Near-field Effects Research Articles

Thermophotonic cells in self-sustaining parallel circuits

Thermophotonic (TPX) cells, which integrate a light-emitting diode (LED) and a photovoltaic (PV) cell, offer potential advantages over thermophotovoltaic (TPV) cells for low-grade waste heat utilization. While TPX cells traditionally operate in an independent circuit, which requires an external battery storage unit, the self-sustaining circuit presents an appealing alternative that eliminates the need for the storage unit. In this study, we conduct a comprehensive analysis of TPX cells operating in the self-sustaining circuit. We provide clear expressions for the electrical currents flowing through the LED and PV cell, considering the different bandgaps of the LED and the PV cell. Our results reveal that the bandgap energy of the LED must exceed that of the PV cell for the TPX cell to function in a self-sustaining parallel circuit. Furthermore, we demonstrate that the performance of the TPX cell in the self-sustaining circuit improves with a narrower bandgap energy for the PV cell and a wider bandgap energy for the LED, regardless of the far-field or near-field configurations. While near-field TPX cells exhibit higher power density, they also experience decreased energy efficiency compared to far-field TPX cells. Additionally, we explore the scenario where the bandgaps of the LED and PV cell are the same, analyzing the performance of a closed-structured self-sustaining parallel circuit with multiple LEDs and PV cells in the near-field configuration. We find that the near-field effect has a marginal impact on TPX cells with identical bandgaps, which means that the near-field TPX cell in the self-sustaining circuit will not be useful in practice. The insights gained from this work serve as valuable design guidelines for TPX cells operating in the self-sustaining parallel circuit, enabling practical operation without the need for an external battery storage unit.

Experimental comparison of active seismic surface wave tests on shallow and deep bedrock sites

Active seismic surface wave tests are tests during which signals are generated by artificial sources such as sledgehammers, drop weights and fixed or variable frequency shakers. A number of active seismic surface wave tests have become popular in the geotechnical industry. Interestingly, different surface wave tests are popular in different regions of the world, presumably because the tests require different equipment, expertise and numerical analysis techniques. Few studies have been done to compare seismic surface wave tests directly. This study investigated the performance of three active seismic surface wave tests, namely the spectral analysis of surface waves (SASW), multi-channel analysis of surface waves (MASW) and continuous surface wave (CSW) tests. This paper reports the performance of the three tests with regard to repeatability, susceptibility to near-field effects, and maximum and minimum measurement depths at both a shallow and a deep bedrock site.

Broadband Enhancement in the Spectral Response of Photovoltaic Modules with Flower-like Silver Particles

Recent research has indicated that metal nanoparticles, known for their unique optical properties, can enhance the spectral response of photovoltaic modules. Since most nanoparticles demonstrate enhancement effects within a specific wavelength range, broadening the spectral response of photoelectric devices is critical for their application in imaging, energy harvesting, and optical communication. In this study, we applied flower-like silver particles to achieve this broadband enhancement. The optical absorption of photovoltaic modules, featuring an amorphous Si p-i-n structure, was improved across a broad wavelength range of 400~2000 nm by integrating these flower-like silver particles, resulting in an approximately tenfold increase in peak spectral responsivity. The theoretical investigation further elaborates that the enhancement originates from the near-field effects of silver particles due to the interaction of different parts of the flower-like silver particles. Through these studies, we demonstrate that utilizing the flower-like silver particles with roughness surface can achieve the spectral response of the photoelectric device enhanced in broadband range, which can improve the utilization efficiency of optical energy for the applications of sensing, imaging, optical communication, and energy harvesting.

