Near-field Method Research Articles

Analysis of the electromagnetic scattering characteristics of large-aperture parabolic surface based on symmetrical time-domain near-field physical optics method

Analysis of the electromagnetic scattering characteristics of large-aperture parabolic surface based on symmetrical time-domain near-field physical optics method

Detection of atomic vortex beam using the near-field diffraction method

We theoretically introduce the method for detecting the atomic vortex (AV) of a helium atomic beam. The AV detection is based on near-field diffraction. The simulations relied on the Feynman path integral. The Gaussian beam and velocity distribution of helium atoms are taken into account. Gravitational field is also included in this study. This research can be useful in the field of matter-wave optics.

Numerical Analysis and Measurement Techniques for Aircraft Cabin Noise Using JAXA's Jet-FTB "HISHO"

The reduction of aircraft cabin noise is one of the important issues not only to increase passenger comfort but also to strengthen international competitiveness in aircraft development. JAXA has been conducting research activities on numerical and experimental prediction techniques of aircraft cabin noise. This paper focuses on the development of our cabin noise prediction tool using the Finite-Difference Time-Domain method, and in-flight noise measurement in the JAXA's Jet-Flying Test Bed (Jet-FTB) "HISHO" with the aim of grasping the cabin noise characteristics. The measurement data include the noise level near interior walls using the near-field acoustic holography method, the wall impedance, and structural vibrations, acquired through some flight test / ground test campaigns. The authors also explain validation results of our numerical prediction tool using microphone data measured in the Jet-FTB.

Passive localization of wideband near-field sources using Sparse Bayesian learning and envelope alignment

Most studies about passive localization of near-field signals are limited to narrowband sources. In this paper, a novel efficient algorithm is developed for direction of arrival (DOA) and range estimation of wideband near-field sources. Using the specific structure of symmetric linear array, the direction information is extracted and the off-grid Sparse Bayesian learning (SBL) framework is established for DOA estimation. The generalized approximate message passing (GAMP) strategy is applied for fast implementation. Then the envelope aligned approach is proposed for the range estimation in time domain and the linear searching concept is applied to accelerate the computation. Compared with the existing wideband near-field localization methods, the proposed algorithm has better performance of accuracy and resolution probability. Simulation results validate the feasibility and effectiveness of the proposed algorithm.

Method of acoustic separation on the surface of cylindrical baffle

This paper suggests a statistically optimal near-field acoustic holography method based on the combination of plane waves and cylindrical waves, which is used for underwater cylindrical baffles. The method addresses the issue that the extraction of scattered acoustic waves from the surface of underwater cylindrical baffles requires too many measurement points in practical applications: separation of incident and dispersed sound fields and near-field measurements of sound fields at baffle surfaces.The underwater detection sound source is situated in the scatterer's far-field region, so in the actual application scenario for the surface acoustic scattering separation of the cylindrical baffle, the plane wave expansion is used to represent the surface incident acoustic wave and the cylindrical wave expansion is used to describe the scattered acoustic wave on the baffle surface. As a result, fewer expansion orders are required to describe the entire sound field on the baffle's surface. It is verified by simulation that the method is valid and efficient for near-field total sound field measurement and dispersed sound field separation of roughly cylindrical targets under underwater far-field incidence situations because the measurement holographic surface of the surface permits unlimited geometries.

Numerical investigation on the rotor-turbulence interaction noise

Noise from a turbomachinery component, such as compressors and turbines, is an important issue to be resolved in aerospace and ocean engineering applications. The rotor of a turbomachinery assembly usually could operate downstream to other flow structures (fuselages, stators, control surfaces, etc.) that produce either large-scale or small-scale turbulence structures, which are highly complex and are being distorted while they are ingested into the rotor, which generate the rotor-turbulence interaction noise. The presented work conducted numerical simulation research on a ten-bladed rotor ingesting different turbulence structures. The large eddy simulation (LES) and Ffowcs-Williams and Hawkings (FW-H) methods were used to predict the flow-induced noise from the rotor. The results show that the time-space evolution of the upstream turbulent structure when it passes towards the rotor. In addition, the differences in the turbulence ingestion noise at different inflow speeds and rotational speeds were studied. Furthermore, the sound generation mechanism of turbulent ingestion noise was explored using a newly proposed near-field sound source analysis method. In summary, this study can help to improve the understanding of the physical mechanisms of unsteady forces and noise generated by a rotor when it interacts with ingesting turbulent flows.

