Finite-difference Time-domain Tool Research Articles

Silicon nanostructure-based photonic MEMS sensor for biosensing application

We explored the design of an optical pressure sensor. The objective is to create a photonic microelectromechanical system-based cantilever sensor design to detect prostate-specific antigen, a protein biomarker associated with prostate cancer. Sensor’s performance for the early detection of cancerous cells is dependent on sensitivity. The designed sensor consists of a hexagonal ring integrated with microcantilevers. Two types of microcantilever such as rectangular profile and V profile with integrated photonic sensing layer are investigated for sensitivity, quality factor, and resonant wavelength. The analysis shows that a geometrical modification of microcantilever has a significant effect on sensitivity enhancement. The finite difference time domain tool is used for designing photonic ring resonator patches. Numerical analysis of microcantilever is considered to investigate surface stress behavior. The cantilever deformation due to applied pressure resulted in a change in the index of refraction. Results show that the sensitivity of 55 nm / MPa for a triangular-shaped microcantilever obtained is higher compared to the rectangular-shaped microcantilever with a sensitivity of 0.19 nm / MPa. The designed structures have significantly higher sensitivities than the previously published sensitivity of 3.27 nm / MPa at wavelength 1550 nm. The maximum quality factor obtained for the rectangular shape is 2852 and 77,510 for the triangular shape. Triangular-shaped microcantilever can minimize winding inflexibility or stiffness. This capability of the triangular-shaped microcantilever enhances feasibility in making the final packaging and future fabrication.

Nonthermal ablation of deep brain targets: A simulation study on a large animal model.

Purpose: Thermal ablation with transcranial MRI-guided focused ultrasound (FUS) is currently limited to central brain targets because of heating and other beam effects caused by the presence of the skull. Recently, it was shown that it is possible to ablate tissues without depositing thermal energy by driving intravenously administered microbubbles to inertial cavitation using low-duty-cycle burst sonications. A recent study demonstrated that this ablation method could ablate tissue volumes near the skull base in nonhuman primates without thermally damaging the nearby bone. However, blood–brain disruption was observed in the prefocal region, and in some cases, this region contained small areas of tissue damage. The objective of this study was to analyze the experimental model with simulations and to interpret the cause of these effects. Methods: The authors simulated prior experiments where nonthermal ablation was performed in the brain in anesthetized rhesus macaques using a 220 kHz clinical prototype transcranial MRI-guided FUS system. Low-duty-cycle sonications were applied at deep brain targets with the ultrasound contrast agent Definity. For simulations, a 3D pseudospectral finite difference time domain tool was used. The effects of shear mode conversion, focal steering, skull aberrations, nonlinear propagation, and the presence of skull base on the pressure field were investigated using acoustic and elastic wave propagation models. Results: The simulation results were in agreement with the experimental findings in the prefocal region. In the postfocal region, however, side lobes were predicted by the simulations, but no effects were evident in the experiments. The main beam was not affected by the different simulated scenarios except for a shift of about 1 mm in peak position due to skull aberrations. However, the authors observed differences in the volume, amplitude, and distribution of the side lobes. In the experiments, a single element passive cavitation detector was used to measure the inertial cavitation threshold and to determine the pressure amplitude to use for ablation. Simulations of the detector’s acoustic field suggest that its maximum sensitivity was in the lower part of the main beam, which may have led to excessive exposure levels in the experiments that may have contributed to damage in the prefocal area. Conclusions: Overall, these results suggest that case-specific full wave simulations before the procedure can be useful to predict the focal and the prefocal side lobes and the extent of the resulting bioeffects produced by nonthermal ablation. Such simulations can also be used to optimally position passive cavitation detectors. The disagreement between the simulations and the experiments in the postfocal region may have been due to shielding of the ultrasound field due to microbubble activity in the focal region. Future efforts should include the effects of microbubble activity and vascularization on the pressure field.

Plasmon resonance hybridization in self-assembled copper nanoparticle clusters: efficient and precise localization of surface plasmon resonance (LSPR) sensing based on Fano resonances.

In this work, we have investigated the hybridization of plasmon resonance modes in completely copper (Cu)-based subwavelength nanoparticle clusters from simple symmetric dimers to complex asymmetric self-assembled structures. The quality of apparent bonding and antibonding plasmon resonance modes for all of the clusters has been studied, and we examined the spectral response of each one of the proposed configurations numerically using the finite-difference time domain (FDTD) method. The effect of the geometric sizes of nanoparticles used and substrate refractive index on the cross-sectional profiles of each of the studied structures has been calculated and drawn. We proved that Fano-like resonance can be formed in Cu-based heptamer clusters as in analogous noble metallic particles (e.g., Au and Ag) by determining the coupling strength and interference between sub-radiant and super-radiant resonance modes. Employing certain Cu nanodiscs in designing an octamer structure, we measured the quality of the Fano dip formation along the scattering diagram. Accurate tuning of the geometric sizes for the Cu-based octamer yields an opportunity to observe isotropic, deep, and narrow Fano minima along the scattering profile that are in comparable condition with the response of other plasmonic metallic substances. Immersing investigated final Cu-based octamer in various liquids with different refractive indices, we determined the sensing accuracy of the cluster based on the performance of the Fano dip. Plotting a linear diagram of plasmon energy differences over the refractive index variations as a figure of merit (FoM), which we have quantified as 13.25. With this method, the precision of the completely Cu-based octamer is verified numerically using the FDTD tool. This study paves the way toward the use of Cu as an efficient, low-cost, and complementary metal-oxide semiconductor (CMOS)-compatible plasmonic material with optical properties that are similar to analogous plasmonic substances.

