Difference Time Domain Numerical Simulations Research Articles

Olver plasmon: an accelerating surface wave with various orders.

In this Letter, we introduce a new, to the best of our knowledge, class of accelerating surface plasmonic wave: the Olver plasmon. Our research reveals that such a surface wave propagates along self-bending trajectories at the silver-air interface with various orders, among which Airy plasmon is regarded as the zeroth-order one. We demonstrate a plasmonic autofocusing hot-spot by the interference of Olver plasmons and the focusing properties can be controlled. Also, a scheme for the generation of this new surface plasmon is proposed with the verification of finite difference time-domain numerical simulations.

Airy-Talbot plasmon: an accelerating self-imaging surface wave.

In this Letter, a new class of an accelerating self-imaging surface plasmonic wave, the Airy-Talbot plasmon, is introduced for the first time, to the best of our knowledge. Our research shows that such a surface wave propagates at the interface between metal (silver) and a dielectric material (air) and causes a strong interference along curved trajectories, which generates the Talbot effect in the surface. The propagation properties have potential value in nanoscale plasmonic devices. A scheme for generating this novel plasmon theoretically is proposed, and we prove it by finite difference time-domain numerical simulations.

GALYUM KATKILI ÇİNKO OKSİT NANOANTEN İLE MİKROLENS

Alternative plasmonics based fractal microlens are investigated. In this context, lensing performance of gallium-doped zinc oxide Sierpinski carpet-based fractal construction functionalized by conformal Talbot effect is analyzed for communication wavelength 1550 nm. Focusing via diffraction from these 2D finite-sized and two-iterated fractal lattice system is computationally demonstrated. In this regard, focusing performance parameters are computationally examined on the basis of geometrical parameter sweep and fractal generation via finite difference time-domain numerical simulations. Focusing efficiency > 50%, absolute 
 efficiency > 18%, and focal depth larger than primary spot size are introduced by all computational samples. Moreover, a conformal Talbot effect is exhibited by this novel alternative plasmonics construction. A novel perspective based on alternative plasmonics by a newly adapted fractal design to optics is proposed. Thus, this fractal microlens is presented as a new planarized focusing platform, acting a conformal transformation optics device for light capturing tolerance and low-cost.

Numerical modeling and analysis of a TM mode-division (de)multiplexer based on grating assisted couplers

In this paper, a mode-division (de)multiplexer capable of multiplexing three transverse magnetic modes simultaneously at the operating wavelength of 1550 nm has been proposed. The device structure is designed using slab waveguides with Si and SiO2 as core and cladding materials, respectively. Grating assisted couplers have been employed to couple the fundamental modes of two single-mode waveguides with the two higher-order modes of a multimode waveguide in the opposite direction. The fundamental mode of the multimode waveguide keeps propagating in the same waveguide without any power coupling to other modes. Using perturbation approach and coupled-mode theory, the proposed structure is theoretically analyzed. 2D Finite Difference Time Domain numerical simulation technique has been used to simulate and analyze the proposed mode division (de)multiplexer. During the demultiplexing operation, the proposed device exhibits insertion loss ranging from − 2.01 to − 0.01 dB, return loss ranging from − 31.07 to − 9.17 dB, and crosstalk in the range of − 41.76 to − 13.58 dB. Whereas for the multiplexing operation, insertion loss, return loss, and crosstalk range from − 1.59 to − 0.01 dB, − 31.07 to − 10.17 dB, and − 41.76 to − 8.05 dB respectively.

Conformal Talbot-effect-focusing performance of nested gallium-doped zinc oxide nanorings at communication wavelength

Metasurfaces, artificial micro- and nano-photonic structures can modify wavefront of electromagnetic waves to the desired form. Metalenses are the metasurfaces assuming the role of focusing light. The focusing performance of nested gallium-doped zinc oxide nanoring-arrangements functionalized by conformal Talbot effect is investigated for communication wavelength 1550 nm. The Talbot-effect-focusing performance of the 2D finite-sized structures are computationally demonstrated in a systematically manner. The impact of geometrical parameters on the primary focal spot and focusing performance parameters such as numerical aperture, full-width half-maximum, focal depth, efficiency, and fill-factor is examined and summarized via finite difference time-domain numerical simulations. All samples presenting an alternative plasmonic option in this work yield a good focusing efficiency of >50%, absolute efficiency of >19%, numerical aperture up to 0.67, focal depth up to 1.4 times of focal length, and a wide focal spot size up to 6.0654 times of operating wavelength. As a result of finite-sized construction, conformal Talbot effect is examined from a novel alternative plasmonics perspective. Thus, a new planarized focusing platform is employed by using a nanostructured microlens, acting conformal transformation optics devices with low-cost and light capturing tolerance.

Tunable plasmonic force switch based on graphene nano-ring resonator for nanomanipulation.