(Invited) Multiphysics Model of the sSNOM Spectroscopy: Can Near-Field Probes Report the Spectrum of Nanomaterials?

Last decade has been celebrated with a substantial increase of publications on imaging of nanomaterials with near-field probes. The latter include Tip-Enhanced Raman and/or Photoluminescence Spectroscopy (TERS/TEPL), various near-infrared (NIR) Atomic Force Microscopy (AFM) based methods, and several scattering Scanning Near-field optical methods (sSNOM). Using various coherent sources, often tunable in terms of excitation, these methods produce high spatial resolution images – near-field scanning maps, in terms of a pixel size claimed to be limited only by the size of the scanning tip.By design, TERS/TEPL detects a spectral response at each point of a map, which is, naively speaking, gives us a local probe of optical material properties within a single pixel (at the fixed excitation wavelength). NIR-AFM and sSNOM (leaving aside the FTIR mode) allow to obtain quasi-spectral information by tweaking the excitation. In this case, the collected (elastic) signal is restricted to the wavelength of the excitation.The essence of the near-field method is in breaking the laws of a plane-wave reflection from the sample, which requires a sub-wavelength separation between the tip and the sample and a strong enough near-field coupling between those. Still, it is routinely assumed that the tip is an “ideal probe” and its role is to “channel” the information about the local optical response to the detector. Up to date, little is known about the spectral response of the tip-sample system from the perspective of quantum physics.In this work, several aspects of the problem: mechanical motion of the scanning tip, electrodynamics of the excitation/scattered light, and quantum solid state description of the sample and the tip – are combined to obtain, within a simple model, a full picture of the near-field detection. The results, though not general and limited by the simplicity of model used for the sake of clarity of derivation, shed the light on the spectral response of a typical near-field microscope and describe effects, earlier called “artifacts”, as natural response for coupled tip-sample elementary excitations. Lastly, an application of the model for an analysis of spectra of nBN heteronanotubes (Y. Feng, et.al, ACS Nano 15, 5600, 2021) will be presented. Acknowledgement: This work has been partially supported by NSF MRSEC (DMR-2011839).Fig. 1. (left) Amplitude and phase of the near-field optical signal vs. the optical frequency: S(ω). (right) Same optical amplitude and phase vs. ω and the phase of mechanical motion of an AFM tip. Figure 1

Numerical simulation study on noise pollution of the solenoid valve in pressure water-air system of pumped storage power station

To explore the noise problem of the solenoid valve in the pressurized water-air system of a pumped storage power station when operating under the discharge condition, a full-channel unsteady numerical simulation was carried out on the medium-pressure gas storage tank and its discharge facilities. Firstly, the large eddy simulation (LES) method was used to calculate the sound source, and then the finite element method (FEM) was used to solve the internal sound field. The flow field and sound field distribution of the solenoid valve in the pressure water-air system are analyzed in detail. Then the near-field noise analysis method is used to trace the source of the noise problem of the solenoid valve. Finally, according to the noise intensity distribution, the location of the noise source and the noise mechanism is clarified, which provides the basis for the selection of a technical route for the subsequent optimization design of the solenoid valve in the exhaust pressure water system.

Reconstruction of transient acoustic field using sparse real-time near-field acoustic holography

