Propagation Direction Research Articles

Coherent radiation of an electron bunch colliding with an intense laser pulse

We study the conditions for coherent radiation of an electron bunch driven by a counterpropagating strong pulsed electromagnetic plane wave. We derive the spectrum of the coherent radiation and show that it is emitted backward with respect to the laser propagation direction and has a very narrow angular spread. We demonstrate that for a solid density plasma coherent radiation extends to frequencies up to hundreds of keV, thereby enhancing the low-frequency part of the spectrum by many orders of magnitude. Our analytical findings are tested with three-dimensional particle-in-cell simulations of an electron bunch passing through a laser pulse, clearly demonstrating how the coherence can essentially modify the observed radiation spectrum. Published by the American Physical Society 2024

Nonreciprocal magnetoacoustic waves with out-of-plane phononic angular momenta.

Surface acoustic wave (SAW) can carry phononic angular momentum, showing great potential as an energy-efficient way to control magnetism. Still, out-of-plane phononic angular momentum in SAW and its interaction with magnetism remain elusive. Here, we studied the SAW-induced magnetoacoustic waves and spin pumping in Ni-based films on LiNbO3 with selected SAW propagation direction. The crystal inversion asymmetry induces circularly polarized phonons with large out-of-plane angular momenta so that up to 60% of the SAW power attenuates nonreciprocally controlled by the out-of-plane magnetization component. The SAW propagation direction dependence of the nonreciprocity verifies the crystal origin of the phononic angular momentum, and a chiral spin pumping demonstrates that the circular polarization can control the spin current generation efficiency. These results provide an additional degree of freedom for the acoustic control of magnetism and open an avenue for applying circularly polarized phonons.

Assessment of Anisotropic Acoustic Properties in Additively Manufactured Materials: Experimental, Computational, and Deep Learning Approaches.

The influence of acoustic anisotropy on ultrasonic testing reliability poses a challenge in evaluating products from additive technologies (AT). This study investigates how elasticity constants of anisotropic materials affect defect signal amplitudes in AT products. Experimental measurements on AT samples were conducted to determine elasticity constants. Using Computational Modeling and Simulation Software (CIVA), simulations explored echo signal changes across ultrasound propagation directions. The parameters A13 (the ratio between the velocities of ultrasonic transverse waves with vertical and horizontal polarizations at a 45-degree angle to the growth direction), A3 (the ratio for waves at a 90-degree angle), and Ag (the modulus of the difference between A13 and A3) were derived from wave velocity relationships and used to characterize acoustic anisotropy. Comparative analysis revealed a strong correlation (0.97) between the proposed anisotropy coefficient Ag and the amplitude changes. Threshold values of Ag were introduced to classify anisotropic materials based on observed amplitude changes in defect echo signals. In addition, a method leveraging deep learning to predict Ag based on data from other anisotropy constants through genetic algorithm (GA)-optimized neural network (NN) architectures is proposed, offering an approach that can reduce the computational costs associated with calculating such constants.

On the orbit-induced spin density of tightly focused optical vortex beams: ellipticity and helicity

It has recently been established that a linearly-polarized optical vortex possesses spin angular momentum density in the direction of propagation (longitudinal spin) under tight-focusing. The helicity of light has long been associated with longitudinal spin angular momentum. Here we show that the longitudinal spin density of linearly-polarized vortices is anomalous because it has no associated helicity. It was also recently determined that the polarization-independent helicity of tightly-focused optical vortices is associated with their transverse spin momentum density. The key finding of this work is the fact that, in general, longitudinal spin can not necessarily be associated with helicity, and transverse spin is in general not associated with a zero helicity, and such extraordinary behaviour manifests most clearly for optical vortices under non-paraxial conditions.

