Finite-difference Time-domain Software Research Articles

VO2-Based Spacecraft Smart Radiator with High Emissivity Tunability and Protective Layer.

In the extreme space environment, spacecraft endure dramatic temperature variations that can impair their functionality. A VO2-based smart radiator device (SRD) offers an effective solution by adaptively adjusting its radiative properties. However, current research on VO2-based thermochromic films mainly focuses on optimizing the emissivity tunability (Δε) of single-cycle sandwich structures. Although multi-cycle structures have shown increased Δε compared to single-cycle sandwich structures, there have been few systematic studies to find the optimal cycle structure. This paper theoretically discusses the influence of material properties and cyclic structure on SRD performance using Finite-Difference Time-Domain (FDTD) software, which is a rigorous and powerful tool for modeling nano-scale optical devices. An optimal structural model with maximum emissivity tunability is proposed. The BaF2 obtained through optimization is used as the dielectric material to further optimize the cyclic resonator. The results indicate that the tunability of emissivity can reach as high as 0.7917 when the BaF2/VO2 structure is arranged in three periods. Furthermore, to ensure a longer lifespan for SRD under harsh space conditions, the effects of HfO2 and TiO2 protective layers on the optical performance of composite films are investigated. The results show that when TiO2 is used as the protective layer with a thickness of 0.1 µm, the maximum emissivity tunability reaches 0.7932. Finally, electric field analysis is conducted to prove that the physical mechanism of the smart radiator device is the combination of stacked Fabry-Perot resonance and multiple solar reflections. This work not only validates the effectiveness of the proposed structure in enhancing spacecraft thermal control performance but also provides theoretical guidance for the design and optimization of SRDs for space applications.

Bioinspired Diffuse Reflectance Coatings for Fabric-Based Radiative Coolers

As global temperatures rise due to anthropogenic climate change, thermal comfort becomes an increasingly critical area of research. While most buildings are equipped to cool and heat their interiors, this process is very energy intensive; a smarter approach is to enable personal thermal management using clothing and textiles. In particular, we are interested in the ability to use textiles and clothing to cool bodies in extreme heat, both to maintain comfort and prevent deaths in urban deserts due to heat stroke. One innovative category in this realm is radiative coolers—surfaces adept at manipulating light to passively regulate temperature. These coolers achieve sub-ambient surface temperatures by selectively reflecting or refracting sunlight wavelengths (200nm-2.5µm) while emitting infrared light (8-13µm). This unique capability allows them to maintain cooler temperatures, even when exposed to direct sunlight and the open sky. Coatings facilitating this cooling action typically consist of a porous media filled with refractive gaps, requiring specific substrates and custom production processes, thereby limiting their applications. Furthermore, the necessity of low light transmission presents issues in creating fabrics that radiatively cool while still retaining high breathability and durability. In this study we present a method whereby an optically active coating comprised of a mixture of CaCO3 and BaSO4 microcrystals was applied to multiple fabric substrates to create a radiative cooler. Through the use of photoinitiated chemical vapor deposition (pICVD), a 5 µm thick layer of a hydrophilic polymer polyhydroxyethyleneacrylate (pHEA) was deposited on the fabric substrate. Subsequently, through serial immersion in solutions containing Ca and Ba ions and solutions containing CO3 and SO4 ions, inorganic microcrystals were grown directly on the surface of the fabric. These microcrystals formed with a large polydispersity, endowing the coating with excellent reflective properties in the solar spectrum. Mie scattering calculations confirmed that the CaCO3 and BaSO4 microcrystals were primarily responsible for scattering. Further simulations using finite difference time domain software revealed, surprisingly, that surface-immobilized crystals provided the highest reflection efficiency, indicating that composite fibers with embedded particles are not as efficient. Upon outdoor testing, the device showed a cooling ability of 8°C compared to an uncoated fabric, achieving a maximum cooling of 4°C below ambient temperature. Washing and durability testing of the coating found no degradation in the material's performance, affirming its resilience and long-term effectiveness.

