g ) was identified using trigonometric maximisation. The average maximum SAR10g across the 10 subjects with 1 W input power per channel was 1.77 W kg-1 . Hotspots were always close to the body surface near the muscle wall boundary. The wocSAR10g across the 10 subjects ranged from 2.3 to 3.2 W kg-1 and was inversely correlated to fat volume percentage (R = 8) and BMI (R = 0.6). The coefficient of variation values in SAR10g due to variations in subject geometry, respiration phase and realistic coil repositioning were 12%, 4% and 12%, respectively. This study found that the variability due to realistic coil repositioning was similar to the variability due to differing healthy subject geometries for abdominal imaging. This is important as it suggests that population-based modelling is likely to be more useful than individual modelling in setting safe thresholds for abdominal imaging.

Plasmonic Coupled Modes in a Metal-Dielectric Periodic Nanostructure.

In this study we investigate the optical properties of a 2D-gap surface plasmon metasurface composed of gold nanoblocks (nanoantennas) arranged in a metal-dielectric configuration. This novel structure demonstrates the capability of generating simultaneous multi-plasmonic resonances and offers tunability within the near-infrared domain. Through finite difference time domain (FDTD) simulations, we analyze the metasurface's reflectance spectra for various lattice periods and identify two distinct dips with near-zero reflectance, indicative of resonant modes. Notably, the broader dip at 1150 nm exhibits consistent behavior across all lattice periodicities, attributed to a Fano-type hybridization mechanism originating from the overlap between localized surface plasmons (LSPs) of metallic nanoblocks and surface plasmon polaritons (SPPs) of the underlying metal layer. Additionally, we investigate the influence of dielectric gap thickness on the gap surface plasmon resonance and observe a blue shift for smaller gaps and a spectral red shift for gaps larger than 100 nm. The dispersion analysis of resonance wavelengths reveals an anticrossing region, indicating the hybridization of localized and propagating modes at wavelengths around 1080 nm with similar periodicities. The simplicity and tunability of our metasurface design hold promise for compact optical platforms based on reflection mode operation. Potential applications include multi-channel biosensors, second-harmonic generation, and multi-wavelength surface-enhanced spectroscopy.

Analytical investigation of unidirectional reflectionless phenomenon near the exceptional points in graphene plasmonic system.

We propose a two-dimensional array made of a double-layer of vertically separated graphene nanoribbons. The transfer matrix method and coupled mode theory are utilized to quantitatively depict the transfer properties of the system. We present a way to calculate the radiative and the intrinsic loss factors, combined with finite-difference time-domain simulation, conducting the complete analytical analysis of the unidirectional reflectionless phenomenon. By adjusting the Fermi energy and the vertical distance between two graphene nanoribbons, the plasmonic resonances are successfully excited, and the unique phenomena can be realized at the exceptional points. Our research presents the potential in the field of optics and innovative technologies to create advanced optical devices that operate in the mid-infrared range, such as terahertz antennas and reflectors.

Optical absorption enhancement of TiO2 via plasmonic effect of gold nanoparticles in volatile organic compounds medium

Controlling volatile organic compounds (VOC) is of utmost importance due to their adverse impacts on human health and environment. Photocatalytic degradation using TiO2 nanoparticles has emerged as a promising approach for VOC control. However, the limited photocatalytic activity of TiO2 under visible light owing to its wide bandgap presents a significant challenge. To address this limitation, the integration of plasmonic materials with TiO2 enables efficient hot electron injection into TiO2 under visible light. In this study, we investigate the influence of gold nanoparticles (AuNP) on TiO2-based structures, namely AuNP-decorated TiO2 thin films, Au@TiO2 core@shell structures, and AuNP-loaded TiO2 nanoparticles, using finite difference time domain (FDTD) simulations. Simulation results demonstrate that the presence of AuNP in TiO2 structures reduces the bandgap and enhances light absorption. Moreover, the localized electric field enhancement observed at the AuNP-TiO2 interface indicates the potential for effective hot electron injection into TiO2. Additionally, simulations conducted in a VOC medium demonstrate higher absorption compared to vacuum conditions. This work provides insights into the mechanisms underlying previously published experiments and proposes an appealing design for the development of highly efficient photocatalytic materials under visible light.

