Broadband Field Research Articles

Source localization based on field signatures: Laboratory ultrasonic validation

Location awareness in wireless networks is essential for emergency services, navigation, gaming and many other applications. This article presents a method for source localization based on measuring the amplitude-phase distribution of the field at the base station. The existing scatterers in the target area create unique scattered field interference at each source location. The unique field interference at each source location results in a unique field signature at the base station which is used for source localization. In the proposed method, the target area is divided into a grid with a step of less than half the wavelength. Each grid node is characterized by its field signature at the base station. Field signatures corresponding to all nodes are normalized and stored in the base station as fingerprints for source localization. The normalization of the field signatures avoids the need for time synchronization between the base station and the source. When a source transmits signals, the generated field signature at the base station is normalized and then correlated with the stored fingerprints. The maximum correlation value is given by the node to which the source is the closest. Numerical simulations and results of experiments on ultrasonic waves in the air show that the ultrasonic source is correctly localized using broadband field signatures with one base station and without time synchronization. The proposed method is potentially applicable for indoor localization and navigation of mobile robots.

Predicting ion cyclotron emission from neutral beam heated plasmas in Wendelstein7-X stellarator

Measurements of ion cyclotron emission (ICE) are planned for magnetically confined fusion plasmas heated by neutral beam injection (NBI) in the Wendelstein 7-X stellarator (W7-X). Freshly injected NBI ions in the edge region, whose velocity-space distribution function approximates a delta-function, are potentially unstable against the magnetoacoustic cyclotron instability (MCI), which could drive a detectable ICE signal. Prediction of ICE from NBI protons in W7-X hydrogen plasmas is challenging, owing to the low ratio of the ions’ perpendicular velocity to the local Alfvén speed, v⊥(NBI)/VA≃0.14 . We address this from first principles, using the particle-in-cell kinetic code EPOCH. This self-consistently solves the Lorentz force equation and Maxwell’s equations for tens of millions of computational ions (both thermal majority and energetic NBI minority) and electrons, fully resolving gyromotion and hence capturing the cyclotron resonant phenomenology which gives rise to ICE. Our simulations predict an ICE signal which is predominantly electrostatic while incorporating a significant electromagnetic component. Its frequency power spectrum reflects novel MCI physics, reported here for the first time. The NBI ions relaxing under the MCI first drive broadband field energy at frequencies a little below the lower hybrid frequency ωLH, across the wavenumber range kωc/VA=40 –60, where ω c and VA denote ion cyclotron frequency and Alfvén velocity. Nonlinear coupling between these waves then excites spectrally structured ICE with narrow peaks, at much lower frequencies, typically the proton cyclotron frequency and its lower harmonics. The relative strength of these peaks depends on the specifics of the NBI ion velocity-space distribution and of the local plasma conditions, implying diagnostic potential for the predicted ICE signal from W7-X.

Plasmonic Bullseye Nanocavities for Broadband Light Localization and Multi‐Wavelength SERS

AbstractPlasmonic nanostructures capable of broadband light trapping and field enhancement have promising applications in a wide range of fields. This study presents a platform for broadband, polarization‐independent field enhancement in the visible regime through the use of width‐graded nanocavities in a bullseye configuration. The fabrication procedure utilizes electron beam lithography (EBL) to achieve fine control over the nanocavity geometry and template stripping to enable rapid and low‐cost production. The utility of these devices as substrates for multi‐wavelength surface enhanced Raman spectroscopy (SERS) is demonstrated through molecular detection in a 10 μM solution at two excitation wavelengths. The impact of bullseye geometry on both the broadband spectral response and multi‐wavelength SERS performance is examined. The measured SERS enhancement factor (EF) is shown to depend primarily on the plasmonically active surface area of the device, regardless of the local electromagnetic field strength within the nanocavities. These results highlight not only the utility of the width‐graded bullseye as a broadband platform for SERS and other applications but also provide design guidelines to optimize the enhancement factor and broadband performance of similar devices.

Terahertz Plasmonic Metagrating Design Simultaneously Enabling Broadband Extraordinary Transmission and Field Enhancement

Terahertz Plasmonic Metagrating Design Simultaneously Enabling Broadband Extraordinary Transmission and Field Enhancement

Measurement of spatial coherence of light [Invited

The most frequently used experimental techniques for measuring the spatial coherence properties of classical light fields in the space-frequency and space-time domains are reviewed and compared, with some attention to polarization effects. In addition to Young's classical two-pinhole experiment and several of its variations, we discuss methods that allow the determination of spatial coherence at higher data acquisition rates and also permit the characterization of lower-intensity light fields. These advantages are offered, in particular, by interferometric schemes that employ only beam splitters and reflective elements, and thereby also facilitate spatial coherence measurements of broadband fields.

