Search Results (561)

A method is developed to make an optical fiber that only transmits fully linearly polarized light and maintains the polarization state. The method for efficient ingesting laser into this fiber is also reported. Using an optical fiber with a prism head, we can compress a plane wave into the thin rectangular cross-section fiber, and the light intensity within the fiber is much larger than that of the incidence wave. Our finite-difference time-domain (FDTD) simulation results show that the compressed light in the fiber becomes fully polarized and maintains the polarization state, and can be well coupled out by the resonance rings. This method is suitable for developing highly efficient polarization-maintaining optical fibers in a much simpler way, for applications in photonic integrated circuits or optical sensors. Full article

In this study, we evaluated the surface plasmon characteristics of periodic silver nanodisk structures fabricated on a dielectric thin-film spacer layer on a Ag mirror substrate (NanoDisk on Mirror: NDoM) through finite difference time domain (FDTD) simulations and experiments involving actual sample fabrication. Through FDTD simulations, it was confirmed that the NDoM structure exhibits two sharp peaks in the visible range, and by adjusting the thickness of the spacer layer and the size of the nanodisk structure, sharp peaks can be obtained across the entire visible range. Additionally, we fabricated the NDoM structure using electron beam lithography (EBL) and experimentally confirmed that the obtained peaks matched the simulation results. Furthermore, we discovered that applying annealing at an appropriate temperature to the fabricated structure enables the adjustment of the resonance peak wavelength and enhances the scattering intensity by approximately five times. This enhancement is believed to result from changes in the shape and size of the nanodisk structure, as well as a reduction in grain boundaries in the metal crystal due to annealing. These results have the potential to contribute to technological advancements in various application fields, such as optical sensing and emission enhancement. Full article

A Si-based photodetector is the core device of Si-based optical interconnection; its material and performance are the key factors restricting its development. This paper conducts theoretical research on the issues of lattice mismatch between heterogeneous materials and low device responsivity in Si-based InGaAs photodetectors for the 1550 nm optical communication band. The material mismatch issue is addressed through the use of the high-aspect ratio trapping (ART) epitaxial technique, enabling the realization of high-performance Si-based III-V materials. By introducing a dielectric metasurface into the top layer of the structure, the light absorption efficiency is enhanced, realizing broadband optical absorption enhancement for Si-based photodetectors. This paper mainly focuses on designing the optimal parameters of the dielectric metasurface structure based on the finite-difference time-domain (FDTD) Solutions to achieve the performance analysis of a high-responsivity 1550 nm Si-based InGaAs photodetector. The results show that the quantum efficiency of the dielectric metasurface structure is theoretically estimated to be 88.8% and the response rate is 1.11 A/W, which is 2%~16% higher than that of the unetched structure in the whole band. The research results of this paper will provide new ideas for the development of novel, high-performance, and miniaturized Si-based photodetectors and lay a theoretical foundation for Si-based optical interconnection. Full article

A dual fan-shaped structure covered with Ag films was investigated for generating twin photonic hooks (t-PHs). The t-PH characteristics of this structure are studied using the Finite-Difference Time-Domain (FDTD) method. The results show that by designing appropriate fan-shaped opening angles and angles of Ag films coverage, the switching between t-PHs, S-shaped t-PHs, and W-shaped t-PHs can be achieved, along with controlling over the bending angles. The maximum first, second, and third bending angles for the obtained W-shaped t-PHs are 51.3°, 36.4°, and 41.8°, respectively, while the Ag films angle is 5°. The investigated tunable morphology t-PHs provide innovative applications in the fields of nanolithography and integrated optics. Full article

In recent years, solar energy has become popular because of its clean and renewable properties. Meanwhile, two-dimensional materials have become a new favorite in scientific research due to their unique physicochemical properties. Among them, monolayer molybdenum disulfide (MoS₂), as an outstanding representative of transition metal sulfides, is a hot research topic after graphene. Therefore, we have conducted an in-depth theoretical study and design simulation using the finite-difference method in time domain (FDTD) for a solar absorber based on the two-dimensional material MoS₂. In this paper, a broadband solar absorber and thermal emitter based on a single layer of molybdenum disulfide is designed. It is shown that the broadband absorption of the absorber is mainly due to the propagating plasma resonance on the metal surface of the patterned layer and the localized surface plasma resonance excited in the adjacent patterned air cavity. The research results show that the designed structure boasts an exceptional broadband performance, achieving an ultra-wide spectral range spanning 2040 nm, with an overall absorption efficiency exceeding 90%. Notably, it maintains an average absorption rate of 94.61% across its spectrum, and in a narrow bandwidth centered at 303 nm, it demonstrates a near-unity absorption rate, surpassing 99%, underscoring its remarkable absorptive capabilities. The weighted average absorption rate of the whole wavelength range (280 nm–2500 nm) at AM1.5 is above 95.03%, and even at the extreme temperature of up to 1500 K, its heat radiation efficiency is high. Furthermore, the solar absorber in question exhibits polarization insensitivity, ensuring its performance is not influenced by the orientation of incident light. These advantages can enable our absorber to be widely used in solar thermal photovoltaics and other fields and provide new ideas for broadband absorbers based on two-dimensional materials. Full article