A sparse real-time near-field acoustic holography method is proposed to precisely and stably reconstruct a transient sound field. By a time-domain impulse response function, a time domain convolution equation between the pressure time-wavenumber spectra on the hologram and reconstruction planes is first established. Then, a smoothed ℓ0-norm optimization algorithm is applied to solve the serious ill-conditioned problem of the inverse process to get the pressure time-wavenumber spectrum of the reconstruction plane. The key part of solving this problem is to approximate replace the discontinuous ℓ0-norm by using a suitable continuous Gaussian function family, and a steepest ascent algorithm is introduced to minimize the continuous function for obtaining the optimal solution. Finally, the pressure time-wavenumber spectra of the reconstruction plane for all wavenumbers at all times are solved, and the corresponding time-dependent pressures are gained by the two-dimensional space inverse Fourier transform. A numerical simulation is conducted to observe the reconstruction capacity of the proposed method. The simulation effect prove that the proposed method can accurately reconstruct the transient sound field, and its reconstruction results are compared to those of the real-time near-field acoustic holography (RTNAH) with Tikhonov regularization and YALL1 modal for verifying the superiority of the proposed method. The YALL1 module utilizes an alternating direction algorithm for solving the ℓ1 minimization problems. Several parameters including the noise, position of the holograph plane and two initialization values are discussed in the simulation. The comparison and discussion results show that the reconstruction effect of the proposed method is much better than those of RTNAH with Tikhonov regularization and YALL1 modal under different parameter configurations. An experiment with a percussive steel plate is implemented for further validating the effectivity of the proposed method.

Near-Field Picometer Vibration Detection Based on Dielectric Microsphere Probe

We propose a near-field picometre vibration detection method based on a dielectric microsphere probe. Using an optical fiber combined with a dielectric microsphere to form a near-field probe. When parallel light is injected into the dielectric microsphere, interaction occurs between the surface of the microsphere and the focused beam in the near-field, at the front end of the dielectric microsphere will generate a highly localized electromagnetic beam, i.e., a photonic nanojet(PNJ). The nanojet of the probe in the near-field can enhance the scattered light of the object under the test surface. We used a differential structure that was formed by two optical fiber probes to detect the picometre vibration in the near-field. From the experimental results, we know that the method can detect the picometre vibration signal, while the minimum vibration signal of 20 pm can be detected. The method has the potential to apply in cellular diagnostics, single-molecule detection, and in situ measurements.

Numerical study on mean wave drift forces and moment of ships using Rankine panel method in frequency domain

A three-dimensional frequency-domain seakeeping model based on the Rankine panel method is developed to compute the mean wave drift forces and moment acting on ships with forward speeds. Two different linearization models (the double-body and Neumann–Kelvin models) are applied to evaluate the mean wave drift forces and moment using a near-field method. In these models, the Rayleigh’s artificial friction is adopted in the free-surface boundary condition to suppress longer wave components in computational regions apart from the ship. A novel upwind difference scheme is introduced to calculate the first-order and second-order derivatives of the velocity potential on the free surface. Wigley I, Wigley III and S175 containership are adopted in numerical simulations to validate the effectiveness of the developed model. In the present study, the motion response amplitude operators in head, beam and oblique seas obtained using the two models are compared, and their contributions to the mean wave drift forces and moment are analyzed. Results show that in head seas, the pitch motions have significant effects on the surge-directional wave drift force Fx. In beam seas, the heave motions have the most effect on the mean wave drift forces and moment (Fx, Fy and Mz).

Terahertz cancer imaging and sensing: open research challenges and opportunities

There has been a rapid development of THz technology—sources, detectors and various THz imaging and sensing techniques. The THz technology demonstrates great potential as a modality for early, label free, non-ionizing and non-invasive detection of cancer. Some progressive technological development milestones have been achieved in this regard, however, to become clinically competitive and to provide the sought after real operational convenience, there is need for further research and development to overcome the existing challenges. This paper provides recent trends and perspectives through identification of existing challenges for the development of THz imaging and sensing systems that can evolve into actual medical modalities. We provide an overview of various aspects of THz technology, including techniques for imaging and sensing, mechanisms for THz image contrast and models for tissue dielectric responses to THz waves. The THz imaging application for detection of various cancers is briefed. The advantages of THz cancer imaging and sensing as well as the existing challenges are identified, with recommendations provided in contribution to future research. Further, some recent THz imaging and sensing developments such as the near-field methods to break the diffraction limit including waveguides, resonance and plasmonic metasurfaces are discussed. We emphasize the contribution of analytical algorithms that are based on machine learning, in particular, deep learning for the development of THz technology.Graphical abstract

Label-free detection and identification of single bacteria via terahertz near-field imaging.

