Near-Field Scattering Research Articles

Measurement of Nanofiber Mechanical Flexural Modes Based on Near-Field Scattering.

We systematically investigated the intrinsic mechanical flexural modes of tapered optical fibers (TOFs) with a high aspect ratio up to 3×10^{4}. Based on the near-field scattering of the hemispherical microfiber tip to the vibrating TOF evanescent field, we detected more than 320 ordered intrinsic mechanical modes through the TOF transmission spectra which was enhanced by 72dB compared to without near-field scattering. The trend of the vibration amplitude with the mode order was similar to pendulum waves. Our results open a pathway to study the mechanical modes of photonic microstructures-nanostructures that are expected to be used in waveguide QED, cavity optomechanical, and optical sensing.

Validation of a Long-Wavelength, Near-Field Scattering Simulator Based on Boundary Relaxation

Validation of a Long-Wavelength, Near-Field Scattering Simulator Based on Boundary Relaxation

Spectral and Near-Field Scattering Characteristics of Two-Layered Metal-Containing Nanoparticles in Absorbing Matrices

Spectral and Near-Field Scattering Characteristics of Two-Layered Metal-Containing Nanoparticles in Absorbing Matrices

Analyzing Three-dimensional Tip Near-field Scattering of Infrared Polaritons through Peak Force Scattering-type Near-field Optical Microscopy.

Analyzing Three-dimensional Tip Near-field Scattering of Infrared Polaritons through Peak Force Scattering-type Near-field Optical Microscopy.

Near Field Scattering Optical Model-Based Catalyst Design for Artificial Photoredox Transformation

The sun offers a clean and renewable energy cornucopia, whereas how to utilize this inexhaustible yet decentralized energy with the assistance of appropriate media is an enduring topic. Plasmonic metal-based nanostructures (mainly Au and Ag) represent a class of fascinating materials for solar energy conversion due to their exclusive surface plasmon resonance (SPR)-enabled light-harvesting capability. In contrast to these plasmonic metals with characteristic SPR peaks, modulating the optical absorption peaks of nonplasmonic metals (e.g., small Pt nanoparticles) in the visible region but without size alteration is a grand challenge. Until recently, we demonstrated a near field scattering (NFS) optical model to manipulate the absorption peaks of Pt nanoparticles by adjusting their dielectric environment, sowing the seeds for tuning the optical absorption property of other metal nanoparticles and even semiconductor quantum dots. It is the purpose of the present Perspective to retrospect the history of NFS optical model and guide the function-oriented design of NFS-based nanostructures for artificial photoredox transformation.

Selective Mapping of Tip-Launched Near-Field Scattering of Surface Plasmon Polaritons for Retrieving Dispersion Relation in Silver Nanoflakes

Scattering-type near-field scanning optical microscopy (s-NSOM) has been beneficial for observing various nano-optical phenomena by overcoming the spatial resolution diffraction limit, creating optical dispersion, and investigating optoelectronic systems. We used s-NSOM on mono-crystalline silver nanoflakes to selectively decompose tip-launched surface plasmon polariton (SPP) signals and suppress the undesired geometrical effects arising from the silver nanoflakes and s-NSOM tips. Comparing the Fourier transform analysis data from all the edges of silver nanoflakes, we successfully identified the momentum of tip-launched SPPs. We have conducted energy spectral measurements on mono-crystalline silver nanoflakes and applied a geometry-independent factor: tip-launched SPP. We neglected the additional complex contribution from various shapes of the nanoflakes in the visible-near infrared regime. These experimentally obtained momentums of SPPs have a good agreement with analytical calculations. We observed that the propagation of the SPP increases with a decrease in excitation energy even without additional destructive fabrications on the surface, although the surface quality of silver nanoflakes is affected by dirt and oxidation. This study provides a precise interpretation of the experimental studies on the various polariton studies using s-NSOM.

