Near-field Reflection Research Articles

Unveiling Local Optical Properties Using Nanoimaging Phase Mapping in High-Index Topological Insulator Bi2Se3 Resonant Nanostructures.

Topological insulators are materials characterized by an insulating bulk and high mobility topologically protected surface states, making them promising candidates for future optoelectronic and quantum devices. Although their electronic properties have been extensively studied, their mid-infrared (MIR) properties and prospective photonic capabilities have not been fully uncovered. Here, we use a combination of far-field and near-field nanoscale imaging and spectroscopy to study chemical vapor deposition-grown Bi2Se3 nanobeams (NBs). We extract the MIR optical constants of Bi2Se3, revealing refractive index values as high as n ∼ 6.4, and demonstrate that the NBs support Mie resonances across the MIR. Local near-field reflection phase mapping reveals domains of various phase shifts, providing information on the local optical properties of the NBs. We experimentally measure up to 2π phase-shift across the resonance, in excellent agreement with finite-difference time-domain simulations. This work highlights the potential of Bi2Se3 for quantum circuitry, nonlinear generation, high-Q metaphotonics, and photodetection.

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.

Distance Measurement by Neural Network Learning of Near-Field Microwave Reflection Spectra

Microwave-based distance measurements are limited depending on the sensing environment, such as the propagation medium and surrounding obstacles, and the complex environment also affects the measurement performance. To tackle this problem, we propose a method for predicting the distance based on the artificial neural network learning of near-field microwave reflection spectra. In principle, the spectral data is expected to contain a signature of the distance of the target object. Based on this, we proposed a two-step neural network to extend the measurable distance range while ensuring prediction performance. The first step is to predict the coarse range of the target by classification, and the next is to predict the precise distance value through multidimensional regression within that coarse range. The method was verified through experiments to predict the position of an object in an underwater environment, which was difficult to measure with conventional methods.

Four-channel display and encryption by near-field reflection on nanoprinting metasurface

Abstract Multichannel metasurfaces become one of the most significant development trends, as they exhibit versatile manipulation abilities on electromagnetic fields and provide a promising approach to constitute compact devices with various complex functions, especially in optical encryption due to its capabilities of multichannel, high complexity, and high concealment. However, the existent multichannel metasurfaces based optical encryption technology can only realize two channels in the near-field, or perform three channels in near- and far-field. In this paper, a four-channel display metasurface used to encrypt information by three optical parameters as security keys is firstly proposed and experimentally demonstrated, which is different from the previous three-channel metasurface combined nanoprinting and hologram in near- and far-field. The novel design strategy of the four-channel metasurface can effectively enhance the information capacity and increase the difficulty of leaks without causing manufacturing challenges and additional costs. In addition, the simulation and experimental results demonstrate that the designed metasurface with four independent channels can separately display distinguishable nanoprinting images under decoding keys of special optical parameters. The proposed four-channel display metasurface with advantages of high capacity and ultracompactness will pave a way for multichannel applications in nano display, information storage, optical anticounterfeiting, and other relevant fields.

Enhancing the hybridization of plasmons in graphene with 2D superconductor collective modes

We explore ways in which the close proximity between graphene sheets and monolayers of 2D superconductors can lead to hybridization between their collective excitations. We consider heterostructures formed by combinations of graphene sheets and 2D superconductor monolayers. The broad range of energies in which the graphene plasmon can exist, together with its tunability, makes such heterostructures promising platforms for probing the many-body physics of superconductors. We show that the hybridization between the graphene plasmon and the Bardasis–Schrieffer mode of a 2D superconductor results in clear signatures on the near-field reflection coefficient of the heterostructure, which in principle can be observed in scanning near-field microscopy experiments.

Roll-to-Roll In-Line Implementation of Microwave Free-Space Non-Destructive Evaluation of Conductive Composite Thin Layer Materials

A free-space microwave nondestructive testing and evaluation module is developed for the low-power, non-ionizing, contactless, and real-time characterization of doped composite thin-film materials in an industrial context. The instrumentation proposed is built up with a handled vector network analyzer interfaced with corrugated horn antennas to measure the near-field complex reflection S11 of planar prepreg composite materials in a roll-to-roll in-line production line. Dedicated modeling and calibrations routines are developed to extract the microwave conductivity from the measured microwave signal. Practical extraction of the radiofrequency (RF) conductivity of thin film prepreg composite materials doped with nano-powders is exemplary shown at the test frequency of 10 GHz.

Near-field transmission and reflection spectroscopy for revealing absorption and scattering characteristics of single silver nanoplates.

Near-field optical microscopy visualizes spatial characteristics of elementary excitations induced in metal nanostructures. However, the microscopy is not able to reveal the absorption and scattering characteristics of the object simultaneously. In this study, we demonstrate a method for revealing the absorption and scattering characteristics of silver nanoplate by using near-field transmission and reflection spectroscopy. Near-field transmission and reflection images show characteristic spatial features attributable to the excited plasmon modes. The near-field refection image near the resonance shows a reversed contrast depending on the observed wavelength. Near-field reflection spectra show unique positive and negative resonant features. We reveal that the optical characteristics and the wavelength dependency of the optical contrast originate from the scattering and absorption properties of the plasmons, with the aid of the electromagnetic simulations.