Spectral coherence of white LEDs

We address space–frequency domain coherence properties of broadband light-emitting diodes (white LEDs) and fields radiated by them. Inverse-source techniques are employed to determine the spectral degree of spatial coherence of an effective planar source representing a real LED, and coherent elementary fields associated with it. By fitting with experimental measurements, we formulate simple analytical coherence models that can be used as a basis for theoretical and experimental studies of the coherence of polychromatic stationary light in free space and in various optical systems. In particular, we find that radiation from white LEDs follows closely Wolf’s scaling law for spectral invariance [Phys. Rev. Lett. 56, 1370 (1986)PRLTAO0031-900710.1103/PhysRevLett.56.1370] in the blue and the phosphor-generated parts of the spectrum separately, but not across the entire white-light spectrum.

Diamond NV Centers Based Quantum Sensor Using a VCO Integrated With Filtering Antenna

This paper presents a diamond nitrogen-vacancy (NV) center based magnetic field sensor using a voltage-controlled oscillator (VCO) integrated with a filtering antenna. In existing NV-based quantum sensing systems, microwave source and delivery units are implemented by a set of bulky and costly instruments. This limits its practical applications as well as portability. Here, a VCO integrated with the filtering antenna is designed, providing a more uniform, efficient, broadband and lower phase noise microwave field coupling to the NV centers. As a result, the sensitivity and the dynamic range have been improved over the conventional scheme using separate microwave components, such as antennas, transmission lines, and resonators. Moreover, in this work, the optic filter, and photodetector are also integrated using the hybrid microwave integrated circuits (HMIC) technique for the first time. This significantly increases the integration level and reduces the cost of the NV-based magnetometers. The experimental results indicate that our VCO integrated filtering antenna achieves a sensitivity of 8.52 μT/Hz^1/2 and a dynamic range up to 32.73 dB, which are both greatly improved compared to the VCO with a single resonator. This validates the effectiveness of the approach.

Laser warning with broadband and large field of view based on black-white grating

Laser warning with broadband and large field of view based on black-white grating

Unsupervised deep learning with higher-order total-variation regularization for multidimensional seismic data reconstruction

In 3D marine seismic acquisition, the seismic wavefield is not sampled uniformly in the spatial directions. This leads to a seismic wavefield consisting of irregularly and sparsely populated traces with large gaps between consecutive sail lines, especially in the near offsets. The problem of reconstructing the complete seismic wavefield from a subsampled and incomplete wavefield is formulated as an underdetermined inverse problem. We have investigated unsupervised deep learning based on a convolutional neural network for multidimensional wavefield reconstruction of irregularly populated traces defined on a regular grid. Our network is based on an encoder-decoder architecture with an overcomplete latent representation, including appropriate regularization penalties to stabilize the solution. We proposed a combination of penalties, which consists of the [Formula: see text]-norm penalty on the network parameters, and a first- and second-order total-variation penalty on the model. We determined the performance of our method on broadband synthetic data and field data represented by constant-offset gathers from a source-over-cable data set from the Barents Sea. In the field data example, we compare the results to a full production flow from a contractor company, which is based on a 5D Fourier interpolation approach. In this example, our approach displays improved reconstruction of the wavefield with less noise in the sparse near offsets compared with the industry approach, which leads to improved structural definition of the near offsets in the migrated sections.

Revisiting statistical properties of surf parameter with characteristic wave parameters for single random waves including spectral bandwidth effects

Results from a comparative study of the joint distribution of surf parameter and wave period are provided. First, two transformed joint distributions of wave height and wave period are compared. One of the distributions is a parametric model originating from a best fit to relatively broad-band field data covering a wide range of wave conditions, whilst the other distribution is theoretically based. It appears that the theoretically based distribution does not represent the features of the parametric model especially well, suggesting that parametric models should be used to describe relatively broad-banded data. Then, the theoretically based joint distribution of wave height and wave period is transformed to the joint distributions of surf parameter with wave height and wave period and it is demonstrated how these distributions are affected by the spectral bandwidth. Finally, the theoretically based distribution of wave height and wave period is also transformed to the joint distribution of wave runup time and wave period due to its relation to the stability of rubble-mound breakwaters. Comparisons are made with a limited set of data representing results from small-scale laboratory experiments related to stability of rubble-mound breakwaters. The agreement between measurements and predictions of the distribution of the surf parameter is fair, whilst the agreement is poorer for the probability of resonance.