Scatterers in the Rx Near Field for RF Energy Harvesting Efficiency Enhancement

In this paper, we investigate the enhancement of RF–RF energy harvesting efficiency (erf–rf) in multipath environments in the context of wireless power transfer (WPT). For this, we used a retrodirective transmitting (Tx) antenna array resonating at 2.4 GHz and a receiving (Rx) antenna surrounded by scatterers placed in the Rx near field. The Rx resides in the Tx far field. We showed that in a medium made of a random distribution of scatterers, a time-reversed wave field interacts with the random medium to regenerate not only the propagating waves but also the evanescent waves required to refocus the energy at the receiver location. The system was enclosed inside a 3 m3 cubical reverberating room to create a strong multipath environment. The study was done for homogeneous (free space) and heterogeneous (multipath environment) media. Different WPT scenarios were considered for different applications: 4 × 1, 4 × 2 and 4 × 4 multi input-multi output (MIMO) systems. The simulation results show that using near-field scattering generates signal focusing at the source location, which increases the RF–RF energy harvesting efficiency, especially in a multipath environment. The average received power in the frequency band 2.4–2.5 GHz was greatly enhanced in the presence of the scatterers. The investigated WPT approach showed encouraging results for charging/powering-up sensors, IoT and smart devices in indoor environments.

An Effective High-Frequency Method for the Near-Field Scattering From an Electrically Large Ship Illuminated by a Hertzian Dipole

This letter is dedicated to develop an effective high-frequency method for the near-field (NF) scattering from an electrically large ship illuminated by a Hertzian dipole. First, for primary illumination, a local expansion technique based on the facets of the target is introduced to account the NF scattering, which has a singularity-free characteristic. Second, for secondary rays, the ray-generation technique on the sphere grid is proposed to reduce the number of ray tubes. And the ray-tracing strategy based on one central ray is proposed to accelerate the tracing process, which benefits to the parallelism by utilizing the graphics processing unit (GPU). Therefore, it is particularly attractive since the proposed method requires very little increase in computational cost and yields a good accuracy in the NF scattering computation of an electrically large ship problem.

A Novel Hybrid Algorithm for Transient Near-Field Scattering from Rough Surface in 2-D Case

A novel hybrid algorithm is proposed to reduce the computation cost of the finite-difference time-domain (FDTD) method in calculating the transient near-field scattering from rough surface. The scattering problem is split into the FDTD calculation of equivalent sources on the contour enclosing rough surface and the calculation of the near-field radiation with the two-dimensional (2-D) time-domain Huygens’ principle. The radiation fields are found from a surface integral of the temporal convolution for which the direct numerical integration of the convolution is computationally expensive. In this paper, the 2-D time-domain Green’s function as the convolution kernel is approximated with a sum of exponential terms by using the Prony’s method. Then, the semianalytical recursive convolution (SARC) approach is applied to complete the update of the near-field radiation. Compared with the traditional FDTD, this hybrid algorithm can significantly reduce the memory usage and run time, especially for the large distance between the rough surface and observation point.

Polaritonic Enhancement of Near-Field Scattering of Small Molecules Encapsulated in Boron Nitride Nanotubes: Chemical Reactions in Confined Spaces

Near-field spectroscopy has been extensively applied recently to analyze collective optical properties of materials at the nanoscale. However, vibrations of small molecules were only recognizable in close proximity to a metallic resonator. We show that encapsulation in boron nitride nanotubes (BNNT) enhances the near-field vibrational spectra of C$_{60}$ fullerene, reaching a sensitivity limit of a few hundred molecules. Furthermore, products of chemical reactions inside the tubes can be identified, so long as their vibrational signatures lie in the polariton gap of the BNNT.

High-Accurate Non-Uniform Grids for System-Combined ADI-FDTD Method in Near-Field Scattering With Proper CFL Factor

In this paper, a high-accurate technique with non-uniform grids is introduced into a system-combined alternative-direction-implicit finite-difference time-domain (SC-ADI-FDTD) algorithm, and then successfully used to analyze electromagnetic propagations. To our knowledge, the conventional FDTD with non-uniform grids can be effectively deal with some edges of the three-dimensional cubes and complicated structures of the tiny objects by modulating the local grid scales, which to the extent improves its reliability and accuracy. However, due to existing the finer grids in the local computational region and the inevitable Courant-Friedrichs-Lewy (CFL) limit in the conventional FDTD, the temporal interval must be determined by the minimum fine spatial grid, resulting in much larger temporal sampling density required during the whole computation process. As the advantage of circumventing the repeated variables, the non-uniform SC-ADI-FDTD (NUSC-ADI-FDTD) cannot only break through the CFL limit to implement the high-efficient computation, but also further save more CPU time in the local microstructure cases. Furthermore, the empirical formula between the spatial sampling density and the CFL factors can be obtained from the numerical fitting method after errors analysis. The numerical simulations of the electromagnetic scattering have been executed to illustrate feasibility and validity of our proposed method.