Using a plasmonic graphene ring resonator of resonant frequency 10.38 THz coupled to a plasmonic graphene waveguide, we design a lab-on-a-chip optophoresis system that can function as an efficient plasmonic force switch. Finite difference time domain numerical simulations reveal that an appropriate choice of chemical potentials of the waveguide and ring resonator keeps the proposed structure in on-resonance condition, enabling the system to selectively trap a nanoparticle. Moreover, a change of 250 meV in the ring chemical potential (i.e., equivalent to 2.029 V change in the corresponding applied bias) switches the structure to a nearly perfect off-resonance condition, releasing the trapped particle. The equivalent plasmonic switch ON/OFF ratio at the waveguide output is -15.519 dB. The designed system has the capability of trapping, sorting, controlling, and separating PS nanoparticles of diameters ≥30 nm with a THz source intensity of 14.78 mW/µm2 and ≥22 nm with 29.33 mW/µm2.

Monitoring of Evolving Laser Induced Periodic Surface Structures

Laser induced periodic surface structures (LIPSS) are generated on titanium and silicon nitride surfaces by multiple femtosecond laser pulses. An optical imaging system is used to observe the backscattered light during the patterning process. A characteristic fringe pattern in the backscattered light is observed and evidences the surface modification. Experiments are complemented by finite difference time domain numerical simulations which clearly show that the periodic surface modulation leads to characteristic modulations in the coherently scattered light field. It is proposed that these characteristic fringe pattern can be used as a very fast and low-cost monitor of LIPSS formation formation during the manufacturing process.

Modeling ultrasound attenuation in porous structures with mono-disperse random pore distributions using the independent scattering approximation: a 2D simulation study

The validity of the independent scattering approximation (ISA) to predict the frequency dependent attenuation in 2D models of simplified structures of cortical bone is studied. Attenuation of plane waves at central frequencies ranging from 1 to 8 MHz propagating in structures with mono-disperse random pore distributions with pore diameter and pore density in the range of those of cortical bone are evaluated by finite difference time domain numerical simulations. An approach to assess the multiple scattering of waves in random media is discussed to determine the pore diameter ranges at which the ISA is applicable. A modified version of the ISA is proposed to more accurately predict the attenuation in porosity ranges where it would traditionally fail. The results show that the modified ISA can model the frequency-dependent attenuation of ultrasonic wave with pore diameter and density ranges comparable to those of cortical bone.

Design and modelling of the photonic crystal fano structure for all optical switching applications

In this paper, the optical Fano switch based on the 2D slab photonic crystal is analyzed. Switching characteristics are improved by manipulating the Q factors of cavities. Modified device with enhanced Fano resonance has lower energy consumption than other previous optical switches. Results show that, the switching contrast is enhanced while the quality factor has remained in its highest value. Therefore, the typical important trade off between switching contrast and quality factor has vanished in the modified structure. The switching contrast is improved from 7 to 34 db in constant $$\Delta \lambda_{peak - peak}$$ of 0.3 nm in the modified structure which leads to decreasing the energy consumption of the device. In the previous devices, the maximum reported contrast is about 20 db for $$\Delta \lambda_{peak - peak}$$ of 0.6 nm. A coupled mode theory for analyzing the structure is developed. The results of finite difference time domain numerical simulation are in good agreement with the theoretical coupled mode theory. Results show that the switching contrast of the modified Fano switch is enhanced two times, consequently the loss is suppressed and the bandwidth is improved.

Light-absorption enhancement design of ultrathin perovskite solar cells with conformal structure

We report a structural design of ultrathin perovskite solar cells based on a conformal structure at the rear surface for potential applications in both single-junction and tandem cells. The light transmittances of the front and the rear surfaces are calculated in the wavelength range of 300–800 nm via the finite difference time domain numerical simulation method. Compared with the reference cell, significant photocurrent density enhancement of 27.5% and 29.7% are achieved when the ratios of height to width of the fluorine doped tin oxide nanoblock are 2 and 3, respectively. For the case with a hole transport material layer, the enhancements of photocurrent density enhancements are 19.2% and 29.0%, respectively. When back Au is removed, the photocurrent density also has notable enhancements of 23.3% and 45.9%, respectively. The achieved results are beneficial for the development of efficient ultrathin single-junction and tandem perovskite solar cells.