Dyadic wavelet analysis of bender element signals in determining shear wave velocity

Determining shear wave velocity is a critical technique in bender element tests, as it can be readily affected by near-field effects, wave reflection, and other factors. This study proposes a new method based on the dyadic wavelet transform modulus maxima. Combining the local modulus maxima of dyadic wavelet transform approximate coefficients at fine decomposition levels and an appropriate threshold value, the proposed method can automatically detect the target point. For validation, a comparative study among the dyadic wavelet transform modulus maxima, peak-to-peak, first arrival, and cross-correlation methods was carried out using 140 sets of bender element signals. The comparison results show that the proposed method not only mitigates the adverse effects of near-field, later major peaks, and noise contamination, but is also more robust in estimating shear wave velocity.

Scanning laser terahertz near-field reflection imaging system

We developed a scanning laser terahertz (THz) near-field reflection imaging system using a fiber-coupled femtosecond laser source for high spatial resolution measurements without any sub-wavelength probes. In this system, THz wave pulses generated at the laser focusing spot in a non-linear optical crystal interact with a sample set on the crystal. Using this system, we obtained the THz reflection images of metallic structures with a resolution of ∼λTHz/34 (λTHz = 680 μm) and also observed paraffin-embedded human breast tissue of several 10 μm in size. These results promise that this system can be suitable for industrial applications which require micron-scale resolution.

Towards an improvement of GPR-based detection of pipes and leaks in water distribution networks

Ground-penetrating radar (GPR) is a non-destructive tool that can be used to help detecting water leaks in water distribution networks. This study, which was undertaken in the framework of the SENSPORT project (Walloon Region, Belgium), aims at improving GPR-based detection of pipes and leaks in water distribution networks using advanced, integrated radar data processing and visualization strategies. The proposed method includes a physically-based near-field antenna effects removal approach and reflection detection and segmentation algorithms. It also involves a quantitative estimation of the medium properties using full-wave inversion. Finally, a specific human-computer interface allowing the end-user to visualize buried utilities and 2-D/3-D processed data on mobile devices is proposed. We successfully validated the general methodology through a laboratory experiment with near-field measurements performed at different times over a leaky pipe buried in a sandbox. This integrated tool appears to be promising to help detecting and monitoring water leaks. Future research will focus on more complex real cases.Index Terms—Full-wave inversion (FWI), ground-penetrating radar (GPR), human-computer interaction, subsurface imaging, water leaks detection.

Development of Aperture-Type Near-Field Reflection Spectroscopy and Its Application to Single Silver Nanoplates

We developed an aperture-type near-field reflection spectral imaging method to obtain optical properties of materials with a nanometer spatial resolution. We adopted a phase-stepping technique to extract genuine near-field signals of a sample from the observed intensity spectra dominated by the large background, in a multiplex manner. We performed near-field reflection spectral imaging of a single silver nanoplate to evaluate the developed system and also to examine near-field optical properties of the nanoplate. The near-field reflection spectrum of the nanoplate shows multiple resonant features that are not observable by conventional methods. The near-field reflection image taken at the resonance shows unique spatial features attributable to the plasmon mode resonantly excited. The developed system is applicable to transparent samples as well as opaque ones and enables spectral as well as spatial characteristics of the samples to be revealed.

Modeling and Calibration in Near-Field Microwave Microscopy for Dielectric Constant and Loss Tangent Measurement

The characterization of liquid samples using evanescent microwave fields is described. A calibration model is developed to relate the measured near-field reflection coefficient to the local properties of the sample under test. As a demonstration, permittivity extraction of different sodium chloride solutions is experimentally determined. The accuracy of 0.3% and 1.3% is obtained for the dielectric constant and the loss tangent at 2.45 GHz.

Using microwave near-field reflection measurements as a non-destructive test to determine water penetration depth of concrete

A non-destructive version of a standard test for concrete permeability has been demonstrated. The method is based on water penetration depth estimation from near-field microwave measurements using an open-ended rectangular waveguide. In particular, water penetration is determined from the differential measurement of reflection coefficient in the S-band before and after injection of water under pressure. Experimental results show a good correlation between the standard destructive test and the new method. Microwave measurements simplify the standard test avoiding the need of drilling out a sample from the structure and thus reducing both the cost and time needed in the analysis.

Wrist Pulse Detection System Based on Changes in the Near-Field Reflection Coefficient of a Resonator

This letter presents a wrist pulse sensor based on reflection coefficient (S11) from a resonator which is applicable to existing wearable communication devices. The compact resonator is designed on flexible substrate as an RF signal radiator. Since the reflection coefficient from the resonator depends on the distance between the resonator and the wall of the major artery, slight changes in radial artery diameter can be utilized to obtain pulse rate at the wrist. The sensor system is implemented and tested at 2.4 GHz ISM band, and reveals excellent performance in terms of sensitivity as well as convenience as a wearable device compared to conventional piezoelectric or photoplethysmography (PPG) sensor.