How do peak-water-levels respond to natural shoreline restoration projects with progressive tide parallel to shore

Facing strict control of natural shoreline retention and the urgent need for mitigation of oceanic hazards, amid climate change, challenges persist in the spatial allocation of nature-based and concrete-based measures in China's coastal protection and restoration plans. This study explores the feasibility of “Operational Landscape Units" (OLUs) and their impact on peak-water-levels (PWL) and tides on the western Yellow Sea inner shelf (WYS), considering the newly published “China Coastal Ecological Zoning on Land and Seas” in 2023. Numerical experiments with a single OLU of county-size operated along the WYS indicate the largest reduction of 10% (0.23m) in PWL on the coast happens when an OLU is set on the section closest to the tidal amphidrome and the shoreline is parallel to the tidal propagation direction. Furthermore, as the restoration section extends on both sides, the spatial scale of the changed PWL expands. Further investigation reveals that OLUs positioned upstream of the tidal propagational direction can more effectively mitigate coastal inundation risks. This is because local tidal damping through bottom friction inside the OLUs overweighs the tidal damping on the inner shelf outside the OLUs. Overall, this study provides insights into high-tidal-flooding mitigation through OLUs in China's coastal zones, offering references for future coastal plans and flood control deployments.

Super-resolution diffractive neural network for all-optical direction of arrival estimation beyond diffraction limits

Wireless sensing of the wave propagation direction from radio sources lays the foundation for communication, radar, navigation, etc. However, the existing signal processing paradigm for the direction of arrival estimation requires the radio frequency electronic circuit to demodulate and sample the multichannel baseband signals followed by a complicated computing process, which places the fundamental limit on its sensing speed and energy efficiency. Here, we propose the super-resolution diffractive neural networks (S-DNN) to process electromagnetic (EM) waves directly for the DOA estimation at the speed of light. The multilayer meta-structures of S-DNN generate super-oscillatory angular responses in local angular regions that can perform the all-optical DOA estimation with angular resolutions beyond the diffraction limit. The spatial-temporal multiplexing of passive and reconfigurable S-DNNs is utilized to achieve high-resolution DOA estimation over a wide field of view. The S-DNN is validated for the DOA estimation of multiple radio sources over 5 GHz frequency bandwidth with estimation latency over two to four orders of magnitude lower than the state-of-the-art commercial devices in principle. The results achieve the angular resolution over an order of magnitude, experimentally demonstrated with four times, higher than diffraction-limited resolution. We also apply S-DNN’s edge computing capability, assisted by reconfigurable intelligent surfaces, for extremely low-latency integrated sensing and communication with low power consumption. Our work is a significant step towards utilizing photonic computing processors to facilitate various wireless sensing and communication tasks with advantages in both computing paradigms and performance over electronic computing.

Thermo-optic effect induced tunable phase controlled propagation of solitons in a Jaynes-Cummings-Hubbard model

The manipulation of light propagation has garnered significant attention in discrete periodic photon structures. In this study, we investigate the impact of an adjustable phase on soliton behavior within a one-dimensional (1D) coupled cavity array. Each cavity is doped with two-level qubits, and the system can be effectively described by a Jaynes-Cummings-Hubbard model (JC-Hubbard model). By numerically exploring the photonic phase, we reveal that it introduces an additional degree of flexibility in controlling soliton propagation. This flexibility encompasses dispersion relations, propagation direction, transverse velocity, and stability conditions. We observe that soliton styles transition with changes in the tunneling phase. At a phase of 0, solitons form due to the delicate balance between spatial dispersion and system nonlinearity. When the phase increases to π/2, solitons vanish because spatial dispersion is significantly suppressed. The underlying theory explains this suppression, which arises from the opposite phase ±θ. Interestingly, standard temporal solitons emerge in the discrete periodic cavity array. Our investigation has broader applicability extending to various discrete structures, encompassing but not limited to waveguide arrays and optomechanical cavity arrays.

Seismic oceanography of internal solitary waves offshore the South Island, New Zealand

Seismic oceanography has been widely used in the study of internal solitary waves (ISWs) in recent years, and has achieved remarkable results. In this paper, we analyzed the multi-channel seismic reflection data in the Canterbury Basin offshore New Zealand from January 9 to January 29, 2000, collected by R/V Maurice Ewing. We observed 4 groups of ISWs (labeled ISW1s to ISW4s) on 4 seismic survey lines. We studied their waveforms and propagation speeds in detail. There are two theoretical structures used to describe the vertical waveform of ISWs: the first-order nonlinear vertical structure and the linear vertical structure. We found that ISW1s fit the nonlinear structure well, ISW3s and ISW4s fit the linear structure, and ISW2 does not fit either one. As the water depth increases, the waveforms of all ISWs gradually widen. Two satellite SAR images reveal that all ISWs generally travel shoreward across the isobaths. However, the propagation direction of ISW1s is about 354°-360° (clockwise from due north), different from the propagation directions of other ISWs (about 22°-26°), which explains why ISW1s have the largest characteristic half-height width. The estimated propagation speeds are close to the theoretical speeds, confirming our speed correction method. In the end, we also discuss the interaction of ISWs and eddies.