Exposure to radio-frequency electromagnetic radiation shortens sleep duration and lengthens sleep latency and sleep inertia in humans living in proximity to the base transceiver stations

Radio-frequency electromagnetic radiations (RF-EMRs) are ubiquitous at present. Therefore, it is essential to assess the impact of RF-EMRs on human health. In this study, we examined the non-thermal effects of RF-EMR expo-sure on behavioral sleep patterns in humans. A total of 1072 randomly se-lected individuals living in the proximity of base transceiver stations (BTS) participated in the study. The sample consisted of 122 subjects from zone A (Inter-tower region), 310 from zone B (0-150 m), 316 from zone C (150-300 m), 197 from zone D (300-500 m), and 127 from the control zone (without BTS installations). We classified the zones as a function of distance from the BTS. We measured electric-field strength at each participant’s house using Narda Broadband Field Meter-550 equipped with EF0-391 probe. We used Munich-Chronotype Questionnaire to determine each subject’s behavioral sleep patterns. ANOVA results revealed the highest E-field strength in zone-A than the other zones and control. Results from ANCOVA, Kruskal-Wallis, and Mann-Whitney U tests showed that the participants from zone A had shorter sleep duration, and longer sleep latency and inertia than those living in other zones. Further, a significant effect of co-factors ‘gender’ and ‘year of resi-dence’ was validated on mid-sleep (work and free days). Compared to wom-en and > 5-year residents, men and 1-5-year residents had delayed mid-sleep. We concluded that RF-EMR might alter the behavioral sleep patterns of subjects living in the vicinity of BTS. However, further confirmatory and extensive studies are necessary, involving a large sample living near many more BTS installations.

Home Use of a Percutaneous Wireless Intracortical Brain-Computer Interface by Individuals With Tetraplegia.

Objective.Individuals with neurological disease or injury such as amyotrophic lateral sclerosis, spinal cord injury or stroke may become tetraplegic, unable to speak or even locked-in. For people with these conditions, current assistive technologies are often ineffective. Brain-computer interfaces are being developed to enhance independence and restore communication in the absence of physical movement. Over the past decade, individuals with tetraplegia have achieved rapid on-screen typing and point-and-click control of tablet apps using intracortical brain-computer interfaces (iBCIs) that decode intended arm and hand movements from neural signals recorded by implanted microelectrode arrays. However, cables used to convey neural signals from the brain tether participants to amplifiers and decoding computers and require expert oversight, severely limiting when and where iBCIs could be available for use. Here, we demonstrate the first human use of a wireless broadband iBCI.Methods.Based on a prototype system previously used in pre-clinical research, we replaced the external cables of a 192-electrode iBCI with wireless transmitters and achieved high-resolution recording and decoding of broadband field potentials and spiking activity from people with paralysis. Two participants in an ongoing pilot clinical trial completed on-screen item selection tasks to assess iBCI-enabled cursor control.Results:Communication bitrates were equivalent between cabled and wireless configurations. Participants also used the wireless iBCI to control a standard commercial tablet computer to browse the web and use several mobile applications. Within-day comparison of cabled and wireless interfaces evaluated bit error rate, packet loss, and the recovery of spike rates and spike waveforms from the recorded neural signals. In a representative use case, the wireless system recorded intracortical signals from two arrays in one participant continuously through a 24-hour period at home.Significance.Wireless multi-electrode recording of broadband neural signals over extended periods introduces a valuable tool for human neuroscience research and is an important step toward practical deployment of iBCI technology for independent use by individuals with paralysis. On-demand access to high-performance iBCI technology in the home promises to enhance independence and restore communication and mobility for individuals with severe motor impairment.

Petahertz-scale nonlinear photoconductive sampling in air

The electric field waveform of a light field can be used to unlock a detailed recording of its interaction with matter, but accessing it requires a measurement with subfemtosecond temporal resolution. We demonstrate nonlinear photoconductive sampling of light fields at optical frequencies in ambient air. The resulting detection method provides broadband electric field measurement in an inexpensive setup using a self-healing medium. A direct comparison is made between detection in air and in quartz, validating the technique up to 0.7 PHz. This provides both a simple route toward field metrology and a new platform for future studies in attosecond physics without the need for complex vacuum setups or sophisticated sample preparation.

A Compact, Broadband, and Calculable Electromagnetic Field Source for Quality Assurance in EMC Testing

The aim of this article is to describe the design, realization, and characterization of a compact, broadband, and calculable electromagnetic field source. The source has been extensively used as a traveling reference radiator in proficiency tests of radiated emission measurements. The result of its characterization has been validated by more than 500 measurements over the frequency range from 50 to 6000 MHz in semianechoic (3 and 10 m) and fully anechoic (3 m) chambers. The use of the source as a reference radiator for emission measurements could be extended to other testing facilities, such as transverse electromagnetic waveguides and reverberation chambers.

Broadband Field Localization, Density of States, and Nonlinearity Enhancement in Nonreciprocal and Topological Hotspots

Nonlinear and quantum optics, and electromagnetic applications such as sensing, benefit from large, broadband electric field enhancements, which are usually achieved through localized resonances. That approach, however, involves a stringent compromise between field enhancement, bandwidth, and overall device size. Here the authors study the unusual phenomena arising at a $n\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}p\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l$ $h\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}p\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}t$, a nonresonant broadband accumulation of energy in an ultrasmall volume at the end of a unidirectional waveguide. These hotspots support larger field enhancements $a\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}d$ much broader bandwidths than conventional systems, providing an attractive alternative for enhancing light-matter interaction.