Low light extraction efficiency (LEE) remains a critical bottleneck in the performance of contemporary micro-light-emitting diodes (micro-LEDs). This study presents an innovative approach to improve the LEE of Gallium nitride (GaN)-based thin-film flip-chip (TFFC) micro-LEDs by integrating an inclined sidewall with photonic crystals (PhCs). Three-dimensional finite-difference time-domain (FDTD) simulations reveal that the inclined sidewall design significantly increases the escape probability of light, thereby improving LEE. Additionally, the PhCs’ structure further improves LEE by enabling more light to propagate into the escape cones through diffraction. Optimal results are achieved when the inclined sidewall angle (θ) is 28° and the PhCs exhibit a period (a) of 220 nm, a filling factor (f) of 0.8, and a depth (d) of 3 μm, resulting in a maximum LEE of 36.47%, substantially surpassing the LEE of conventional planar TFFC micro-LEDs. These results provide valuable design guidelines for the development of high-efficiency GaN-based micro-LEDs. Full article

The paper describes an axisymmetric Finite-Difference Time-Domain computer model of a coaxial Switched Wave Oscillator integrated with a dipole antenna. The model analyzes the operation of a realistic circuit model of a multistage Marx generator that charges the oscillator to a high voltage. The initial field distribution is calculated with an electrostatic finite-difference method solver to speed up the time-domain analysis. The work presents the results of our circuit model of a Marx generator’s simulations of the charging phase, followed by the results from the discharge phase, using our axisymmetric Finite-Difference Time-Domain model. Our work describes new and fast numerical solvers that can observe the operation of Switched Wave Oscillator systems (also considering the connected antenna). The codes could be used in the process of designing such a system. Advanced boundary conditions modeling the spark gap and oscillator’s excitation set our work apart from the other attempts in the literature. Full article

Terahertz (THZ) spectroscopy has emerged as a superior label-free sensing technology in the detection, identification, and quantification of biomolecules in various biological samples. However, the limitations in identification and discrimination sensitivity of current methods impede the wider adoption of this technology. In this article, a meticulously designed metasurface is proposed for molecular fingerprint enhancement, consisting of a periodic array of lithium tantalate triangular prism tetramers arranged in a square quartz lattice. The physical mechanism is explained by the finite-difference time-domain (FDTD) method. The metasurface achieves a high quality factor (Q-factor) of 231 and demonstrates excellent THz sensing capabilities with a figure of merit (FoM) of 609. By varying the incident angle of the THz wave, the molecular fingerprint signal is strengthened, enabling the highly sensitive detection of trace amounts of analyte. Consequently, cinnamoylglycine can be detected with a sensitivity limit as low as 1.23 μg·cm−2. This study offers critical insights into the advanced application of THz waves in biomedicine, particularly for the detection of urinary biomarkers in various diseases, including gestational diabetes mellitus (GDM). Full article

In this paper, a comparative analysis and numerical simulation of operation of two types of amplitude binary gratings (conventional and fork), both in the focal plane and near-field diffraction under illumination by mode beams with different polarization states, were performed. The simulation of the field formation in the focal plane was performed using the Richards–Wolf formalism. The diffraction calculation in the near-field diffraction was performed based on the FDTD method, considering the 3D structure of optical elements. The possibility of multiplying the incident beam in different diffraction orders of binary gratings and the polarization transformation associated with spin–orbit interaction at tight focusing were shown. In this case, various polarization transformations were formed in ±1 diffraction orders of the fork grating due to different signs of the introduced vortex-like phase singularity. The obtained results can be useful for the laser processing of materials and surface structuring. Full article

Surface enhanced Raman spectroscopy (SERS) is gaining importance as sensing tool. However, wide application of the SERS technique suffers mainly from limitations in terms of uniformity of the plasmonics structures and sensitivity for low concentrations of target analytes. In this work, we present SERS specimens based on periodic arrays of 3D-structures coated with silver, fabricated by silicon top-down micro and nanofabrication (10 mm × 10 mm footprint). Each 3D-structure is essentially an octahedron on top of a pyramid. The width of the top part—the octahedron—was varied from 0.7 µm to 5 µm. The smallest structures reached an analytical enhancement factor (AEF) of 3.9 × 10⁷ with a relative standard deviation (RSD) below 20%. According to finite-difference time-domain (FDTD) simulations, the origin of this signal amplification lies in the strong localization of electromagnetic fields at the edges and surfaces of the octahedrons. Finally, the sensitivity of these SERS specimens was evaluated under close-to-reality conditions using a portable Raman spectrophotometer and monitoring of the three vibrational bands of 4-nitrobenzenethiol (4-NBT). Thus, this contribution deals with fabrication, characterization and simulation of multiscale 3D-structures with SERS activity. Full article