Geometrical Optics-Based Model for Dielectric Constant and Loss Tangent Free-Space Measurement

A microwave free-space reflection method for determining the complex permittivity of planar dielectric materials is demonstrated. The method makes use of the measurement of the near-field microwave reflection coefficient of a metal-backed sample. The modeling of the structure and its calibration are based on geometrical optics considering spherical electromagnetic waves propagating through the material. The technique that presents a number of features such as low-cost, compactness, robustness, and reliability is a good candidate for industrial applications. As a demonstration, dielectric parameters extraction of building materials is experimentally demonstrated for wireless local area network operations in the 2.45- and 5-GHz bands.

Defining regions of interest for microwave imaging using near-field reflection data

Microwave imaging benefits from information on the internal structure of the object of interest. Previously, we developed a tool to define regions dominated by a particular material and tested this approach with 2-D simulations of breast models. In this paper, we apply the technique to experimental data with the goal to extract an object's internal structural information using radar-based techniques. Specifically, the technique extracts information from microwave reflection data in order to identify discontinuities in material properties. By combining observations from multiple antenna locations, this information is used to estimate regions of interest. The utility of the tool is demonstrated in practical scenarios where the data are generated experimentally from cylindrical models using an ultra-wideband sensor.

Technique to Decompose Near-Field Reflection Data Generated From an Object Consisting of Thin Dielectric Layers

Extracting properties of hidden structures using ultra-wideband (UWB) radar is evolving into a promising technology. For these applications, a short-duration electromagnetic wave is transmitted into an object or structure of interest and the backscattered fields that arise due to dielectric contrasts at interfaces are measured. The time-of-arrival (TOA) between reflections and the amplitude of the reflections may be used to infer the geometrical and dielectric properties of hidden structures or objects. For electrically thin layers, the limited bandwidth of the illuminating signal typically gives rise to overlapping reflections, necessitating the use of high-resolution techniques. We investigate an iterative nonlinear parameter estimation technique that may be used for near-field applications. The effectiveness of the algorithm to decompose the reflection data is evaluated using numerical data generated from 2D and 3D dielectric slabs and experimental data from multi-layered slabs.

Regional estimation of the dielectric properties of inhomogeneous objects using near-field reflection data

We present a new inversion strategy that integrates radar-based methods with microwave tomography (MT) to efficiently provide low resolution information about an object's structure and average dielectric properties. For this preliminary investigation, we assume that the object may be characterized as having three regions: a thin outer layer and an interior with two inhomogeneous regions having dissimilar average dielectric properties. Our aim is not to reconstruct a detailed image of an object, but rather to provide information about its basic structure, including the geometric and mean dielectric properties of regions predominantly composed of a given material. The inversion technique is carried out in two steps. First, a reconstruction model indicating the locations and spatial features of the three regions of interest is constructed efficiently and quickly using ultrawideband (UWB) reflection data. The reconstruction model formed using radar-based techniques is then incorporated into the second step of the procedure which estimates the mean dielectric properties over each region using MT methods. Identifying the three homogeneous regions with radar-based techniques provides a priori information about an object's internal geometry and significantly simplifies the parameter space structure so that the inverse scattering problem solved with MT is not as ill-posed as those typically encountered. The performance of the proposed technique is first evaluated with both reflection and transmission data generated by progressively more complex 2D numerical models. Microwave breast imaging approaches would benefit from the internal structural information extracted by the algorithm, so a practical application is explored using 2D breast models formed from the magnetic resonance (MR) scans of a patient study. The algorithm's ability to infer the breast's basic internal structure is demonstrated with these examples.

A tapered parallel-plate-waveguide probe for THz near-field reflection imaging

We demonstrate a broad-band low-loss THz near-field imaging technique based on a tapered parallel-plate waveguide. This technique works in a reflection geometry, which avoids the problem of low energy throughput due to the impedance mismatch and consequently limited spatial resolution, in the transmission geometry. Images reconstructed by the filtered-back-projection algorithm are able to resolve features of size ∼100 μm using radiation with an average wavelength of 1.5 mm.

Detection and Characterization of Buried Macroscopic Cracks Inside Dielectric Materials by Microwave Techniques and Artificial Neural Networks

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The detection and characterization of macroscopic cracks inside dielectric materials is an important practical issue. Thus, there is a need to establish evaluation techniques, which can be used to characterize buried cracks; indeed, the knowledge of the geometrical configuration of a hidden crack is a key factor for fatigue crack engineering. Therefore, a microwave method for nondestructive characterization of macroscopic cracks inside dielectric materials is presented in this paper. This nondestructive and noncontact technique is based on the determination of the near-field reflection coefficient of an open-ended rectangular waveguide. The measurements are achieved by means of a microwave six-port-based system that operates at 35 GHz. We show that relatively small defects are detectable and demonstrate that the association of signal processing tools to this characterization method enables the retrieval of the crack profile in an acceptable manner. The reconstruction of a 1-D buried crack profile is performed by means of a multiple-multilayer-perceptron (MLP) approach. Several cases are investigated to demonstrate the capabilities of the method. </para>