Seismic oceanography of internal solitary waves offshore the South Island, New Zealand

Seismic oceanography has been widely used in the study of internal solitary waves (ISWs) in recent years, and has achieved remarkable results. In this paper, we analyzed the multi-channel seismic reflection data in the Canterbury Basin offshore New Zealand from January 9 to January 29, 2000, collected by R/V Maurice Ewing. We observed 4 groups of ISWs (labeled ISW1s to ISW4s) on 4 seismic survey lines. We studied their waveforms and propagation speeds in detail. There are two theoretical structures used to describe the vertical waveform of ISWs: the first-order nonlinear vertical structure and the linear vertical structure. We found that ISW1s fit the nonlinear structure well, ISW3s and ISW4s fit the linear structure, and ISW2 does not fit either one. As the water depth increases, the waveforms of all ISWs gradually widen. Two satellite SAR images reveal that all ISWs generally travel shoreward across the isobaths. However, the propagation direction of ISW1s is about 354°-360° (clockwise from due north), different from the propagation directions of other ISWs (about 22°-26°), which explains why ISW1s have the largest characteristic half-height width. The estimated propagation speeds are close to the theoretical speeds, confirming our speed correction method. In the end, we also discuss the interaction of ISWs and eddies.

The model for the repeated propagation of information from constant spreaders based on individual cognition levels

On social media platforms, high-impact disseminators act as fixed sources of crucial news, consistently affecting information dissemination. Throughout the process of repeated information dissemination, the audience’s cognition gradually evolves, thereby shaping the pattern and effectiveness of information dissemination. In this study, we present a comprehensive analytical framework and a model for repeated information dissemination, to explain the dynamics of information flow from fixed communicators on social media platforms as well as the dynamic evolution of audience cognition. The model incorporates periodic information sources and considers the directionality of propagation. Through quantitative analysis, we assess the evolution of audience cognition during repeated dissemination and investigate the impact of source complexity, dissemination periodicity, audience psychological fatigue, and cognitive disparities on dissemination effectiveness. Theoretical analysis reveals periodic solutions and threshold conditions for persistence in information dissemination. Furthermore, our research finds that moderate propaganda frequency and intensity can enhance dissemination effectiveness; whereas excessive propaganda may induce fatigue and reduce audience cognition efficacy. Empirical validation of the model demonstrates its effectiveness, offering new perspectives and scientific basis for understanding mechanisms of social media information dissemination and optimizing strategies for news dissemination.

Impact of electric body force on lifted flame displacement speed: Numerical insights

Recent advancements in experimental capabilities have unveiled that externally applied electric fields primarily affect flames through flow modification induced by an electric body force. Specifically, the displacement speed of a flame could be significantly increased (decreased) by a parallel (orthogonal) electric field to the flame's propagation direction, while the flame's propagation speed remained negligibly influenced by an electric field. Here we present numerical findings attributing a modified upstream velocity to an electric body force created by redistributed space charges and a resulted electric field. Our investigation involved both simulation and experiment on propagating edge-flames from a stationary lifted flame exposed to the parallel external electric field. Nitrogen diluted methane jet in a coflow air was employed, and seven charged species (electron, H3O+, O2–, O–, OH–, CHO3–, and CO3–) were included in a reduced methane kinetic mechanism. Both the experiment and simulation consistently demonstrated that enhanced flame displacement speeds resulted from a reverse flow in the upstream, created by the electric body force. A stronger reverse flow, and thus more enhanced flame displacement, was achieved when the electric field direction was aligned with the flame propagation direction compared to the opposite direction. This difference in the reverse flow was attributed to variations in the electric field, caused by the different electric field screening zone within the flame. Surprisingly, space charge densities in the upstream were similar in both polarities, challenging previous hypothesis. The rapid transformation of electrons into O2–, CHO3–, and CO3– imposed a substantial number of charges in the upstream. This insight enhanced our understanding of how flame displacement can be controlled by applying electric fields. Importantly, we suggested that the use of a simple ionic kinetic model may fail to predict the flow field and, consequently, cannot adequately investigate modified flame characteristics.