Unsophisticated LSPR biosensors: Optical characterization based on 3D modeling-simulation and experimental study

Multiple applications can be developed and produced with the LSPR biosensor. For these, in addition to experimental investigation, detailed modeling and simulation are investigational priorities. This study utilizes an inexpensive microchannel substrate to produce the biosensor via thermal evaporation without annealing. In addition to the experimental study, advanced 3D modeling besides FDTD optical simulation is used to investigate sensor characteristics. Initially, the extinction spectrum of the observed and simulated samples is matched well. In addition, the bulk RI sensitivity of the sensor was calculated to be 125 nm/RIU by the simulation procedure. Moreover, by developing the proposed model for considering local RI sensitivity, the 40 nm/RIU was simulated for local sensitivity. Furthermore, the simulation of the near-field effect highlights the significance of the microchannel substrate, as it offers a greater variety of spatial interaction possibilities between the recognition agents and the hot spots. For the fabricated sensor, the LOD of 1.14 μg/mL was determined for sensing VB12 in water at concentrations between 0.25 and 4 μg/mL. Therefore, due to the introduction of strong modeling and simulation, simple and practical plasmonic biosensors can be better understood and, as a result, designed more appropriately.

Balancing Near-Field Enhancement and Hot Carrier Injection: Plasmonic Photocatalysis in Energy-Transfer Cascade Assemblies.

Photocatalysis stands as a very promising alternative to photovoltaics in exploiting solar energy and storing it in chemical products through a single-step process. A central obstacle to its broad implementation is its low conversion efficiency, motivating research in different fields to bring about a breakthrough in this technology. Using plasmonic materials to photosensitize traditional semiconductor photocatalysts is a popular strategy whose full potential is yet to be fully exploited. In this work, we use CdS quantum dots as a bridge system, reaping energy from Au nanostructures and delivering it to TiO2 nanoparticles serving as catalytic centers. The quantum dots can do this by becoming an intermediate step in a charge-transfer cascade initiated in the plasmonic system or by creating an electron-hole pair at an improved rate due to their interaction with the enhanced near-field created by the plasmonic nanoparticles. Our results show a significant acceleration in the reaction upon combining these elements in hybrid colloidal photocatalysts that promote the role of the near-field enhancement effect, and we show how to engineer complexes exploiting this approach. In doing so, we also explore the complex interplay between the different mechanisms involved in the photocatalytic process, highlighting the importance of the Au nanoparticles' morphology in their photosensitizing capabilities.

Wave Reflection Effects on Bender Element Test of Very Stiff Clay

A common source of error in bender element testing lies in the determination of the shear wave arrival time arising from the near-field effect, which is attributed to shear wave–like motion of the compression wave obscuring the arrival of the shear waves, and can be mitigated by using a sufficiently high travel-distance-to-wavelength (L/λ) ratio. However, the L/λ ratio also is relevant for signal interference due to side reflections, and its role in this important mechanism is far less understood. Stiff materials such as cement-treated soils, which have a very short shear wave travel time, are likely to exacerbate these errors. This paper examined the causes of travel time errors in cement-treated soil using bender element tests of standard-sized triaxial specimens, large block samples, and three-dimensional finite-element analyses. The results show that the reflection of shear waves from sample boundaries generates a second pulse in the received signal, resulting in signal distortion. This can be ameliorated at sufficiently high L/λ ratios. The reflection of the compressional waves generates spurious prearrivals and distorts the profile of the direct shear wave even at high L/λ ratios. Shear and compressional wave reflections can be mitigated by increasing the specimen diameter to decouple the latter from the direct shear wave arrival.

Manipulating multiple plasmon modes by coupling fields for broadband filtering in terahertz metamaterials

Multiple modes resonance provides the possibility of broadband filtering and has great potential applications in terahertz devices, including miniaturized spectrometers, hyperspectral imagers and optical switches in communication systems. However, it is difficult to further expand the bandwidth range due to the inability to stack an infinite number of resonant structures in a finite space. Here, an innovative method is reported for manipulating two localized surface plasmon polaritons (LSPP) modes by multiscale-coupled plasmonic fields to realize broadband filtering at terahertz (THz) frequencies. The interaction between two LSPP modes was verified theoretically and experimentally, exhibiting a prominent Rabi splitting and two peaks with high transmittance. Particularly, broadband filtering is accomplished by utilizing a double-layer structure that can be regarded as a cavity for inducing far-field interference. Experimental result demonstrates that the maximum transmission range of the bilayer metamaterial reaches 0.63 THz. Besides, the two wide bands can be integrated into one broadband through near-field coupling effects. The integration of strong coupling and far-field interference expands the transmission range of the double-layer metamaterial, further enhancing its capability for broadband filtering. This unique approach for manipulating multi-mode plasmonic coupling forges a convenient route for realizing broadband filtering.