Simultaneous enhancements on emissions from quantum dot and quantum well by Ag nanoparticles for color conversion

Quantum dots (QDs) have been paid much attention on the color conversion for light-emitting diode (LED) in micro-display recently. However, it is hard to achieve high color conversion efficiency in a thin QD layer. In this paper, we fabricated silver nanoparticles (Ag NPs) with radii ranging mostly from 25 to 35 nm on a blue LED with a peak wavelength of 450 nm, then spin-coated QDs with a peak wavelength of 565 nm. Scanning electron microscopy, cathodoluminescence, photoluminescence (PL), and time-resolved PL measurements were performed. The PL emissions from quantum wells (QWs) of blue LED and QDs were enhanced by 10% and 32%, respectively, when the Ag NPs were included. The PL lifetimes of QWs and QDs were reduced by 10 and 6 times, respectively, compared to their initial states. Finite difference time domain software and the perturbation method were used to simulate the PL measurements and variable separation. It was concluded that the coupling of QDs and QWs with localized surface plasmon improves the external quantum efficiency and enhances the spontaneous emission rate in both QWs and QDs. This paper provides a new idea for designing high-efficiency color conversion micro-LED.

Analysis of Graphene Pythagoras Tree Fractal Antenna with Thin SiO2 Substrate in Terahertz Regime

Abstract The Pythagoras Tree Fractal patch is considered on a SiO2 substrate thickness of 1.5 µm to radiate at four frequencies of 4.975THz, 5.38THz, 6.73THz, and 7.61THz with Voltage Standing Wave Ratio (VSWR) ≤ 2. This is a very creative fractal design of a multiband THz antenna, based on a very thin graphene layer, the study proved a value in terms of high radiation efficiency and high gain, at the level of current research we made a comparison study where the design has evidently huge potential regarding applicability for telecommunication technology in the terahertz regime. The demand for high-performance terahertz (THz) antennas has increased significantly in recent years due to their potential applications in various fields such as medical imaging, security screening, and wireless communications. In this paper, the authors present an analysis of a graphene Pythagoras Tree Fractal (GPTF) antenna with a thin SiO2 substrate for THz regime. The GPTF antenna is designed using a fractal geometry approach, which provides multiple resonant frequencies and enhances the overall radiation efficiency. The thin SiO2 substrate is used to reduce the substrate losses and improve the radiation performance of the antenna. The authors use the Finite-Difference Time-Domain (FDTD) software to simulate the performance of the proposed antenna. The results show that the proposed antenna exhibits high gain, low return loss, and wide bandwidth, making it a promising candidate for THz applications.

T-shaped tunable infrared optical switch based on vanadium dioxide with dual-wavelength channels

A T-shaped tunable infrared optical switch based on a plasmonic structure composed of metal nanorods and vanadium dioxide film is proposed. The dynamic adjustment strategy based on the insulator–metal transition of vanadium dioxide and polarization-sensitive properties of the switch is thoroughly investigated to achieve the modulation of the wavelength and spectral reflectance intensity. Benefiting from the phase transition of vanadium dioxide, the optical switch can achieve two switching modes of ON and OFF states for both TE and TM polarizations of the incident light. Simulation results performed by finite-difference time-domain software show that the modulation depth of the switch can reach up to 85% at the resonance wavelength of 4270 nm under TM incident light. While the switch is illuminated with TE incident light, the modulation depth of the switch can reach 73% at the resonance wavelength of 2643 nm. In addition, when the polarizing angle of the incident light varies from 0 to 90°, the intensity of the spectral reflectance of the switch monotonically decreases at the wavelength of 4270 nm. Meanwhile, the spectral reflectance monotonically increases at the wavelength of 2643 nm. Moreover, the resonance wavelength of the optical switch can be tuned by altering the length of the metal nanorods. Those dynamic adjustment approaches will provide potential applications in optical modulation and solar photovoltaic fields.