Spectrally enhanced near-field radiation transfer using nanometer-sized pillar array structured surfaces

This study shows that near-field radiation transfer is spectrally enhanced using nanometer-sized square pillar array structured surfaces, which are faced toward each other with a vacuum gap of a few hundred nanometers. In this case, the emitter exhibits a temperature of 1000K, while the receiver exhibits a temperature of 300K. Moreover, the pillar array structured surfaces made of aluminum-doped zinc oxide (AZO) were assumed. The electromagnetic fields inside AZO and in the vacuum gap were calculated using the finite difference time domain numerical simulation method, which uses a spherical emission source with a sinusoidal modulated Gaussian pulse distributed inside the AZO. As a result, there are local maximum radiation fluxes at the fundamental frequency originating from the Fabry–Pèrot interference between the fundamental wavelength and the pillar height, at its second and third harmonic frequencies, and at the asymptote frequency of the surface wave relating to the plasma frequency. The most striking feature is that the radiation flux at the fundamental frequency becomes more than 30 times higher than that for the far-field blackbody radiation transfer by using a pillar width and a channel width of 80nm, because the electromagnetic energy emitted spherically is collimated in the direction from the emitter to the receiver.

Characterization of the Lung Parenchyma Using Ultrasound Multiple Scattering

The purpose of the study described here was to showcase the application of ultrasound to quantitative characterization of the micro-architecture of the lung parenchyma to predict the extent of pulmonary edema. The lung parenchyma is a highly complex and diffusive medium for which ultrasound techniques have remained qualitative. The approach presented here is based on ultrasound multiple scattering and exploits the complexity of ultrasound propagation in the lung structure. The experimental setup consisted of a linear transducer array with an 8-MHz central frequency placed in contact with the lung surface. The diffusion constant D and transport mean free path L* of the lung parenchyma were estimated by separating the incoherent and coherent intensities in the near field and measuring the growth of the incoherent diffusive halo over time. Significant differences were observed between the L* values obtained in healthy and edematous rat lungs in vivo. In the control rat lung, L* was found to be 332 μm (±48.8 μm), whereas in the edematous lung, it was 1040 μm (±90 μm). The reproducibility of the measurements of L* and D was tested in vivo and in phantoms made of melamine sponge with varying air volume fractions. Two-dimensional finite difference time domain numerical simulations were carried out on rabbit lung histology images with varying degrees of lung collapse. Significant correlations were observed between air volume fraction and L* in simulation (r = −0.9542, p < 0.0117) and sponge phantom (r = −0.9932, p < 0.0068) experiments. Ex vivo measurements of a rat lung in which edema was simulated by adding phosphate-buffered saline revealed a linear relationship between the fluid volume fraction and L*. These results illustrate the potential of methods based on ultrasound multiple scattering for the quantitative characterization of the lung parenchyma.

Morphological Design of Gold Nanopillar Arrays and Their Optical Properties

The fabrication of large areas of gold nanopillar arrays by combining nanosphere lithography and electrodeposition of metals is demonstrated here for the first time. Both the morphology and the surface density of the nanopillars can be finely tuned by controlling the size of the colloidal beads used in the template, the sintering time of the template and the electrodeposition time of metals. The colloidal templates are removed after electrodeposition, and the nanopillar arrays can be transferred onto a transparent polydimethylsiloxane substrate and optically characterized. Our focus here is on the ease and versatility of this technique to prepare large two-dimensional arrays of plasmonic gold nanopillars and their resulting optical properties. By making use of finite difference time domain numerical simulations, the excitation of localized surface plasmon resonances of the individual nanopillar with a strong dipolar character is observed, which is characteristic of such uncoupled metallic nanostructures. ...

Complex dielectric properties of anhydrous polycrystalline glucose in the terahertz region

We utilized terahertz time-domain spectroscopy to investigate the complex dielectric properties of solid polycrystalline material of anhydrous glucose. The imaginary part of the complex dielectric function of polycrystalline glucose showed that there were multiple characteristic absorption peaks from 0.2 to 2.2 THz. Moreover, for a given characteristic absorption peak, the real part of the complex dielectric function showed anomalous dispersion within the full width half maximum of the absorption peak. Both finite difference time-domain numerical simulations and experimental results showed that the complex dielectric function of glucose fits well with the Lorentz dielectric model. The plasma oscillation frequency was below the frequency of the light waves suggesting that the light waves passed through the polycrystalline glucose pellets. Calculations of multi-molecules of glucose based on density functional theory interpreted approximately that the characteristic absorption peaks of polycrystalline glucose originated mainly from collective intermolecular vibrations such as hydrogen bonds and crystal phonon modes.

Near-field distribution and propagation of scattering resonances in Vogel spiral arrays of dielectric nanopillars

In this work, we employ scanning near-field optical microscopy, full-vector finite difference time domain numerical simulations and fractional Fourier transformation to investigate the near-field and propagation behavior of the electromagnetic energy scattered at 1.56 μm by dielectric arrays of silicon nitride nanopillars with chiral α1-Vogel spiral geometry. In particular, we experimentally study the spatial evolution of scattered radiation and demonstrate near-field coupling between adjacent nanopillars along the parastichies arms. Moreover, by measuring the spatial distribution of the scattered radiation at different heights from the array plane, we demonstrate a characteristic rotation of the scattered field pattern consistent with net transfer of orbital angular momentum in the Fresnel zone, within a few micrometers from the plane of the array. Our experimental results agree with the simulations we performed and may be of interest to nanophotonics applications.