Sunlight harvesting in thin-film silicon solar cells using latticed Al2O3 anti-reflection coating, Ag disk NPs, and Ag back-reflector

Solar energy is one of the renewable energy sources that has been made available to humans at no cost. Therefore, solar cells are a suitable alternative candidate for fossil fuels. Thin-film solar cells, despite being a cost-effective technology, have low efficiency. Therefore, in this research work, we provided a new architecture for a 200 nm Si solar cell, in which a latticed Al2O3 ARC is used as an antireflection, and Ag disk shape nanoparticles are placed on the front surface of the active layer in the center of the lattices. Furthermore, an Ag film is used as a back-reflector in the rear surface of the cell. By this design, because of the forward light scattering and near-field enhancement plasmonic effect of the Ag disk, reducing the Fresnel reflection from the cell surface by latticed ARC, and recycling the unabsorbed transmitted photons by BRS, the short-circuit current density and the efficiency can be reached to 14.67 mA cm−2, and 7.11%, respectively. These parameters equal 3.94 mA cm−2, and 1.78% for the bare cell, respectively.

The Far-/Near-Field Beam Squint and Solutions for THz Intelligent Reflecting Surface Communications

Terahertz (THz) and intelligent reflecting surface (IRS) have been regarded as two promising technologies to improve the capacity and coverage for future 6G networks. Generally, IRS is usually equipped with large-scale elements when implemented at THz frequency. In this case, the near-field model and beam squint should be considered. Therefore, in this paper, we investigate the far-field and near-field beam squint problems in THz IRS communications for the first time. The far-field and near-field channel models are constructed based on the different electromagnetic radiation characteristics. Next, we first analyze the far-field beam squint and its effect for the beam gain based on the cascaded base station (BS)-IRS-user channel model, and then the near-field case is studied. To overcome the far-field and near-field beam squint effects, we propose to apply delay adjustable metasurface to IRS, and develop a scheme of optimizing the reflecting phase shifts and time delays of IRS elements, which effectively eliminates the beam gain loss caused by beam squint. Finally, simulations are conducted to demonstrate the effectiveness of our proposed schemes in combating the far-field and near-field beam squint.

A thermal contact model encompassing near-field effects of multi-interface TEGs in hypersonic conditions

High efficiency heat harvest of hypersonic vehicles claims elaborate design of thermoelectric (TE) conversion process. A thermoelectric generator (TEG) contains multi-interfaces in the heat transfer path and the interfacial thermal contacts with certain amount of nanoscale effects have significant influence on its TE conversion process. In this paper, a thermal contact model encompassing near-field thermal radiation (NFTR) effects of TEGs with heterogeneous interfaces between substrates, electrodes and TE legs is established and experimentally validated in hypersonic conditions. First, a real-topology-based thermal contact resistance (TCR) prediction model is developed and the far- and near-field radiative heat transfer at heterogeneous interfaces is considered by the fluctuational electrodynamics, and the model is validated by direct and indirect experimental measurements. Second, the effects of average interfacial temperature, clearance distance and opposite heat transfer paths on the thermal radiation are analyzed, and the influences of average interfacial temperature, loading pressure and clearance mediums on the TCR are then clarified. Third, the TCR is equivalently considered in a TEG performance evaluation model and the NFTR effects is quantitatively counted. The result shows that: with the consideration of the NFTR, the TCRs between TE legs and electrodes, substrates and electrodes decrease by about 3.90% to 17.0% and 0.510% to 6.40% under vacuum clearance, 1.30% to 5.50% and 0.220% to 2.00% under air clearance, respectively; and the maximum output power of the TEG increases by about 8.02% and 1.29% under vacuum and air clearance (0.1 MPa, 650 K), respectively.