Calculate The Resonance Angle of Surface Plasmon Resonance Gold film Configured with Kretschmann

The current work includes measurements of the resonance angle and reflectance for p-polarization of the electric field by using the Fresnel equation at a given length. Gold's surface plasmon wave can be seen at the metal-to-air boundary. We try to determine the greatest (SPR) angle for a metal thin layer that is most suitable for the surface plasmon excitation while stimulated by a laser. SPR was performed of a single film of gold placed on a glass prism; there are SPR modes in this structure, which match the surface plasmon. We also suggest that the SPR mode associated with the Au surface, which is extremely sensitive to changes in the surrounding environment, particularly (dielectric). A few considerations to be taken into account to attain the SPR, like the incident angle of light rays addressed and analyzed for the purpose of finding the essential value for the plasmon to be emerge; the gold/air resonance angle. Furthermore, we can compare our result with other work, which was performed by using the Finite-Element-Method (FEM), the simulation is done by FDTD (Finite Difference Time Domain) software. SPR was applied in a variety of domains, containing biomedicine science, optics, biomedicine, photo-thermal plasmon, and health.

Ultra-short bilayer graphene optical fiber intensity modulator based on silver-semicircular hybrid plasmonic waveguide

The intensity modulator is a key component in optical fiber communication. In this paper, an ultra-short graphene light intensity modulator based on hybrid plasmonic waveguide (HPW) array is proposed. Al2O3 embedded with double-layer graphene at a certain interval is placed on the side polishing surface of the microfiber Mach–Zehnder modulator (MZM), and the silver-semicircular array is placed on the surface of the transition material as hybrid plasmonic waveguide for the first time in all-fiber modulator. Analyzing by building a 3D model in finite difference time domain software, the hybrid surface plasmon polariton (HSPP) between silver-semicircles significantly enhances the subwavelength confinement and the light–graphene interaction. The modulation range of the real part of effective refractive index (Neff) can reach to 0.01984 by adjusting the Fermi level of graphene. Therefore, π phase change can be achieved only in a modulation length of 45 μm, and the insert loss can be controlled below 0.0159 dB/μm through the analysis of the imaginary part of the Neff, which also leads to a low total loss of 0.7155 dB. Owing to the excellent phase modulation efficiency, high extinction ratios of 39.39 dB and 47.77 dB can be reached for port 1 and port 2, respectively. Double-layer graphene forms the flat capacitor, which brings high modulation efficiency while achieving a high modulation bandwidth of 59.945 GHz. The application of HPW in graphene optical fiber modulator greatly improves the working efficiency, reduces the difficulty of integration, overcomes high insert loss, improves the extinction ratio, this modulator can be widely used in the future optical fiber communication system.

Which method is more efficient on enhancing light absorption for silicon nanowires array based solar cells: Plasmonic metal nanoparticles or narrow-bandgap semiconductor quantum dots?

Being the unique physical structure and excellent optoelectronic properties, silicon nanowires (Si-NWs) array is considered to be a key nanostructure to build low-cost and high-efficiency solar cells, showing excellent performance of photons capturing and anti-reflection. In this work, the light absorption enhancement of Si-NWs array based solar cells by filling plasmonic metal nanoparticles (NPs), such as Au, Ag and Al, and/or PbS quantum dots (QDs) in the gaps of Si-NWs or depositing on the tips of Si-NWs, are simulated by using finite-difference time-domain (FDTD) software, respectively. Based on the proposed geometry of periodic Si-NWs array, our simulation demonstrates that the light absorption in the long-wavelength region can be greatly enhanced by filling suitable metal NPs or PbS QDs to form a certain thickness layer or depositing metal NPs on the tips of Si-NWs, by making good use of the localized surface plasmons and scattering generated around metal NPs, or the high absorption of PbS QDs in near-infrared region. Also, our simulation is in agreement with experimental data reported in literatures. Therefore, our work provides a valuable theoretical basis for designing high-performance Si-NWs array based solar cells.

Tunable Infrared Optical Switch Based on Vanadium Dioxide.