Efficient single-photon directional transfer between waveguides via two giant atoms

We investigate the single-photon transport properties in a double-waveguide quantum electrodynamic system. We force the energy degeneracy of the collective states by adjusting the direct coupling strength between the two giant atoms. Our results indicate that resonant photons can be completely transferred between the two waveguides owing to the scattering interference of eigenstates, which also results in the directional propagation of resonant photons in the output waveguide. Perfect transfer occurs when the two scattering states degenerate in the energy and decay rates. We further propose a simple scheme to realize the efficient photon transfer with directional control. This study has potential applications in quantum networks and integrated photonic circuits.

Efficient single-photon directional transfer between waveguides via two giant atoms

We investigate the single-photon transport properties in a double-waveguide quantum electrodynamic system. We force the energy degeneracy of the collective states by adjusting the direct coupling strength between the two giant atoms. Our results indicate that resonant photons can be completely transferred between the two waveguides owing to the scattering interference of eigenstates, which also results in the directional propagation of resonant photons in the output waveguide. Perfect transfer occurs when the two scattering states degenerate in the energy and decay rates. We further propose a simple scheme to realize the efficient photon transfer with directional control. This study has potential applications in quantum networks and integrated photonic circuits.

Simplified identification of fire spread risk in building clusters based on digital image processing technology

ABSTRACT Due to factors such as smoke, the non fire identification rate and fire identification accuracy in building fire identification are low, resulting in poor identification results. Therefore, a simplified identification method for building fire spread risk based on digital image processing technology is proposed. A fire spread model is constructed by considering both thermal radiation and thermal plumes, and based on this model, changes in smoke, temperature, and other factors during the occurrence of a fire are obtained. Using K-means to improve the dark channel defogging algorithm, the image after defogging is obtained, making the colour of the image more prominent and the feature information more abundant. Using the area threshold method to further extract the flame target contour in the defogged image, and determine whether the suspected flame area is a flame. Finally, four aspects of colour, sharp corner characteristics, area growth characteristics, and automatic estimation of propagation direction were identified to obtain simplified identification results for fire propagation risk. The experimental results show that the average non fire recognition rate of the proposed method reaches 95.8%, with good fire recognition accuracy and recognition effect, which verifies its feasibility.

Uncertainty quantification by direct propagation of shallow ensembles

Statistical learning algorithms provide a generally-applicable framework to sidestep time-consuming experiments, or accurate physics-based modeling, but they introduce a further source of error on top of the intrinsic limitations of the experimental or theoretical setup. Uncertainty estimation is essential to quantify this error, and to make application of data-centric approaches more trustworthy. To ensure that uncertainty quantification is used widely, one should aim for algorithms that are accurate, but also easy to implement and apply. In particular, including uncertainty quantification on top of an existing architecture should be straightforward, and add minimal computational overhead. Furthermore, it should be easy to manipulate or combine multiple machine-learning predictions, propagating uncertainty over further modeling steps. We compare several well-established uncertainty quantification frameworks against these requirements, and propose a practical approach, which we dub direct propagation of shallow ensembles, that provides a good compromise between ease of use and accuracy. We present benchmarks for generic datasets, and an in-depth study of applications to the field of atomistic machine learning for chemistry and materials. These examples underscore the importance of using a formulation that allows propagating errors without making strong assumptions on the correlations between different predictions of the model.

Asymmetric transmission for linearly polarized terahertz waves without polarization conversion

In this study, a novel metamaterial (MM) architecture is presented, which comprises cut wires and split-ring resonators (SRRs) and demonstrates asymmetric transmission characteristics. Unlike previously reported multi-layer chiral structures, spatial asymmetry is introduced by aligning the cut wires and SRRs with the propagation direction of electromagnetic waves. This innovative alignment effectively disrupts time-reversal symmetry, enabling asymmetric transmission. A thorough analysis of the electromagnetic field distribution and surface current patterns offers insights into the fundamental mechanisms underlying this asymmetric transmission. Notably, the ability of this MM design to maintain the polarization state of transmitted waves positions it as a promising candidate for terahertz systems applications.