An Alternative Method to Analyze Waveforms from Bender Element Tests in Soft Clays

Bender element (BE) testing has become a widely used technique to evaluate S-wave velocities and to derive shear moduli from them in laboratory tests. Results of BE tests depend heavily on the proper identification of the S-wave travel time from waveforms often affected detrimentally by factors such as the near-field effect, crosstalk, noise, or drift. The authors have performed BE tests on soil specimens of Texcoco Clay over the last few years in triaxial cells and oedometers and have sometimes dealt with waveforms influenced by those undesirable factors thus, it was observed that the using of different available techniques to evaluate the S-wave travel time are often prone to errors. This paper shows that the S-wave travel time can be assessed unambiguously from the distance between the inflection points present in both the source and the received signals when waveforms generated with different input frequencies are superimposed, even in signals affected by the near-field effect, crosstalk, noise, or drift. This approach, named here the Wave Inflection Method (WI method), yields good results, at least when applied to data obtained from BE tests on soft Texcoco Clays. The method was validated by applying it to synthetic signals and experimental waveforms obtained from triaxial and oedometer tests equipped with BE. When applied to previously published waveforms on a wide range of soil types, our results suggest that the WI method significantly reduces subjectivity and produces improved reliability in evaluating the S-wave travel time.

Investigation of near-field optical tweezers based on the edge effect of extraordinary optical transmission in thin microcavity

Optical tweezers are powerful tools capable to trap and manipulate particles directly. However, using conventional optical tweezers for nanosized objects remains a formidable challenge due to the optical diffraction limits and high-power levels required for nanoscale trapping, which usually causes irreversible damage to the captured particles. In this paper, we investigate the near-field edge effect of thin microcavity due to macroscopic quantum effect, and the highly enhanced electric field can reach 2.4 times. Thus, a dual near-field optical trap potential well is generated at the edge of the thin microcavity. We theoretically show that this near-field potential well can stably capture nanoparticles smaller than 10 nm while keeping the incident optical power level below 100 mW. Besides, the relationship between size of the microcavity and optical gradient force has also been carefully studied. Finally, the theoretical model of near-field optical tweezers with double thin microcavity is established, and the electric field magnitude of the double microcavity model is enhanced by 4.5 times compared with single microcavity model, in which the coupling effect of double hole makes smaller particles be stably trapped. Our research presents a huge potential for optical trapping and separation of nanoparticles and biomolecules.

Near-field thermophotovoltaic energy conversion analysis based on enhanced radiative absorption distribution

Near-field radiation has been widely shown to greatly boost the electrical power of thermophotovoltaic (TPV) cells. However, there is a lack of theoretical analysis exploring the important influences of near-field effects on radiative absorption distributions as well as TPV energy conversion performances. This work investigates the electrical performances of near-field TPV cells made of InGaSb coupled with different practical emitters such as plain tungsten (W), indium tin oxide (ITO) film, and alternate W and alumina multilayer in detail. A comprehensive analysis is conducted to systematically compare the impacts of evanescent wave tunneling, surface plasmon resonance, and hyperbolic modes on spatial distributions of radiative absorption and the profiles of local carrier concentrations. The detailed and accurate analysis reveals the crucial role of near-field radiation emitted by various emitters in charge collection efficiency, thermal photon flux penetration depth, and photocurrent generation. Thus, the results certify that the electric power could be enhanced by utilizing ITO and multilayer emitters instead of a plain W emitter. The efficiency for an ITO emitter increases with decreasing vacuum gap owing to the suppressed bulk recombination but decreases when the vacuum gap falls below 18 nm due to increased surface recombination. While the efficiency for a multilayer emitter is comparatively lower due to the larger sub-bandgap photons and inefficient n-region. Furthermore, we verify the strategies for performance improvement via decreasing the surface recombination and optimizing the p-region thickness. The underlying mechanism is interpreted based on the spatial distribution and the collection efficiency of minority carriers.