Engineering Surface-Enhanced Raman Scattering Probe for Enhanced Detection of Acute Diseases with Paper Microfluidics

Effective and fast detection of acute diseases is still a main challenge for early diagnosis and monitoring treatment. In this work, surface-enhanced Raman scattering (SERS) probes are incorporated into a microfluidic paper strip for antigen biomarker detection. First, a new material is designed to tune plasmon band position and intensity. Second, geometry is optimized to maximize the SERS enhancement factor. The Finite-Difference Time-Domain simulation has confirmed that more hotspots are generated by the above optimization. As a results, the paper microfluidic strip has achieved the limit of detection at pg/mL level toward antigen detection in blood plasma. The paper microfluidic device shows a great potential in early detection of acute diseases at home, clinics, and remote areas with limited resources.

Optical Characterisation of Plasmonic Indium Lattices Fabricated Via Electrochemical Deposition

Indium is an emerging plasmonic material that offers to extend the plasmonic applications given by gold and silver from the visible to the ultraviolet spectral range, with applications in imaging, sensing, and lasing. However, due to the high-vapor pressure/low melting temperature of indium, nanofabrication of pre-defined metallic indium nanostructures is non-trivial.In this work, we show the potential of selective area electrochemical deposition for the manufacturing of large-area arrays of indium pillars, which can be used for plasmonic applications. We combine electrochemical deposition with substrate-conformal imprint lithography to realize these large-scale indium nanoparticle arrays, having well-defined dimensions. This template growth results into well-defined hexagonal array of pillars with a mean height of 235 nm having a standard deviation of 60 nm.To highlight the potential of this electrochemical-based technique for plasmonic applications, we have performed angle-dependent extinction measurements using s-polarized light and using a RI matching oil. We found that these indium nanopillar arrays exhibit a strong plasmonic response due to the coupling to the plasmonic surface lattice resonances (SLRs). Furthermore, we have simulated the optical response of this indium nanopillar array using finite difference time domain (FDTD) simulations. In these simulations, we included the morphology obtained from SEM and the electrical permittivity of elemental indium. We found that the simulated extinction spectra agree well with the experimentally obtained spectra at 0˚ and 8˚ incidence, despite the polydispersity of the indium pillar height and defects in the hexagonal array.Our results therefore demonstrate the possibility of realizing large-area fabrication of indium plasmonic nanoparticles, which can be used for plasmonic applications. Given the low-cost and scalable nature of this technique, this work opens new possibilities in the design and fabrication of large-area plasmonic metasurfaces that operate in the UV spectral range. Future extensions of this work may leverage the indium mask-constrained growth to tailor even further the shape of the nanoparticle and hence give a powerful tuning knob to tailor plasmonic resonances.

Tailoring Geometry of Large-Area Hexagonal Close-Packed Plasmonic Nano-Array Structures for Biosensing

Generally, a large area of plasmonic nanoarray patterns are employed for biosensing. Electromagnetic (EM) field and Quality-factor (Q-factor, the ratio of the resonance wavelength over its bandwidth) of plasmonic patterns, are important to the sensitivity of plasmon-enhanced biosensors, for example, fluorescence and surface enhanced Rama spectroscopy (SERS) biosensors. Hexagonal close-packed plasmonic nanostructures arranged in nano-disk, nano-ring, nano-disk/ring patterns are prevailing configurations using batch processes (e.g., anodic aluminum oxide (AAO)). Different patterns due to size/geometry difference can have different Q-factors. However, the geometry effect on Q-factor in such configurations is not clear. Therefore, we adopted Finite-difference time-domain (FDTD) simulation to numerically study how EM field and Q-factor are modulated by geometric design of large-area hexagonal close-packed gold disk/ring structures. The simulation results provide guidelines to increase Q-factor to improve the sensitivity of plasmon-enhanced biosensors.

Pore evolution and size effect of nanoporous silver on SERS for malachite green detection

The nanoporous silver (NPS) films with the average pore size ranging from 51 to 246 nm were synthesized by dealloying co-sputtered AgxAl100-x (x = 30–56 at%) precursor films in 1 M HCl for 30–120 min. The structure of the NPS films, the dissociation of Al, and the aggregation of Ag were evaluated by field emission scanning electron microscopy as a function of dealloying time. The results showed that the pore size first decreased as the dealloying time increased, followed by pore coarsening, regardless of the relative density of NPS films. The finite-difference time-domain (FDTD) simulation further indicated that the smaller the nanopores, the higher the hot-spot area and surface electric field. As the average pore size of NPS film decreased to 51 nm, the enhanced Raman intensity of rhodamine 6G (R6G) using the NPS film as substrate was at least two orders higher than that using pure Ag film. Meanwhile, the detection limit of malachite green, a wildly used chemical in aquaculture, was 10-5 M with the enhancing factor 1.58 × 105.