In recent years, terahertz (THz) imaging has attracted much attention because of its ability to obtain physical and chemical information in a label-free, noninvasive and nonionizing manner. However, the low spatial resolution of traditional THz imaging systems and the weak dielectric response of biological samples hinder the application of this technology in the biomedical field. In this paper, we report a new THz near-field imaging method for a single bacteria, through the coupling effect of nanoscale radius of probe and platinum gold substrate, which greatly enhances THz near-field signal of biological samples. A THz super-resolution image of bacteria has been successfully obtained by strictly controlling the relevant test parameters such as tip parameters and driving amplitude. By analyzing and processing the THz spectral image, the morphology and inner structure of bacteria have been observed. The method has been used to detect and identify Escherichia coli represented by Gram-negative bacteria and Staphylococcus aureus represented by Gram-positive bacteria. This application provides a new label-free, noninvasive and nonionizing testing protocol for the detection of single bacteria.

Flexible Humidity Sensor with High Sensitivity and Durability for Respiratory Monitoring Using Near-Field Electrohydrodynamic Direct-Writing Method.

The humidity of breath can serve as an important health indicator, providing crucial clinical information about human physiology. Significant progress had been made in the development of flexible humidity sensors. However, improving its humidity sensing performance (sensitivity and durability) is still facing many challenges. In this work, near-field electrohydrodynamic direct writing (NFEDW) was proposed to fabricate humidity sensors with high sensitivity and durability for respiration monitoring. Due to the applied electric field, dense carbon nanotube/cellulose nanofiber (CNT/CNF) networks formed during the printing process that enhance the sensitivity of the sensor. The prepared sensor showed excellent humidity responses, with a maximum response value of 61.5% (ΔR/R0) at 95% relative humidity (RH). Additionally, the sensitivity film prepared by the NFEDW method closely fits the poly(ethylene terephthalate) (PET) substrate, endowing the sensor with outstanding bending (with a maximum curvature of 4.7 cm-1) and folding durability (up to 50 times). The sensitivity of the prepared sensor under different simulated conditions, namely, nose breathing, mouth breathing, coughing, yawning, breath holding, and speaking, was excellent, demonstrating the potential of the sensor for the real-time monitoring of human breath humidity. Thus, the high-performance flexible humidity sensor is suitable for human respiration and health monitoring.

Label-free super-resolution microscopy based on non-diffraction superoscillation beam (NDSB) illumination

Due to the diffraction properties of light waves, optical microscopy suffers from a fundamentally limited resolution of ∼λ/2, where λ relates to the illumination wavelength. Though near-field methods capturing the evanescent waves can reach a resolution of several nanometers, the extremely short working distance has restrained their applications. Typical far-field super-resolution microscopies using fluorescence to monitor the labelled samples also have their limitations in applications because of their phototoxicity and complex preprocessing. We hereby proposed a label-free super-resolution microscopy based on non-diffraction superoscillation beam (NDSB) illumination. We developed a transmission-mode confocal optical microscopy and experimentally demonstrated an imaging resolution of 0.285 λ at the illumination wavelength of 632.8 nm. As NDSB owns the capabilities of self-healing and deep penetration in dense media, we could clearly visualize the fine complex structures while NDSB illuminated from the backside of a 175-µm-thick glass substrate. As a result of NDSB's excellent penetration and superoscillatory transverse size, the 3D super-resolution capability of the proposed microscopy was experimentally verified. Owing to the advantages of label-free imaging, far field, super resolution, self-healing behaviour and deep penetration, we believe the proposed NDSB microscopy bears the potential to become a promising tool for 3D super-resolution visualization in both biological and nonbiological samples.

Near-field antenna measurement based on Rydberg-atom probe.

Current near-field antenna measurement methods are commonly based on metal probes, with the accuracy limited and hard to be optimized due to the drawbacks they suffered, such as large volume, severe metal reflection/interference and complex circuit signal processing in parameter extracting. In this work, a novel method is proposed based on Rydberg atom in the near-field antenna measurement, which can offer a higher accuracy due to its intrinsic character of traceability to electric field. Replacing the metal probe in near-field measurement system by Rydberg atoms contained in a vapor cell (probe), amplitude- and phase- measurements on a 2.389 GHz signal launched out from a standard gain horn antenna are conducted on a near-field plane. They are transformed to far-field pattern and agree well with simulated results and measured results by using a traditional metal probe method. A high precision in longitudinal phase testing with an error below 1.7% can be achieved.