Near-Field Coupling Effect Analysis of SMD Inductor Using 3D-EM Model

In this paper, we propose a methodology for analyzing the near-field coupling between two surface mount device (SMD) inductors using a 3-dimensional electromagnetic (3D-EM) model. To develop the 3D-EM model, we first constitute the equivalent circuit of the SMD inductor from the measured impedance and derive the loss tangent using circuit parameters. Secondly, the loss tangent using damped harmonic oscillator model is introduced to extract the effective permeability of core magnetic material in the SMD inductor. The optimization algorithm is used to compare the two loss tangents. Then the effective permeability is used in the magnetic material for the 3D-EM modeling of the SMD inductor. The validity of the proposed 3D-EM model is confirmed by comparing the impedance and S-parameters obtained from both measured and EM-simulated values for the two near-field coupled SMD inductors. Finally, the near-field coupling effects between the two adjacent SMD inductors are visualized in terms of coupling path visualization (CPV) using the proposed 3D-EM model, which demonstrates its usefulness for near-field coupling analysis.

Investigation on the near-field cutoff effect in a subwavelength plasma shell with near-zero permittivity.

Although the plasma-induced receiving and radiating near-field cutoff phenomena in the subwavelength regime are found of crucial importance in electromagnetic (EM) signal transmissions and plasma property studies, their mechanisms to a large extent remain unclear and undistinguished. In this paper, in the perspective of field and energy transfer, it is demonstrated that the cutoff in the near-field regime is completely different from that in the geometric optical regime. Results show that, for the receiving mode, epsilon-near-zero (ENZ) plasmas can be treated as a nearly ideal EM fluid, and thus, EM waves are restricted into the plasma channel. For the radiating mode, on the other hand, it is the destructive interference between the electric dipole fields of the antenna and the ENZ plasma that results in vanishing far-field radiation. As an important supplement to the existing cutoff theories, our results not only offer clearer physical insights into the near-field cutoff effect but also provide a helpful reference for cutoff-related practical applications in various frequency bands.

Near-Field Rainbow: Wideband Beam Training for XL-MIMO

Wideband extremely large-scale multiple-input-multiple-output (XL-MIMO) plays an important role in boosting the data rate for 6G networks. Because of the huge bandwidth and the large number of antennas, wideband XL-MIMO introduces a significant near-field beam split effect, where beams at different frequencies are focused on different locations. This effect results in a severe array gain loss, and existing works mainly consider to compensate for this loss by utilizing time-delay (TD) beamforming. This paper demonstrates that despite degrading the array gain, the near-field beam split effect can also contribute to the fast near-field beam training. Specifically, we first reveal the controllable near-field beam split effect. This effect indicates that TD beamforming can control the degree of the near-field beam split effect, i.e., beams at different frequencies can flexibly occupy the desired location range. Due to the similarity with the dispersion of natural light caused by a prism, we also call this effect as “near-field rainbow”. Then, by taking advantage of the near-field rainbow, a fast wideband beam training scheme is proposed to generate beams focusing on multiple locations at multiple frequencies with the help of TD beamforming. Finally, simulation results demonstrate that the proposed scheme is able to realize efficient near-field beam training with low training overheads.

Near-Field Channel Estimation in Mixed LoS/NLoS Environments for Extremely Large-Scale MIMO Systems