A tunable infrared optical switch based on a plasmonic structure consisting of aluminum nanoarrays with a thin film of vanadium dioxide is proposed. This optical switch can realize arbitrary wavelength-selective optical switching in the mid-infrared region by altering the radii of the aluminum nanoarrays. Furthermore, since vanadium dioxide transforms from its low-temperature insulator phase to a high-temperature metallic phase when heated or applied voltage, the optical switch can achieve two-way switching of its “ON” and “OFF” modes. Finite-difference time-domain software is used to simulate the performance of the proposed infrared optical switch. Simulation results show that the switch offers excellent optical performances, that the modulation depth can reach up to 99.4%, and that the extinction ratio exceeds −22.16 dB. In addition, the phase transition time of vanadium dioxide is on the femtosecond scale, which means that this optical switch based on a vanadium dioxide thin film can be used for ultrafast switching.

A Dual-Band and Low-Cost Microstrip Patch Antenna for 5G Mobile Communications

This paper investigates the numerical and experimental analysis of a low-cost and dual-band microstrip patch antenna for the fifth generation (5G) mobile communications. The numerical analysis of the proposed antenna is performed using the computational electromagnetic simulator (CEMS) software which is based on the finite-difference time-domain (FDTD) and CST software which is based on the finite integration technique (FIT). The performance of the proposed antenna designed and fabricated on a low-cost FR-4 substrate is verified with the simulated and measured results. The antenna operates at dual frequency bands which are 24 and 28 GHz. The antenna maximum gain values are 3.20 dBi and 3.99 dBi in the x-y plane at 24 and 28 GHz, respectively. The proposed antenna provides almost omni-directional patterns suitable for 5G mobile communication devices.

A simulation study on the effect of size gold nanoparticles on broadband light absorption in dye-sensitised solar cells

Plasmon-assisted energy conversion in dye-sensitised solar cells (DSSCs) has been achieved by applying gold (Au) nanoparticles (NPs) inside Titania (TiO2) photoanodes. Gold nanoparticles (GNPs) were introduced into DSSCs to further enhance their power conversion efficiency (PCE). In this research work, an effort has been made to enhance the optical absorption and improve the performance of DSSCs. By utilising finite-difference time-domain (FDTD) software, GNPs with radii of 15, 25, 35, 45, 55, 65, 75 and 85 nm were produced and introduced into the TiO2 photoanode. The optimum radius for the optical absorption enhancement was found to be 85 nm because the effect of plasmon coupling is more significant for metal nanoparticle sizes > 60 nm. The effect of various sizes of GNPs on light scattering has also been presented in this study. Moreover, the investigation has focused on the role of incident angle of light source on the absorption in TiO2 films. It was found that the optimum incident angle for the enhancement of broadband optical absorption in the wavelength range of 450–800 nm is 70°.

Tutorial on finite-difference time-domain (FDTD) methods for room acoustics simulation

Wave-based simulation models for acoustics have remained an active area of research and development for the past three decades. Wave-based methods aim to solve the 3D wave equation directly and therefore have large computational costs relative to conventional ray-based methods, which tend to simplify wave-diffraction effects. However, wave-based methods offer the potential of complete numerical solutions, including all wave-scattering and diffraction effects over the full audible bandwidth. Additionally, wave-based methods are highly parallelisable, making them amenable to parallel computing architectures such as graphics processing units, which can greatly cut down lengthy simulation times. This tutorial will give an introduction to wave-based methods for room acoustics with a focus on the finite-difference time-domain (FDTD) method. The basic concepts behind FDTD methods, along with practical implementation issues, will be discussed and illustrated with examples. The relationship of FDTD methods to other wave-based methods, along with differences to ray-based methods, will be explained. Also, the use of graphics processing units (GPUs) with freely available FDTD software will be discussed.

Sunflower-Like Nanostructure with Built-In Hotspots for Alpha-Fetoprotein Detection.