In vivo13C magnetic resonance spectroscopy of human brain on a clinical 3 T scanner using [2‐13C]glucose infusion and low‐power stochastic decoupling

This study presents the detection of [2-(13)C]glucose metabolism in the carboxylic/amide region in the human brain, and demonstrates that the cerebral metabolism of [2-(13)C]glucose can be studied in human subjects in the presence of severe hardware constraints of widely available 3 T clinical scanners and with low-power stochastic decoupling. In the carboxylic/amide region of human brain, the primary products of (13)C label incorporation from [2-(13)C]glucose into glutamate, glutamine, aspartate, gamma-aminobutyric acid, and N-acetylaspartate were detected. Unlike the commonly used alkanyl region where lipid signals spread over a broad frequency range, the carboxylic carbon signal of lipids was found to be confined to a narrow range centered at 172.5 ppm and present no spectral interference in the absence of lipid suppression. Comparison using phantoms shows that stochastic decoupling is far superior to the commonly used WALTZ sequence at very low decoupling power at 3 T. It was found that glutamine C1 and C5 can be decoupled using stochastic decoupling at 2.2 W, although glutamine protons span a frequency range of approximately 700 Hz. Detailed specific absorption rate analysis was also performed using finite difference time domain numerical simulation.

High-frequency wave propagation in suspensions of elastic particles: Experimental validation of a time domain numerical simulation.

Sound propagation in suspensions of elastic particles immersed in a fluid is a difficult problem due to (i) the random distribution of the particles and (ii) possible multiple scattering and mode conversion effects. The ultrasonic propagation in suspensions has mostly been studied in the frequency domain and time domain approaches may also be of interest. Here, two‐dimensional finite difference time domain numerical simulations of ultrasonic propagation in suspensions of polystyrene 6‐μm‐diameter microspheres immersed in water were performed around 50 MHz. The numerical results were compared with the Faran analytical model, leading to a maximum difference of 15%, which constitutes a validation of the numerical approach. Experiments were carried out in transmission and backscattering modes. The attenuation coefficient 50 MHz [α(50)], the ultrasonic velocity (V) and the relative backscattered intensity (IB) were measured for different concentrations (from 2 to 25 mg/ml). Each experimental ultrasonic paramet...

Verifying the attenuation of earplugs in situ: Method validation using artificial head and numerical simulations

The use of in situ measurements of hearing protectors' (HPD's) attenuation following the microphone in real ear (MIRE) protocol is increasing. The attenuation is hereby calculated from the difference in sound levels outside the ear and inside the ear canal behind the HPD. Custom-made earplugs have been designed with an inner bore that allows inserting a miniature microphone. A thorough understanding of the difference, henceforth called transfer function, between the sound pressure of interest at the eardrum and the one measured at the inner bore of the HPD is indispensable for optimizing the MIRE technique and extending its field of application. This issue was addressed by measurements on a head-and-torso-simulator and finite difference time domain numerical simulations of the outer ear canal occluded by an earplug. Both approaches are in good agreement and reveal a clear distinction between the sound pressure at the MIRE microphone and at eardrum, but the measured transfer functions appear to be stable and reproducible. Moreover, the most striking features of the transfer functions can be traced down to the geometrical and morphological characteristics of the earplug and ear canal.

Finite difference time domain numerical simulation of ultrasonic propagation in coated contrast agents

Acoustic wave propagation in solutions of ultrasound contrast agents (UCA) designed for in situ drug delivery remains unclear due to the complex interaction between ultrasound and the coated particles (mode conversion, scattering, multiple reflections). To address this problem, two-dimensional Finite Difference Time Domain numerical simulations of ultrasound transmission and reflection were performed in solutions of an aqueous suspension of spheres made of fluid surrounded by a polymeric capsule. Simulations were made for particle concentrations equivalent ranging from 2 to 87 mg/ml. For each concentration 15 aqueous solutions with randomly located particles were considered. The speed of sound c, the attenuation coefficient and the integrated backscattered coefficient were computed from the transmitted and backscattered signals. The values of c decrease from 1498 m/s to 1432 m/s when the concentration increases, in good agreement with an effective medium model. The attenuation coefficient at 5 MHz is an increasing function of the concentration, from 0.05 to 1.2 dB/cm. The integrated backscattered coefficient increases with the concentration for low concentration values but decreases for high concentration (with a maximum value for a concentration of about 50 mg/ml). Our numerical simulations successfully mimic experimental results of ultrasound propagation in solutions of UCA.