Time Domain Sampling of the Radial Functions in Spherical Harmonics Expansions

Spherical harmonics representations are widely adopted in applications such as analysis, manipulation, and synthesis of wave fields that are either captured or simulated. Recent studies have shown that, for broadband fields, a time domain representation of spherical harmonics expansions can benefit from computational efficiency and favorable transient properties. For practical usage, an accurate discrete-time modeling of spherical harmonics expansions is indispensable. The main challenge is to model the so called radial functions which describe the radial and frequency dependencies. In homogeneous cases, they are commonly realized as finite impulse response filters where the coefficients are obtained by sampling the continuous-time representations. This article investigates the temporal and spectral properties of the resulting discrete-time radial functions. The spectral distortions caused by aliasing are evaluated both analytically and numerically, revealing the influence of the distance from the expansion center, sampling frequency, and fractional sample delay. It is also demonstrated how the aliasing can be reduced by employing a recently introduced band limitation method.

Broadband Field Enhancement and Giant Nonlinear Effects in Terminated Unidirectional Plasmonic Waveguides

Unidirectional wave propagation in nonreciprocal structures enables exciting opportunities to control and enhance wave-matter interactions in extreme ways. Within this context, here we investigate the possibility of using terminated unidirectional plasmonic waveguides to enhance typically weak nonlinear effects by orders of magnitude. We theoretically demonstrate remarkable levels of electric field enhancement and confinement (field hot spots) when the unidirectional waveguiding structure is terminated with a suitable boundary that fully stops the one-way mode. Such a large field enhancement, originating from a nonresonant effect, is fundamentally different from the narrow-band field concentration effects in resonant plasmonic structures. Instead, it is analogous to the broadband response of plasmonic tapers, but without the need for any adiabatic impedance matching. We show that this effect can indeed lead to a substantial boosting of nonlinear light-matter interactions, exemplified by an improvement of several orders of magnitude in the third-harmonic-generation efficiency, which is of large significance for several applications. More broadly, our findings show the potential of extreme nonreciprocal configurations for enhanced wave-matter interactions.

Harmonic optical tomography of nonlinear structures

Second-harmonic generation microscopy is a valuable label-free modality for imaging non-centrosymmetric structures and has important biomedical applications from live-cell imaging to cancer diagnosis. Conventional second-harmonic generation microscopy measures intensity signals that originate from tightly focused laser beams, preventing researchers from solving the scattering inverse problem for second-order nonlinear materials. Here, we present harmonic optical tomography (HOT) as a novel modality for imaging microscopic, nonlinear and inhomogeneous objects. The HOT principle of operation relies on inter-ferometrically measuring the complex harmonic field and using a scattering inverse model to reconstruct the three-dimensional distribution of harmonophores. HOT enables strong axial sectioning via the momentum conservation of spatially and temporally broadband fields. We illustrate the HOT operation with experiments and reconstructions on a beta-barium borate crystal and various biological specimens. Although our results involve second-order nonlinear materials, we show that this approach applies to any coherent nonlinear process.

Moiré Hyperbolic Metasurfaces.

Recent advances in twistronics of low-dimensional materials, such as bilayer graphene and transition-metal dichalcogenides, have enabled a plethora of unusual phenomena associated with moiré physics. However, several of these effects require demanding manipulation of superlattices at the atomic scale, such as the careful control of rotation angle between two closely spaced atomic lattices. Here, we study moiré hyperbolic plasmons in pairs of hyperbolic metasurfaces (HMTSs), unveiling analogous phenomena at the mesoscopic scale. HMTSs are known to support confined surface waves collimated toward specific directions determined by the metasurface dispersion. By rotating two evanescently coupled HMTSs with respect to one another, we unveil rich dispersion engineering, topological transitions at magic angles, broadband field canalization, and plasmon spin-Hall phenomena. These findings open remarkable opportunities to advance metasurface optics, enriching it with moiré physics and twistronic concepts.

Validity of the dispersion relations in magnetotellurics: Part I\u2014theory

Application of the dispersion relations (DR) in magnetotellurics (MT) is an efficient tool of post-processing and quality assessment of broadband field data. The main limitation of the approach is that it requires the observed transfer functions to be causal and minimum-phase (MP), which is formally secured only for 1-D and some types of 2-D impedances. As a consequence, many MT practitioners involuntarily restrict the DR application to apparent resistivity curves acquired in relatively simple geological conditions. In the present research, we show how an inherently non-MP or non-causal transfer function could be recognized, and propose a universal technique, which makes it possible to correctly apply the DR virtually to any set of field MT data.