In the present study, a sunflower-like nanostructure array composed of a series of synaptic nanoparticles and nanospheres was manufactured through an efficient and low-cost colloidal lithography technique. The primary electromagnetic field contribution generated by the synaptic nanoparticles of the surface array structures was also determined by a finite-difference time-domain software to simulate the hotspots. This structure exhibited high repeatability and excellent sensitivity; hence, it was used as a surface-enhanced Raman spectroscopy (SERS) active substrate to achieve a rapid detection of ultra-low concentrations of Alpha-fetoprotein (AFP). This study demonstrates the design of a plasmonic structure with strong electromagnetic coupling, which can be used for the rapid detection of AFP concentration in clinical medicine.

Development of a Microplate Platform for High-Throughput Sample Preparation Based on Microwave Metasurfaces.

Sample preparation is one of the most time-consuming steps in diagnostic assays, particularly those involving biological samples. In this paper we report the results of finite-difference time-domain (FDTD) simulations and thermographic imaging experiments carried out with the intent of designing a microplate for rapid, high-throughput sample preparation to aid diagnostic assays. This work is based on devices known as microwave lysing triangles (MLTs) that have been proven capable of rapid sample preparation when irradiated in a standard microwave cavity. FDTD software was used to model a microplate platform as a polystyrene substrate with an array of various passive scattering elements (PSEs) of different sizes, shapes, and interelement spacings in a 2.45 GHz field identical to that of a common microwave oven. Based on the FDTD modeling, several PSE arrays were fabricated by cutting PSEs out of aluminum foil and adhering them to the bottom of regular polystyrene microplates to make prototypes. Each prototype microplate was then irradiated in a microwave cavity for a range of different times, powers, and source angles and the heating effects were observed via a forward looking infrared (FLIR) camera. Based on the results, two prototype microplate platforms have been shown to demonstrate electromagnetic and thermal enhancements similar to those seen with MLTs as well as tunable thermal responses to radio frequency (RF) irradiation.

Room temperature terahertz detector based on single silicon nanowire junctionless transistor with high detectivity

In this work, the n-type single silicon nanowire (NW) based junctionless field-effect transistor (FET) is demonstrated as an efficient terahertz (THz) detector. For the effective coupling of the THz radiations with NW junctionless FET, the lobes of the rounded bow-tie antenna are connected to the gate and source terminals of the device. The antenna design is optimized with proper impedance matching conditions to achieve maximum power transfer between antenna and detector. The simulated antenna resonates at 0.43 THz frequency with 19 GHz bandwidth. Further simulations have been done on Lumerical finite difference time domain software to analyze the electric field distribution profile. To investigate the optical response of this optimized antenna design, an array of the simulated antenna has been fabricated and its transmission spectra are measured. Finally, the simulated antenna has been integrated with the n-type NW junctionless transistor. A maximum responsivity of 468 V W−1 at 0.425 THz frequency and noise-equivalent-power of ∼ 10−9W/Hz1/2 is obtained at room temperature. The complementary metal-oxide-semiconductor’s compatibility, ease of integration on chips, possibility to realize multiple pixel arrays, andscalability to higher frequencies, make this device promising for THz electronics.

Parametric optimization of visible wavelength gold lattice geometries for improved plasmon-enhanced fluorescence spectroscopy.

We report the exploitation of spectroplasmonics for innovations in optical transducer development, specifically in the well-established application of labeled fluorescent analytes known as fluorescence spectroscopy. Presented herein are comprehensive analyses of nanoscale plasmonic lattice feature geometries using finite-difference time-domain software to determine the largest surface electric ($E$E) field enhancement resulting from localized surface plasmon resonance for reducing the limit of detection of plasmon-enhanced fluorescence. This parametric optimization of the critical dimensions of the plasmon resonance of noble metal nanostructures will enable improved excitation and emission enhancement of fluorophores used in visible wavelength fluorescence spectroscopy.