Dynamics of polarization-tuned mirror symmetry breaking in a rotationally symmetric system

Lateral momentum conservation is typically kept in a non-absorptive rotationally symmetric system through mirror symmetry via Noether’s theorem when illuminated by a homogeneous light wave. Therefore, it is still very challenging to break the mirror symmetry and generate a lateral optical force (LOF) in the rotationally symmetric system. Here, we report a general dynamic action in the SO(2) rotationally symmetric system, originating from the polarization-tuned mirror symmetry breaking (MSB) of the light scattering. We demonstrate theoretically and experimentally that MSB can be generally applied to the SO(2) rotationally symmetric system and tuned sinusoidally by polarization orientation, leading to a highly tunable and highly efficient LOF (9.22 pN/mW/μm−2) perpendicular to the propagation direction. The proposed MSB mechanism and LOF not only complete the sets of MSB of light-matter interaction and non-conservative force only using a plane wave but also provide extra polarization manipulation freedom.

A climatology of stratospheric gravity waves induced by tropical cyclones on the northwest Pacific Ocean

A climatology of stratospheric gravity waves (SGWs) induced by tropical cyclones (TC-SGWs) is necessary for stratospheric aviation safety and accurate numerical weather prediction. Few previous works have characterized the climatology of TC-SGWs, and the relative importance of the source and background wind for TC-SGWs has not been clearly established. Here we present the distribution of TC-SGWs formed over the northwest Pacific Ocean based on 30 years of ERA5 reanalysis data. Using explainable artificial intelligence, we found the background wind in the lower stratosphere to be more important than TCs in controlling the distribution pattern of TC-SGWs. The background wind shear can influence the direction of TC-SGW propagation by refraction and filtering, which influences the distribution pattern of the TC-SGWs. The TC intensity only influences the TC-SGW intensity. Our results provide information on TC-SGW hotspots and suggest the importance of the background wind shear.

An imaging condition for acoustic reverse time migration with implicit angle‐dependent weighting factors and its extended applications for imaging elastic P–P and S–S data

AbstractThe imaging condition is a crucial component of the reverse time migration. In its conventional form, it involves cross‐correlating the extrapolated source‐ and receiver‐side wavefields. Effective imaging conditions are usually developed to suppress imaging artefacts (e.g. low‐wavenumber artefacts) and enhance the image quality. For acoustic reverse time migration, not only the scalar pressure but also their spatial and/or time derivatives are used in the imaging condition, similar to the gradient terms of adjoint tomography. These operations implicitly introduce additional angle‐domain weighting factors to the image results. In this study, based on an analysis of angle‐dependent properties of the existing imaging conditions, we propose a new imaging condition tailored for acoustic reverse time migration. It can be implemented efficiently using the variables within the finite‐difference solver. Without explicitly measuring wave propagation directions, the proposed imaging condition can naturally suppress the low‐wavenumber artefacts while maintaining a relatively wider imaging aperture, thereby corresponding to a broader wavenumber sampling range. Additionally, the evolved imaging conditions for imaging elastic P–P and S–S scattering and reflections are also formulated. In the angle domain, we conduct a comparative analysis between existing imaging conditions and the newly proposed ones. Various numerical examples are provided to demonstrate the advantages of the new imaging conditions. A comprehensive understanding of their angle‐domain properties may be further beneficial to constructing reasonable inversion strategies for full waveform inversion.

Low-frequency non-reciprocal sound propagation features in thermoacoustic waveguide.

Thermoacoustic waveguides are systems of hollow tubes and thermally graded porous segments that can operate as active materials where acoustic waves receive energy from an external heat source. This work demonstrates that by adjusting the pore geometry several unique low-frequency propagation features arise from the complex-valued band structure of periodic thermoacoustic waveguides that reflect into the acoustic pressure field within finite-length systems. Numerical methods have been employed to model waveguides with porous segments constituted by cylindrical inclusions (parallel pins). In periodic structures, a critical frequency emerges where the sign of the refractive index in one direction of propagation changes, thus zero- and negative-unidirectional refractive index, unidirectional energy transport, and amplification/attenuation crossover effects may take place. On the other hand, the study of the acoustic pressure field shows that, for wave packets with either direction of propagation, finite-length waveguides may behave as active acoustic metamaterials with zero- or negative-refractive index. The acoustic pressure field in the waveguide, generated by an upstream source, may exhibit increasing amplitude and phase recovery farther away from the source, mimicking the field created by a downstream source, propagating upstream in a non-active medium.