Full-wave Finite-difference Time-domain Research Articles

Development of a SERS Probe for Selective Detection of Healthy Prostate and Malignant Prostate Cancer Cells Using ZnII.

Even in the 21st century, prostate cancer remains the second leading cause of cancer-related death for men. Since a normal prostate gland has a high ZnII content and there are huge differences in ZnII content between healthy and malignant prostate cancer cells, mobile zinc can be used as a biomarker for prostate cancer prediction. A highly efficient surface enhanced Raman spectroscopy (SERS) probe using a p-(imidazole)azo)benzenethiol attached gold nanoparticle as a Raman reporter, which has the capability to identify prostate cancer cells based on ZnII sensing, has been designed. A facile synthesis, characterization and evaluation of a ZnII sensing Raman probe are described. Reported data indicate that after binding with ZnII , Raman reporter attached to a gold nanoparticle forms an assembly structure, which allows selective detection of ZnII even at 100 ppt concentration. Theoretical full-wave finite-difference time-domain (FDTD) simulations have been used to understand the enhancement of the SERS signal. The SERS probe is highly promising for in vivo sensing of cancer, where near-IR light can be easily used to avoid tissue autofluorescence and to enhance tissue penetration depth. Reported data show that the SERS probe can distinguish metastatic cancer cells from normal prostate cells very easily with a sensitivity as low as 5 cancer cells mL-1 . The probe can be used as a chemical toolkit for determining mobile ZnII concentrations in biological samples.

Large enhancement of third-order nonlinear effects with a resonant all-dielectric metasurface

A novel low-profile nonlinear metasurface, consisting of a single-layer of all-dielectric material, is proposed and numerically investigated by a nonlinear full-wave finite-difference time-domain (FDTD) method. The proposed metasurface is transparent for low, and opaque for high values of incident light intensity. The metasurface design is broadly applicable to enhancement of intrinsic nonlinearities of any material with a sufficiently high refractive index contrast. We illustrate the ability of this design to enhance intrinsic nonlinear absorption of a transition metal oxide, vanadium pentoxide (V2O5), with resonant metasurface elements. The complex third-order nonlinear susceptibility (χ(3)) for V2O5, representing both nonlinear refraction and absorption is considered in FDTD simulations. Our design achieves high initial transparency (>90%) for low incident light intensity. An order of magnitude decrease in the required input light intensity threshold for nonlinear response of the metasurface is observed in comparison with an unpatterend film. The proposed all-dielectric metasurface in this work is ultrathin and easy to fabricate. We envision a number of applications of this design for thin film coatings that offer protection against high-power laser radiation.

Large-area, lithography-free, low-cost SERS sensor with good flexibility and high performance

Cost-effective, sensitive and bio-compatible surface-enhanced Raman spectroscopy (SERS) substrate has been in high demand since the Raman spectrum was designated as a significant tool for analyzing the composition of liquids, gases and solids in 1998 []. In this research, we presented the design, fabrication and characterization of an improved gold-based SERS substrate. With fine tuning of the SiO2 thickness we achieved a 3.391 times improvement and achieved an enhancement factor of 1.55 * 107 which is 15 times better than the current gold-standard Klarite substrate. Such improvement is ascribed to the localized surface plasmon resonance (SPR) and propagating SPR, which is proved by full-wave finite-difference time-domain simulations.

Electromagnetic resonances of solar-selective absorbers with nanoparticle arrays embedded in a dielectric layer

We numerically investigate a solar-selective absorber with tungsten core–shell nanoparticle arrays embedded in an SiO2 layer. The 3D full-wave finite-difference time-domain (FDTD) simulations are performed to investigate the geometric effects of different types of solar-selective absorbers. Consequently, broadband light absorption was achieved with either a tungsten nanoparticle array or a tungsten core–shell nanoparticle array because of the strong electric field enhancement in the gap between the core nanoparticles. The solar performance of the proposed structure is shown for high-efficiency solar light absorption. This study enhances understanding of the light absorption mechanism of metallic nanoparticle/dielectric composite and facilitates the design of high-efficiency solar-selective absorbers.