Optimization of Laser Recrystallization Process for GeSn Films on Si Substrates Based on Finite Difference Time Domain and Finite Element Method

GeSn alloy on Si substrate has the advantages of high carrier mobility, high radiation recombination efficiency, compatibility with the Si process, and is widely used in the field of semiconductor optoelectronics. However, due to the high lattice mismatch between the GeSn epitaxial layer and the Si substrate, how to prepare a perfect GeSn film on the Si substrate is an issue. The 808 nm continuous wave laser recrystallization technology can significantly improve the quality of the GeSn alloy epitaxial layer by melting and recrystallization, which provide another technical way for solving this problem. Optimized laser recrystallization related process parameters is necessary when laser recrystallization technology is used to prepare high quality GeSn alloy on Si substrate. For this purpose, the absorption, reflection and transmission models of GeSn alloy epitaxial layer/Si substrate system irradiated by 808 nm continuous wave laser are established using finite difference time domain software FDTD Solutions. The thickness-related process parameters of GeSn alloy epitaxial layer and SiO2 capping layer are optimized. In addition, the temperature distribution model of 808 nm continuous wave laser irradiation of GeSn alloy epitaxial layer on Si substrate system is obtained by COMSOL Multiphysics simulation. The process parameters related to laser recrystallization temperature are optimized and listed, which can be used as important technical references for the growth of low defect density GeSn layer on Si substrate assisted by the laser recrystallization technology.

Linewidth study of pixelated aluminum nanowire gratings on polarization performance.

Nowadays, nanowire gratings are widely used in various applications such as imaging sensors and high-resolution microscopes. Structure parameters are the main factors that affect the optical performance of the gratings. This work aims to present the influence of the linewidth of pixelated aluminum nanowire gratings with a fixed period on the transmittance and extinction ratio in the visible region. By controlling the exposure doses of electron beam lithography (EBL), different linewidths of pixelated aluminum nanowire gratings with a period of 170 nm were fabricated. The significant effects of linewidth difference on the polarization performance were verified by the simulations of finite-difference time-domain (FDTD) software. The simulations were divided into two parts: the discussion of the pure aluminum without considering oxidation and the discussion of the surface aluminum being oxidized into the aluminum oxide. An optical system was built to evaluate the performance of the fabricated structures. The results show that the trends of the measurement results are consistent with that of simulation. This work will give a guide to the fabrication and evaluation of the nanowire gratings.

An enhanced finite difference time domain method for two dimensional Maxwell's equations

AbstractAn enhanced finite‐difference time‐domain (FDTD) algorithm is built to solve the transverse electric two‐dimensional Maxwell's equations with inhomogeneous dielectric media where the electric fields are discontinuous across the dielectric interface. The new algorithm is derived based upon the integral version of the Maxwell's equations as well as the relationship between the electric fields across the interface. To resolve the instability issue of Yee's scheme (staircasing) caused by discontinuous permittivity across the interface, our algorithm revises the permittivities and makes some corrections to the scheme for the cells around the interface. It is also an improvement over the contour‐path effective permittivity algorithm by including some extra terms in the formulas. The scheme is validated in solving the scattering of a dielectric cylinder with exact solution from Mie theory and is then compared with the above contour‐path method, the usual staircasing and the volume‐average method. The numerical results demonstrate that the new algorithm has achieved significant improvement in accuracy over other methods. Furthermore, the algorithm has a simple structure and can be merged into current FDTD software packages easily. The C++ source code for this paper is provided as supporting information for public access.

Metalens for coaxial double wavelength focusing

We propose a metalens for coaxial double wavelength focusing. One focusing spot is a circular solid spot, and the other focusing spot is a doughnut-shaped spot that is circling the solid spot. The designed metalens was composed of a meta-molecular nanostructured cell array. Each meta-molecular nanostructured cell was divided into four squares. Two slots with exactly the same shape, but usually with the rotation angle measured clockwise from the positive x axis, are etched into the gold film in two diagonally connected squares. Another two slots with the same shape but with the rotation angle measured counter-clockwise from the positive x axis are etched into another two diagonally connected squares in the same cell. The lasers with two different wavelengths are transformed into right-handed and left-handed circularly polarized beams, respectively. The two sets of slots with different azimuthal rotations modulated the phases of incident right-handed and left-handed circularly polarized beams independently. The numerical simulation with finite-difference time-domain (FDTD) software was carried out, and the experimental verification was also implemented. Both the experimental result and the numerical simulation agree well with the theoretical design.