On the Reflectivity Extraction Based on Partial Bistatic Near-Field Scattering From Microwave Blackbody

This work reports numerical investigations on the scattering from microwave blackbody, for developing a national standard facility on the emissivity determination in China. Based on the Kirchhoff’s law of thermal equilibrium, the standard facility follows a bistatic scattering measurement configuration and covers a wide frequency range. In general, the reflectivity is to be extracted based on partially collected scattering in the near-field region. The whole electromagnetic process, from the quasi-optical antenna illumination, to target scattering, and finally the aperture coupling reception, is modeled to be analyzed. It is shown in the simulation results that the Floquet scattering properties of the array-shaped blackbody can be obtained without fulfilling the far-field condition. Then, evaluations on the reflectivity extraction are performed at 54, 89, 118, and 183 GHz, addressing typical blackbody structures of coated cones and coated pyramids. The factors of referencing targets, curve integration, and the oblique incidence are discussed, as well as the case of geometry flawed blackbody. This work offers important reference and specific suggestions for achieving accurate and stable reflectivity determination in the bistatic near-field configuration.

Determination of particle size distribution based on dynamic light scattering measurements in the near field

Particle size distribution (PSD) measurement is important for improving product quality, inhibiting environmental pollution, and safeguarding human health. The near-field scattering (NFS) technique offers many substantial advantages over conventional techniques. However, the reconstruction equations are ill-posed and it is difficult to get a reliable solution. In this paper, we propose a new method that can solve the reconstruction equations. This method combines the Backus-Gilbert eclectic theory with truncated singular value decomposition (TSVD) method to obtain a non-negative approximation solution as the initial value of Chahine algorithm for fine calculation. Our method exhibited good performance with several numerical cases of unimodal and bimodal PSDs. It was then used for PSD measurement of two standard particles (GBW(E) 120,028 and 120,047) using the NFS techniques. The relative errors of the length mean diameter, with respect to the peak size of these two standard particles, were 0.18% and 0.76%, respectively.

Near-Field Scattering Prediction Based on Refined Time-Domain High-Frequency Method

The theoretical implementation aspects of near-field wideband electromagnetic (EM) scattering prediction based on refined time-domain physical optics (TDPO) and time-domain equivalent edge currents (TDEEC) integral are proposed in this letter. First, the closed-form expressions in far-field assumption of TDPO and TDEEC integrals are derived based on a Radon transform interpretation. Then, to solve the difficulties of the traditional TDPO and TDEEC in the near-field scattering computation, a refined integral representation is formulated. Afterward, by introducing the concept of the locally expanded approximations both in phase and amplitude, a more accurate expression of the Green function in frequency domain and time domain is developed. In this way, the near-field versions of TDPO and TDEEC are proposed to predict the scattered EM field of electronically large target in the near-field zone while retaining the simplicity of the conventional forms. The validity and applicability of the proposed refined TD method in near-field calculation are demonstrated by some numerical examples in the last part.

Adaptive beamforming with unknown scattering coefficients of near‐field scatterers

Conventionally, near-field scattering effect was considered in antenna measurement, which requires a great amount of measuring efforts and re-measurement if the operational platform has changed. In this study, the authors consider the near-field scattering problem in the framework of adaptive array beamforming theory, which avoids re-measurement and can be adaptive to the arbitrary scenario. Generally, the near-field scattering can result in severe performance degradation in adaptive beamforming applications. They propose a robust adaptive beamforming approach that can maintain the performance in the presence of near-field scatterers with unknown scattering coefficients. In the authors’ approach, the near-field scatterering signal component is incorporated into the presumed steering vector. Thus, it is treated as useful information in this study instead of nuisance interference in the literature. In particular, the properties of the far-field direct-path signal and near-field signal are explored and the large uncertainty set is divided into two small ones describing the far-field and near-field steering vectors, respectively. Simulation examples are provided to show the performance improvement by making use of the near-field scattering signals.