Low-Profile Antenna With Elevated Toroid-Shaped Radiation for On-Road Reader of RFID-Enabled Vehicle Registration Plate

A low-profile antenna installed on the road surface for RFID-enabled vehicle communication is presented. The antenna in this application operates as on-road sensor for vehicles identification. To that end, the antenna is required to have an elevated toroid-shaped radiation, so that the reader can easily interrogate an RFID-enabled vehicle license. The antenna is designed using a capacitive-loaded inverted-F radiator, which has an omnidirectional radiation in the azimuth plane. To adjust the radiation pattern to the required elevation above the road surface, and isolate the radiation from the effects of variable road conditions, a corrugated ground plane with perturbed surface impedance is used underneath the radiator. The antenna’s configuration is verified by full-wave finite difference time domain simulations with various road surface conditions. A prototype antenna is then developed and tested in realistic field scenarios as on-road sensor for vehicles identification. The results show that the antenna possess more than 15 dB return loss, more than 1.5 dBi gain, vertical polarization, and desirable radiation pattern from 880 to 960 MHz, which is the standard band licensed for RFID-enabled plates. The antenna also successfully communicates with an RFID-enabled vehicle registration plate.

A theoretical investigation on the parametric instability excited by X‐mode polarized electromagnetic wave at Tromsø

AbstractRecent ionospheric modification experiments performed at Tromsø, Norway, have indicated that X‐mode pump wave is capable of stimulating high‐frequency enhanced plasma lines, which manifests the excitation of parametric instability. This paper investigates theoretically how the observation can be explained by the excitation of parametric instability driven by X‐mode pump wave. The threshold of the parametric instability has been calculated for several recent experimental observations at Tromsø, illustrating that our derived equations for the excitation of parametric instability for X‐mode heating can explain the experimental observations. According to our theoretical calculation, a minimum fraction of pump wave electric field needs to be directed along the geomagnetic field direction in order for the parametric instability threshold to be met. A full‐wave finite difference time domain simulation has been performed to demonstrate that a small parallel component of pump wave electric field can be achieved during X‐mode heating in the presence of inhomogeneous plasma.

Hyperlensing at NIR frequencies using a hemispherical metallic nanowire lens in a sea-urchin geometry.

Label-free and real time far-field super-resolution imaging (hyperlensing) at the nanoscale is of significant interest where sub-λ imaging remains a constraint because of Abbe's diffraction limit. Though by utilizing anisotropic permittivities, metal-dielectric multilayers have been successful in reconstructing high-frequency components of sub-λ objects, yet they remain cumbersome and expensive to make. Most of the multilayer structures require multiple vacuum deposition cycles and are plagued by stringent requirements on the surface roughness of metallic layers. In contrast to the multilayer structure here we propose a 3D hyperbolic metamaterial model composed of metallic nanorods arranged in a sea-urchin geometry as a hyper-lensing device, which is capable of projecting and magnifying diffraction limited information into the far-field at Near-infrared (NIR) frequencies. The hyperlens generates a band of flat hyperbolic dispersions in spherical coordinates, which in turn supports the propagation of high wave-vector spatial harmonics leading to far-field super-resolution imaging. Using full-wave finite-difference time-domain (FDTD) simulations with diffraction limited trimer, quadrumer and ringed objects etched on thin perfect electric conductor (PEC) films, we show that the hyperlens model can achieve magnification factors of up to 10× in the far-field (∼4.5λ from the object's surface) under a light source with a wavelength of 1 μm, with successful resolution down to 220 nm (∼λ/5). The magnified image field distribution projected into the far-field is shown to follow the object under a reduction in the symmetry. These results are important for making progress in the realization of real-time biomolecular imaging systems, eliminating the need for near-field scanning, destructive electron microscopy and various image post-processing techniques.