Magnetic resonance imaging (MRI) is a non-invasive diagnostic technique able to provide information about the anatomical, structural, and functional properties of different organs. A magnetic resonance (MR) scanner employs radiofrequency (RF) coils to generate a magnetic field to excite the nuclei in the sample (transmit coil) and pick up the signals emitted by the nuclei (receive coil). To avoid trial-and-error approaches and optimize the RF coil performance for a given application, accurate design and simulation processes must be performed. We describe the full-wave simulation of a Helmholtz coil for high-field MRI performed with the finite-difference time-domain (FDTD) method, investigating magnetic field pattern differences between loaded and unloaded conditions. Moreover, the self-inductance of the single loops constituting the Helmholtz coil was estimated, as well as the frequency splitting between loops due to inductive coupling and the sample-induced resistance. The result accuracy was verified with data acquired with a Helmholtz prototype for small phantom experiments with a 3T MR clinical scanner. Finally, the magnetic field variations and coil detuning after the insertion of the RF shield were evaluated. Full article

Photonic circuits find applications in biomedicine, manufacturing, quantum computing and communications. Photonic waveguides are crucial components, typically having cross-section orders of magnitude inferior when compared with other photonic components (e.g., optical fibers, light sources and photodetectors). Several light-coupling methods exist, consisting of either on-plane (e.g., adiabatic and end-fire coupling) or off-plane methods (e.g., grating and vertical couplers). The grating coupler is a versatile light-transference technique which can be tested at wafer level, not requiring specific fiber terminations or additional optical components, like lenses, polarizers or prisms. This study focuses on fully-etched grating couplers without a bottom reflector, made from hydrogenated amorphous silicon (a-Si:H), deposited over a silica substrate. Different coupler designs were tested, and of these we highlight two: the superimposition of two lithographic masks with different periods and an offset between them to create a random distribution and a technique based on the quadratic refractive-index variation along the device’s length. Results were obtained by 2D-FDTD simulation. The designed grating couplers achieve coupling efficiencies for the TE-like mode over −8 dB (mask overlap) and −3 dB (quadratic variation), at a wavelength of 1550 nm. The coupling scheme considers a 220 nm a-Si:H waveguide and an SMF-28 optical fiber. Full article

Ground-penetrating radar (GPR) has been widely used for subsurface detection and testing. Numerical simulations of GPR signal are commonly performed to aid the interpretation of subsurface structures and targets in complex environments. To enhance the accuracy of GPR simulations on heterogeneous medium, this paper proposes a hybrid modeling method that combines the discrete element method with a component fusion strategy (DEM–CFS). Taking the asphalt pavement as an example, three 3D stochastic models with distinctly different porosities are constructed by the DEM–CFS method. Firstly, the DEM is utilized to establish the spatial distribution of random coarse aggregates. Then, the component fusion strategy is employed to integrate other components into the coarse aggregate skeleton. Finally, the GPR response of the constructed asphalt models is simulated using the finite-difference time-domain method. The proposed modeling method is validated through both numerical and laboratory experiments and demonstrates high precision. The results indicate that the proposed modeling method has high accuracy in predicting the dielectric constant of heterogeneous media, as generated models are closely aligned with real-world conditions. Full article

Converting carbon dioxide (CO₂) into high-value-added chemicals using solar energy is a promising approach to reducing carbon dioxide emissions; however, single photocatalysts suffer from quick the recombination of photogenerated electron–hole pairs and poor photoredox ability. Herein, silver (Ag) nanoparticles featuring with localized surface plasmon resonance (LSPR) are combined with g-C₃N₄ to form a Schottky junction for photothermal catalytic CO₂ reduction. The Ag/g-C₃N₄ exhibits higher photocatalytic CO₂ reduction activity under UV-vis light; the CH₄ and CO evolution rates are 10.44 and 88.79 µmol·h⁻¹·g⁻¹, respectively. Enhanced photocatalytic CO₂ reduction performances are attributed to efficient hot electron transfer in the Ag/g-C₃N₄ Schottky junction. LSPR-induced hot electrons from Ag nanoparticles improve the local reaction temperature and promote the separation and transfer of photogenerated electron–hole pairs. The charge carrier transfer route was investigated by in situ irradiated X-ray photoelectron spectroscopy (XPS). The three-dimensional finite-difference time-domain (3D-FDTD) method verified the strong electromagnetic field at the interface between Ag and g-C₃N₄. The photothermal catalytic CO₂ reduction pathway of Ag/g-C₃N₄ was investigated using in situ diffuse reflectance infrared Fourier transform spectra (DRIFTS). This study examines hot electron transfer in the Ag/g-C₃N₄ Schottky junction and provides a feasible way to design a plasmonic metal/polymer semiconductor Schottky junction for photothermal catalytic CO₂ reduction. Full article

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. Full article