The Study on Near-Field Scattering of a Target Under Antenna Irradiation by TDSBR Method

An efficient time domain shooting and bouncing ray (TDSBR) method is proposed to analyze the transient near-field scattering from an electrically large complex object illuminated by a far-field antenna source. The time-domain far-field incident sources can be derived by a convolution of pulsed excitation with the inverse Fourier transform of frequency domain (FD) far-fields radiated from an antenna. In order to obtain accurate near-field scattering results, time domain physical optics (TDPO) near-field integral representations are proposed and reduced to closed-form expressions to improve computing efficiency by applying locally expanded Green function approximation. Different from the plane wave incident situation, in the case of antenna radiation, the incident fields of each individual facet in the ray path are represented by the radiation fields from the equivalent mirror antenna derived in this paper. Since the radiation fields naturally contains the time delay term, the complicated processing of the time delay problem can be avoided by solving the equivalent mirror antenna radiation fields. It is simpler and more accurate than the traditional time domain geometrical optics (TDGO) method. Numerical examples are presented to demonstrate the efficiency and accuracy of the proposed method.

On the Modeling of Near-Field Scattering of Vehicles in Vehicle-to-X Wireless Channels Based on Scattering Centers

Efficient and reliable vehicle-to-X (V2X) wireless communication requires a deep understanding of the associated propagation channels. However, V2X channels are complicated due to the complex traffic phenomena, and thus extensive propagation studies are required to characterize these channels. This paper considers the effects of vehicle scattering on V2X channels. Most existing techniques that address vehicle scattering are imperfect, due to deficiencies in their computational accuracy and efficiency. In this paper, the tradeoff between computational accuracy and efficiency is addressed by proposing an analytical model based on the concept of scattering centers (SCs). The model can efficiently predict near-field bi-static scattering of vehicles with good accuracy. Also, to make the model widely acceptable, a general methodology of extracting SCs is carefully derived. Then, three standard vehicles (car, van, and truck) are employed for evaluating the performance of the model. The results indicate that the model is useful for reconstructing vehicle scattering in V2X channels.

MIMO Radar Calibration and Imagery for Near-Field Scattering Diagnosis

Multiple-input multiple-output (MIMO) radar is an enabling technique for high-resolution imaging, which is especially useful for near-field electromagnetic scattering diagnosis of complex targets. Among others, high sidelobes and radar cross section (RCS) calibration uncertainty are the major challenges for such applications, due to array nonuniformity, imperfect channels, and antenna pattern tapering. These shortcomings prevent a MIMO radar from obtaining high-quality images with enough dynamic range and RCS accuracy. In this paper, we develop a complete solution for these problems. A novel adaptive weighting technique is proposed, where the complex weights are optimized for exact amplitude and phase error calibration of a MIMO array and for azimuth sidelobe reduction. A MIMO filtered backprojection algorithm is developed for image formation with improved RCS calibration accuracy, where propagation path-loss, antenna pattern tapering, and phase distortion due to the near-field spherical wave front are compensated. Both indoor and outdoor field test results are presented to show the high-quality images obtained using the proposed techniques, demonstrating the applicability of a MIMO radar for diagnostic RCS imaging of complex targets.

SBR Method for Near-Field Scattering of an Electrically Large Complex Target Illuminated by Dipole Sources

An accurate and efficient physical-optics (PO) near-field integral representation is proposed and introduced into shooting and bouncing ray (SBR) to evaluate the electric and magnetic near-fields scattered from electrically large complex objects illuminated by dipole sources. The PO near-field integral expressions can be reduced to closed-form expressions by applying locally expanded phase approximations and surface partitioning. By introducing the proposed PO near-field integral expressions, SBR can be used to solve near-field scattering problems from electrically large complex objects. Simple and complex numerical examples are presented to demonstrate the efficiency and accuracy of the proposed method.

Near-field Scattering of Light from a Hollow Particle with Semisoft Boundary

Within the validity of the first-order Born approximation, the near-field evanescent wave is analytically obtained from the scattering of a monochromatic plane wave from a spatially deterministic hollow particle with a semisoft boundary. The scattered spectral density shows the fluctuating profile in any cross-sectional plane. The increase of either the outer or the inner radius of the scattering potential accelerates the decay of the spectral density value. Given a smaller integer number in the summation index of the scattering potential, the scattered wave experiences less spreading in the near field. The obtained results have potential applications to generate the unique scattered patterns in the near field and the guidance of tiny particles along the desired trajectories.