Plasmon-controlled excitonic emission from vertically-tapered organic nanowires.

Organic molecular nanophotonics has emerged as an important avenue to harness molecular aggregation and crystallization on various functional platforms to obtain nano-optical devices. To this end, there is growing interest to combine organic molecular nanostructures with plasmonic surfaces and interfaces. Motivated by this, herein we introduce a unique geometry: vertically-tapered organic nanowires grown on a plasmonic thin film. A polarization-sensitive plasmon-polariton on a gold thin-film was harnessed to control the exciton-polariton propagation and subsequent photoluminescence from an organic nanowire made of diaminoanthraquinone (DAAQ) molecules. We show that the exciton-polariton emission from individual DAAQ nanowires can be modulated up to a factor of 6 by varying the excitation polarization state of surface plasmons. Our observations were corroborated with full-wave three-dimensional finite-difference time-domain calculations performed on vertically-tapered nanowire geometry. Our work introduces a new optical platform to study coupling between propagating plasmons and propagating excitons, and may have implications in emerging fields such as hybrid-polariton based light emitting devices and vertical-cavity nano-optomechanics.

The effect of street canyon design on traffic noise exposure along roads

Abating road traffic noise pollution is one of the main urban environmental challenges nowadays. However, architects and urbanists take decisions on urban regulations that define the shape of streets and buildings without taking this aspect into account. Furthermore, there is little information about the influence of urban geometry on traffic noise exposure in streets. In this study, the effect of street canyon design on sound pressure level distribution is numerically studied in high detail with the full-wave finite-difference time-domain method (FDTD). The CNOSSOS equivalent source power spectra were used to approach road traffic noise sources along two traffic lanes. Receivers both along the façades and the sidewalks have been considered in 42 cases. Numerical results demonstrate that building shape, street geometry and the presence of street furniture can have a strong impact on people's noise exposure. Building shape can be responsible for variations of up to 7.0 dB(A) at pedestrians. Building-façade design can reduce the average exposure at windows with 12.9 dB(A). It was further predicted that street geometry can enhance the positive effect of low barriers to 11.3 dB(A) along sidewalks. It was therefore concluded that carefully designing building façades and street geometry could improve the sound climate for people living and walking along busy urban streets and should be considered in future urban street design.

FDTD Study of Half Cloak in Bipolar Cylindrical Shape With Compressed Geometry and Complementary Media

Full-wave finite-difference time-domain (FDTD) analysis of an invisibility cloak with the cloaking shell, not necessarily enclosing the cloaking region completely, is presented for bipolar cylindrical geometry. The permittivity and the permeability tensors for the cloaking shell are obtained from an effective medium approach in general relativity. The cloak contains the complementary region adjacent to the cloaking shell. The role of the complementary medium is to cancel the phase vectors of the illuminated electromagnetic waves across the boundaries of the line segment. By intelligent design of a cloaking shell and a complementary medium, it is shown that the same cloaking performance as the conventional cloak can be achieved by using only the half of the cloaking shell required for the conventional invisibility cloak.

Finite-difference time-domain study of modulated and disordered coupled resonator optical waveguide rotation sensors.

We present a full-wave finite difference time domain (FDTD) study of a coupled resonator optical waveguide (CROW) rotation sensor consisting of 8 doubly degenerate ring resonators. First we demonstrate the formation of rotation-induced gap in the spectral pass-band of the CROW and show the existence of a dead-zone at low rotation rates which is mainly due to its finite size and partly because of the individual cavities losses. In order to overcome this deficiency, we modulate periodically the refractive indices of the resonators to effectively move CROW's operating point away from this dead-zone. Finally, we analyze the performance of a structurally disordered CROW to model the unavoidable fabrication errors and inaccuracies. We show that in some cases structural disorder can increase the sensitivity to rotation by breaking the degeneracy of the resonators, thus making such CROW even more sensitive to rotation than its unperturbed ideal counterpart.