Accurate channel model and channel estimation are essential to empower extremely large-scale MIMO (XL-MIMO) in 6G networks with ultra-high spectral efficiency. With the sharp increase in the antenna array aperture of the XL-MIMO scenario, the electromagnetic propagation field will change from far-field to near-field. Unfortunately, due to the near-field effect, most of the existing XL-MIMO channel models fail to describe mixed line-of-sight (LoS) and non-line-of-sight (NLoS) path components simultaneously. In this paper, a mixed LoS/NLoS near-field XL-MIMO channel model is proposed to match the practical near-field XL-MIMO scenario, where the LoS path component is modeled by the geometric free space propagation assumption while NLoS path components are modeled by the near-field array response vectors. Then, to define the range of near-field for XL-MIMO, the MIMO Rayleigh distance (MIMO-RD) and MIMO advanced RD (MIMO-ARD) is derived. Next, a two stage channel estimation algorithm is proposed, where the LoS path component and NLoS path components are estimated separately. Moreover, the Cramér-Rao lower bound (CRLB) of the proposed algorithm is derived in this paper. Numerical simulation results demonstrate that, the proposed two stage scheme is able to outperform the existing methods in both the theoretical channel model and the QuaDRiGa channel emulation platform.

Self-organized patterning on azo molecular glass film via optical near-field effect

Self-organized surface patterning has attracted great research interest ranging from scientific understanding to various applications. Here, we report a unique patterning behavior observed on an azo molecular glass film with polystyrene microspheres on the surface upon irradiation with a circularly polarized laser beam. Photoinduced submicron hillocks are found to organize into concentric arrays around isolated polystyrene spheres, while more complex patterns are observed around two or more spheres close to each other. The irradiation first creates surface relief rings encircling the spheres and then the rings split into hillocks arranged in circular arrays with increasing exposure time. Pattern formation is shown to be controlled by synergy from the optical near-field of polystyrene spheres with the spontaneous hillock formation characteristics of the molecular glass film upon light irradiation. This work is beneficial to deepening the understanding of light–matter interactions and is expected to be applied in different areas.

Multi-panel extra-large scale MIMO based joint activity detection and channel estimation for near-field massive IoT access

The extra-large scale multiple-input multiple-output (XL-MIMO) for the beyond fifth/sixth generation mobile communications is a promising technology to provide Tbps data transmission and stable access service. However, the extremely large antenna array aperture arouses the channel near-field effect, resulting in the deteriorated data rate and other challenges in the practice communication systems. Meanwhile, multi-panel MIMO technology has attracted extensive attention due to its flexible configuration, low hardware cost, and wider coverage. By combining the XL-MIMO and multi-panel array structure, we construct multi-panel XL-MIMO and apply it to massive Internet of Things (IoT) access. First, we model the multi-panel XL-MIMO-based near-field channels for massive IoT access scenarios, where the electromagnetic waves corresponding to different panels have different angles of arrival/departure (AoAs/AoDs). Then, by exploiting the sparsity of the near-field massive IoT access channels, we formulate a compressed sensing based joint active user detection (AUD) and channel estimation (CE) problem which is solved by AMP-EM-MMV algorithm. The simulation results exhibit the superiority of the AMP-EM-MMV based joint AUD and CE scheme over the baseline algorithms.

A Novel Beam Domain Channel Model for B5G Massive MIMO Wireless Communication Systems

In this paper, a novel beam domain channel model (BDCM) is proposed for beyond fifth generation (B5G) massive multiple-input multiple-output (MIMO) wireless communication systems. Different from conventional massive MIMO BDCMs which assumed the far-field plane wavefront effect, the proposed BDCM considers more realistic spherical wavefront caused by near-field effect. We transform a massive MIMO geometry-based stochastic model (GBSM) from the antenna domain to the beam domain through specific algorithms to obtain the novel BDCM. The space-time-frequency correlations of both the GBSM and BDCM are studied, and the correlations for both models at the cluster level are similar. We also compare the quasi-stationary distance (QSD), computational complexity, and channel capacity for both models. Results show that in comparison to the GBSM, the novel BDCM has lower complexity and similar accuracy if the number of beams is sufficiently large. Furthermore, we compare the singular value spreads (SVSs) of both channel models with channel measurements under the same conditions. Both the novel BDCM and GBSM are close to the measurement. Through the above analysis, the novel BDCM is proved to be more convenient for information theory and signal processing researches than the conventional GBSMs.