An iteratively reweighted time-domain acoustic method for reconstructing the transient acoustic field

An improved iterative reweighted least-squares based on real-time near-field acoustic hologram method (RTNAH) is proposed to reconstruct the transient acoustic field. First, time-domain convolution relation formula between the pressure time-wavenumber spectra on the hologram and reconstruction planes is first established by an impulse response function, and the corresponding matrix equation is obtained. Then, for solving the pressure spectra on the reconstruction plane, an improved iterative reweighted least-squares algorithm is applied for solving the minimization problem of the sum of 2-norm residual and 2-norm weighted solution under the weighted and sparse representation framework. By controlling the iteratively updated weighting matrix, the sparsity of the solution is promoted. Finally, the pressure spectra of the reconstruction plane are estimated, and the corresponding time-varying pressures are acquired by the two-dimensional inverse Fourier transform. A numerical simulation is conducted to observe the reconstruction capacity of the proposed method. The simulation results demonstrate that the proposed method can reconstruct the transient acoustic field in time-space domains very well. Meanwhile, several vital parameters are discussed, and the superiorities of the proposed method are testified by comparing to RTNAH with Tikhonov regularization and YALL1 model. An experiment is implemented for further validating the effectivity of the proposed method.

Mixed source localization considering mutual coupling and unknown nonuniform noise under exact spatial geometry

A mixed far-field (FF) and near-field (NF) source localization method in the presence of unknown mutual coupling and nonuniform noise is proposed in this paper, where the exact spatial geometry considering propagation attenuation is adopted. First, the nonuniform noise covariance and FF direction of arrival (DOA) are estimated by the iterative least-squares subspace (ILSS) technique and the principle of rank reduction (RARE), respectively. Then, the NF source components are extracted by applying the oblique projection technique. Finally, by choosing three relative sensor positions and constructing four special cumulant matrices properly, NF DOA and range information are obtained in closed-form expressions, where a scheme to avoid ambiguity is exploited. Simulation result show that the proposed method can yield an improved performance in comparison with existing state-of-the-art methods.

A Cylindrical Near-Field Acoustical Holography Method Based on Cylindrical Translation Window Expansion and an Autoencoder Stacked with 3D-CNN Layers.

The performance of near-field acoustic holography (NAH) with a sparse sampling rate will be affected by spatial aliasing or inverse ill-posed equations. Through a 3D convolution neural network (CNN) and stacked autoencoder framework (CSA), the data-driven CSA-NAH method can solve this problem by utilizing the information from data in each dimension. In this paper, the cylindrical translation window (CTW) is introduced to truncate and roll out the cylindrical image to compensate for the loss of circumferential features at the truncation edge. Combined with the CSA-NAH method, a cylindrical NAH method based on stacked 3D-CNN layers (CS3C) for sparse sampling is proposed, and its feasibility is verified numerically. In addition, the planar NAH method based on the Paulis-Gerchberg extrapolation interpolation algorithm (PGa) is introduced into the cylindrical coordinate system, and compared with the proposed method. The results show that, under the same conditions, the reconstruction error rate of the CS3C-NAH method is reduced by nearly 50%, and the effect is significant.

In Situ Measurement of Absorbing Properties of Materials Based on Near-Field Reflection Method

To meet the requirements of the in-situ measurement of the absorbing properties of materials and break through the limitations of the transmission line probe method in ultra-wideband and high-precision measurement, this paper presents in-situ measurement technology for the absorbing properties of materials based on near-field reflection testing. A miniaturized ultra-wideband antenna, a decoupling structure and a shielding cover are designed, and an in-situ measurement probe is constructed. By integrating the probe with a microwave transceiver module as well as a control processing module, a handheld broadband microwave reflectivity test instrument is developed and its performance is verified. The measurement accuracy of the proposed instrument is higher than that of transmission line probes, the deviation of its test results from those of standard arch systems is less than 1.5 dB in the reflectivity range of 0 to -20 dB; its test frequency band is wider, and its frequency band covers 2-18 GHz. The instrument is suitable for accurately measuring the absorbing properties of materials with thicknesses of less than 1.5 mm.