Compact-range accuracy improvement: ground-reflection-error mitigation using surface patterning [AMTA corner

Compact-range measurement facilities have been used successfully for many years to characterize antenna performance as well as radar signatures. This paper investigates strategies for improving compact-range-measurement accuracy by mitigating errors associated with ground reflections inherent in most range designs. A methodology is developed for strategically modifying, or patterning, the surface between the range's source antenna and the reflector to reduce error terms, thereby increasing measurement accuracy. Candidate patterns were evaluated using a full-wave computational Finite-Difference Time-Domain (FDTD) model at VHF/UHF frequencies to determine baseline performance, and to develop trade rules for more advanced designs. Physical Optics (PO) models were used to analyze the final design at the frequencies of interest.

A Systematic Electromagnetic-Circuit Method for EMI Analysis of Coupled Interconnects on Dispersive Dielectrics

This paper presents a systematic electromagnetic-circuit method for efficient simulation of coupled interconnects on dispersive dielectrics in the presence of electromagnetic interference. The dispersive substrate of coupled interconnects is characterized by the multiterm Debye or Lorentz formula. According to the differential equations derived from the equivalent circuit modeling of dispersive media, the full-wave finite-difference time-domain algorithm is utilized to capture accurate field in a dispersive substrate. The effects of incident field, under the quasi-TEM assumption, are then represented by the distributed sources along transmission lines. An efficient algorithm based on the waveform relaxation with transverse partitioning (WR-TP) is employed for fast simulation of coupled transmission lines. A simple and clear derivation of the relaxation sources in WR-TP is described and the compact results are given. The impact of dispersive dielectrics on the incident field, and thus to the transient response of interconnects, is investigated by comparison with conventional techniques. Several numerical examples are given to illustrate the accuracy and efficiency of the proposed method.

3D Nanopillar optical antenna photodetectors

We demonstrate 3D surface plasmon photoresponse in nanopillar arrays resulting in enhanced responsivity due to both Localized Surface Plasmon Resonances (LSPRs) and Surface Plasmon Polariton Bloch Waves (SPP-BWs). The LSPRs are excited due to a partial gold shell coating the nanopillar which acts as a 3D Nanopillar Optical Antenna (NOA) in focusing light into the nanopillar. Angular photoresponse measurements show that SPP-BWs can be spectrally coincident with LSPRs to result in a x2 enhancement in responsivity at 1180 nm. Full-wave Finite Difference Time Domain (FDTD) simulations substantiate both the spatial and spectral coupling of the SPP-BW / LSPR for enhanced absorption and the nature of the LSPR. Geometrical control of the 3D NOA and the self-aligned metal hole lattice allows the hybridization of both localized and propagating surface plasmon modes for enhanced absorption. Hybridized plasmonic modes opens up new avenues in optical antenna design in nanoscale photodetectors.

Dispersive finite-difference time-domain (FDTD) analysis of the elliptic cylindrical cloak

A dispersive full-wave finite-difference time-domain (FDTD) model is used to calculate the performance of elliptic cylindrical cloaking devices. The permittivity and the permeability tensors for the cloaking structure are derived by using an effective medium approach in general relativity. The elliptic cylindrical invisibility devices are found to show imperfect cloaking, and the cloaking performance is found to depend on the polarization of the incident waves, the direction of the propagation of those waves, the semi-focal distances and the loss tangents of the meta-material. When the semi-focal distance of the elliptic cylinder decreases, the performance of the cloaking becomes very good, with neither noticeable scatterings nor field penetrations. For a larger semi-focal distance, only the TM wave with a specific propagation direction shows good cloaking performance. Realistic cloaking materials with loss still show a cloak that is working, but attenuated back-scattering waves exist.