Effect of using reflective target materials on enhancing UV nanosecond laser machining of sapphire

The machining of sapphire, a hard and transparent material, is of significant interest in various industries due to its exceptional properties. Traditional machining methods for sapphire suffer from drawbacks such as low efficiency and poor processing quality. Laser machining, particularly using ultraviolet (UV) nanosecond lasers, has emerged as a promising alternative for machining sapphire. In this article, we explored the use of reflective target materials to enhance laser energy absorption during the machining of sapphire. The effect of different reflective target materials, including aluminum and silicon, on the laser machining process was examined. Parametric calculations and finite-difference time-domain (FDTD) simulation were performed to evaluate the laser intensity at the top surface of sapphire when a reflective target material was employed. The results demonstrated that the use of reflective target materials leads to increased laser intensity due to the interference of incident and reflected laser beams, thus improving material removal efficiency. Moreover, experimental observations confirmed the effectiveness of using reflective target materials in enhancing the machined depth of sapphire. For example, the machined depth of 21.6 µm with a power density of 1.7 × 108 W/cm2 increased to 23.3 µm and 26.5 µm by using silicon and aluminum target materials. It is found that using aluminum target led to higher machined depth than silicon owing to its higher reflectivity.

Performance of finite-size metal-dielectric nanoslits metasurface optical filters.

In this work, we analyze the performance of finite-size metal-dielectric nanoslits guided mode resonance metasurface optical filters by using finite-difference time-domain simulations and spatial Fourier transform analysis. It is shown that in the direction of the nanoslits period, the critical size required to maintain the performance of the corresponding infinite size filter is the product of the nanoslits period and the quality factor of the infinite size filter. Size reduction in this dimension below the critical dimension reduces the peak transmittance and broadens the spectral linewidth of the filter. In the dimension orthogonal to the nanoslits period direction, the critical dimension size required is not related to the quality factor of the corresponding infinite size filter. Our analysis shows that the critical size is 12 times the filter peak wavelength in the orthogonal dimension for maintaining the filter performance. The 12 times filter wavelength requirement corresponds to the second zero of the Fourier transform of the aperture function.

A Combined Plasmonic and Electrochemical Aptasensor Based on Gold Nanopit Arrays for the Detection of Human Serum Albumin.

Electrochemical and optical platforms are commonly employed in designing biosensors. However, one signal readout can easily lead to inaccuracies due to the effect of nonstandard test procedures, different operators, and experimental environments. We have developed a dual-signal protocol that combined two transducer principles in one aptamer-based biosensor by simultaneously performing electrochemical- and extraordinary optical transmission (EOT)-based plasmonic detection using gold nanopit arrays (AuNpA). Compared with full hole structures, we found that nanopits, that did not fully penetrate the gold film, not only exhibited a better plasmonic bandwidth and refractive index sensitivity both in the finite-difference time-domain simulation and in experiments by shielding the gold/quartz mode but also enlarged the electrochemical active surface area. Therefore, the periodic non-fully penetrating AuNpA were modified with ferrocene-labeled human serum albumin aptamer receptors. The formation of the receptor layer and human serum albumin binding complex induced a conformational change, which resulted in variation in the electron transfer between the electro-active ferrocene units and the AuNpA surface. Simultaneously, the binding event caused a surface plasmon polaritons wavelength shift corresponding to a change in the surface refractive index. Interestingly, although both transducers recorded the same binding process, they led to different limits of detection, dynamic ranges, and sensitivities. The electrochemical transducer showed a dynamic detection range from 1 nM to 600 μM, while the optical transducer covered high concentrations from 100 μM to 600 μM. This study not only provides new insights into the design of plasmonic nanostructures but also potentially opens an exciting avenue for dual-signal disease diagnosis and point-of-care testing applications.

Mid-Infrared Optical Force Chromatography of Microspheres Containing Siloxane Bonds.

Recent interest in particle sorting using optical forces has grown due to its ability to separate micro- and nanomaterials based on their optical properties. Here, we present a mid-infrared optical force manipulation technique that enables precise sorting of microspheres based on their molecular vibrational properties using a mid-infrared quantum cascade laser. Utilizing the optical pushing force driven by a 9.3 μm mid-infrared evanescent field generated on a prism through total internal reflection, a variety of microspheres, including those composed of Si-O-Si bonds, can be separated in accordance with their absorbance values at 9.3 μm. The experimental results are in good agreement with the optical force calculations using finite-difference time-domain simulation. Thus, each microsphere's displacement and velocity can be predicted from the absorbance value; conversely, the optical properties (e.g., absorbance and complex refractive index in the mid-infrared region) of individual microspheres can be estimated by monitoring their velocity.