Direct laser writing of symmetry-broken nanocorrals and their applications in SERS spectroscopy

We propose a simple and fast approach to prepare surface-enhanced Raman scattering (SERS) substrates over a large area with high flexibility by using direct laser writing (DLW) technique. The proposal is demonstrated by the direct fabrication of an array and a complex of symmetry-broken nanocorrals with DLW followed by a metal deposition process. SERS measurements show significant SERS enhancement, which can be controlled through engineering the focused “hot spots” by changing the structural parameters. The experimental observations are further confirmed by our simulations with a finite-difference time-domain tool. The studies can be extended to versatile SERS substrates with arbitrary geometries.

Analysis of a UWB Hemispherical Antenna Array in FDTD With a Time Domain Huygens Method

Electromagnetic modeling is needed in a wide variety of large and complex situations. Existing numerical techniques such as finite difference time domain (FDTD) can, in principle, solve any problem no matter how large or complicated given a sufficiently powerful computer. However, in practice, there remain problems which require more computer power than is available. Recently, the time-domain Huygens (TDH) approach has been shown to be effective for enabling large problems such as propagation on body area networks (BANs) to be modeled in FDTD. In this paper, it is shown that the much larger problem of the conformal antenna array, which is used in the MARIA breast cancer tumor detection system, is also very amenable to this technique with even greater savings in computer resources. Accurate results are obtained using less that a tenth of the resources needed to solve the same problem using existing advanced FDTD tools. The details of how this method is applied, and the choices which need to be made, are discussed.

Modeling and simulation of multiple resonant Lorentz dispersive materials for integrated optics

Complex materials are of increasing interest in Finite-Difference Time-Domain (FDTD) modeling. For example, when the particle density becomes large, collisional fluid models of plasmas are an attractive alternative to particle in cell methods. Further, frequency dispersive meta-materials are of increasing interest. Thus, Finite-Difference Time-Domain (FDTD) models are derived for magnetized plasmas and for the Lorentz and Drude material models. This paper models the Lorentz material using the FDTD tool. The modal analysis and various types of simulation results are also presented for these materials.

Computational and experimental optimization of a double‐tuned 1H/31P four‐ring birdcage head coil for MRS at 3T

To optimize the homogeneity and efficiency of the B(1) magnetic field of a four-ring birdcage head coil that is double-tuned at the Larmor frequencies of both (31)P and (1)H and optimized to acquire magnetic resonance spectroscopy (MRS) data at 3T for the study of infants. We developed a finite difference time domain (FDTD) tool in-house to iteratively compute and seek the range of geometric and electromagnetic parameters of a dual-tuned, four-ring birdcage coil that would produce the desired resonance patterns, optimize homogeneity of the B(1)-field, and maximize efficiency of the coil. To demonstrate the validity of our computational results, we constructed three RF coils: one dual-tuned coil that was based on the calculated optimized parameters and two single-tuned coils that had dimensions similar to those of the dual-tuned coil, but tuned at the Larmor frequencies of both (31)P and (1)H, respectively. We then tested and compared the performances of the dual-tuned coil and single-tuned coils at both of these frequencies. We found that a dual-tuned, four-ring birdcage coil with a diameter of 180 mm, an inner birdcage length of 100-300 mm, and an outer birdcage length of 25-100 mm produces the desired resonance patterns. For the use of this coil with human infants, optimization of the homogeneity of the B(1) field, combined with improved coil efficiency, yielded a dual-tuned birdcage coil with diameter of 180mm, an inner birdcage length of 150 mm, an outer birdcage length of 25 mm, and corresponding inner and outer capacitances of 17.2 pF and 7.6 pF, respectively. The experimental results from a constructed coil having the same parameters with the modeled coil agreed well with the computational results from the modeled coil. This optimized design overcame the deficiencies of existing dual-tuned, four-ring birdcage coils. The homogeneity and efficiency of the B(1) field for (31)P/(1)H dual-tuned, four-ring birdcage coils can be optimized well using our FDTD tool, especially at high static magnetic fields (B(0)).

Detailed FD-TD analysis of electromagnetic fields penetrating narrow slots and lapped joints in thick conducting screens

The physics of electromagnetic-wave transmission through narrow slots and tapped joints in thick conducting screens is examined in detail by applying numerical models to compute both field distributions within the slots and joints, and fields transmitted to the shadow region. The primary modeling tool is the finite-difference time-domain (FD-TD) method, using a Faraday's-law contour integral approach to modify the basic FD-TD algorithm to properly model the slot physics, even when the slot gap width is much less than one space lattice cell. Finely sampled method-of-moments (MM) models are used to validate the FD-TD tool for relatively simple straight slots; FD-TD is then used to explore properties of more complicated lapped joints which are widely used for shielding purposes at junctions of metal surfaces. It is found that previously reported slot resonances occur in a more general sense for lapped joints as path-length resonances. In addition to greatly enhanced power transmission, path-length resonances can result in total fields within the joint exceeding the incident fields by more than one order of magnitude. Sample field distributions for this case are given.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>