Extreme control of light in metamaterials: Complete and loss-free stopping of light

Abstract We present an overview of recent advances within the field of slow- and stopped-light in metamaterial and plasmonic waveguides. We start by elucidating the mechanisms by which these configurations can enable complete stopping of light. Decoherence mechanisms may destroy the zero-group-velocity condition for real-frequency/complex-wavevector modes, but we show that metamaterial and nanoplasmonic waveguides also support complex-frequency/real-wavevector modes that uphold the light-stopping condition. A further point of focus is how, by using gain, dissipative losses can be overcome in the slow- and stopped-light regimes. To this end, on the basis of full-wave finite-difference time-domain (FDTD) simulations and analytic transfer-matrix calculations, we show that the incorporation of thin layers made of an active medium, placed adjacently to the core layer of a negative-refractive-index waveguide, can fully remove dissipative losses – in a slow- or stopped-light regime where the effective index of the guided lightwave remains negative.

Improving the Accuracy of FDTD Approximations to Tangential Components of the Coupled Electric and Magnetic Fields at a Material Interface

The finite-difference time-domain (FDTD) method is one of the most popular tools used for modeling important electromagnetic wave propagation and scattering problems. Of the many variations on the basic formulation, conventional orthogonal schemes remain prevalent because of their high level of accuracy and relative ease of implementation. Inaccuracies do persist, however, and in this paper a simple, computationally efficient remedy to an error that occurs when simulating waves that impinge on a material interface is identified. The detail of when and how the error occurs is illustrated using a simplistic interface scenario, with an analytic electric field being used to drive a single FDTD magnetic field update step. The resulting FDTD approximations are compared to the analytic solution to reveal the extent of the error. The proposed modification is then introduced and its remedial effect shown by repeating the above comparison using the modified FDTD equations. The broader effects of the proposed modification are demonstrated using a practical, 3D, full-wave FDTD forward modeling tool, taking scenario examples from the application area of ground penetrating radar (GPR). It is concluded that the overall accuracy of conventional, orthogonal FDTD schemes is improved, without unduly increasing their computational burden or algorithmic complexity.

Characterization of large array of plasmonic nanoparticles on layered substrate: dipole mode analysis integrated with complex image method

In this paper, an efficient analytical method for characterizing large array of plasmonic nanoparticles located over planarly layered substrate is introduced. The model is called dipole mode complex image (DMCI) method since the main idea lies in modeling a subwavelength spherical nanoparticle at its electric scattering resonance with an induced electric dipole and representing the electromagnetic (EM) fields of this electric dipole over the layered substrate in terms of finite complex images. The major advantages of the proposed method are its accuracy and rapid calculation in characterizing various kinds of large periodic and aperiodic arrays of nanoparticles on layered substrates. The computational time can be reduced significantly in compared to the traditional methods. The accuracy of the theoretical model is validated through comparison with numerical integration of Sommerfeld integrals. Moreover, the analytical results are compared well with those determined by full-wave finite difference time domain (FDTD) method. To demonstrate the capability of our technique, the performances of large arrays of nanoparticles on layered silicon substrates for efficient sunlight energy incoupling are studied.

Broadband Electromagnetic Radiation Modulated by Dual Memristors

Using dual high-speed memristors, we report on an efficient broadband electromagnetic radiation from a narrowband microstrip patch antenna. The directly modulated microstrip patch antenna system with dual memristors is calculated by using an integrated full-wave finite-difference time-domain method with an embedded SPICE3 solver. Nonlinear transient electromagnetic response is analyzed. The radiation frequency spectrum demonstrates the broadband radiation performance from the narrowband antenna system.

Simulation of reflectometry Bragg backscattering spectral responses in the absence of a cutoff layer

Experimental reflectometry signals obtained in the absence of a cutoff layer, with the possibility of interferometric operation excluded, show a coherent and recurrent frequency spectrum signature similar to an Alfvén cascade signature. A possible explanation resides in the modulation of a resonant Bragg backscattering response by an Alfvén mode structure located at the center of the plasma whose frequency of oscillation modulates the backscattered signal in a conformable way. This situation is modeled and simulated using an O-mode full-wave Maxwell finite-difference time-domain code and the resulting signatures are discussed.