arXiv subjects

Kang Lin

Publications and source records attributed to Kang Lin.

18 recordsLinked to original sources

arXiv2026

Ultrafast electron vortex produced by a grating made of light

The generation of vortex matter waves carrying quantized orbital angular momentum is challenging and relies heavily on the material nanofabrication methods due to their extremely small de-Broglie wavelengths. Here, we introduce an all-optical method for generating an electron vortex by diffraction through a grating made of light. We realize the orbital angular momentum transfer between free electrons and photons by stimulated Compton scattering. The transferred angular momentum quantum number can be freely tuned. The method can be generalized to a broad range of charged particles, neutral atoms, and molecules of diverse masses. Our results open up novel opportunities for applications in free electron lasers and ultrafast electron microscopy by utilizing the orbital angular momentum degree of freedom of free electrons.

physics.atom-ph↗

arXiv2025

Dynamical phase evolution of Coulomb-focused electrons in strong-field ionization probed by a standing light wave

We investigate the dynamical phase evolution of Coulomb-focused electrons in strong-field ionization. We diffract the electrons with an ultrashort standing light wave to track their time-dependent phase. Our findings show that low-energy electrons exhibit a unique chromosome-shaped diffraction pattern, distinct from higher-energy electrons. Our numerical model quantitatively reproduces the experimental results, confirming this pattern maps the electron's time-dependent phase evolution as it escapes from a Coulomb potential. Our pulsed diffraction grating technique offers a new way to sense an electron's quantum phase without interfering its release mechanism.

physics.atom-ph↗

arXiv2025

Beyond Regularity: Modeling Chaotic Mobility Patterns for Next Location Prediction

Next location prediction is a key task in human mobility analysis, crucial for applications like smart city resource allocation and personalized navigation services. However, existing methods face two significant challenges: first, they fail to address the dynamic imbalance between periodic and chaotic mobile patterns, leading to inadequate adaptation over sparse trajectories; second, they underutilize contextual cues, such as temporal regularities in arrival times, which persist even in chaotic patterns and offer stronger predictability than spatial forecasts due to reduced search spaces. To tackle these challenges, we propose \textbf{\method}, a \underline{\textbf{C}}h\underline{\textbf{A}}otic \underline{\textbf{N}}eural \underline{\textbf{O}}scillator n\underline{\textbf{E}}twork for next location prediction, which introduces a biologically inspired Chaotic Neural Oscillatory Attention mechanism to inject adaptive variability into traditional attention, enabling balanced representation of evolving mobility behaviors, and employs a Tri-Pair Interaction Encoder along with a Cross Context Attentive Decoder to fuse multimodal ``who-when-where'' contexts in a joint framework for enhanced prediction performance. Extensive experiments on two real-world datasets demonstrate that CANOE consistently and significantly outperforms a sizeable collection of state-of-the-art baselines, yielding 3.17\%-13.11\% improvement over the best-performing baselines across different cases. In particular, CANOE can make robust predictions over mobility trajectories of different mobility chaotic levels. A series of ablation studies also supports our key design choices. Our code is available at: https://github.com/yuqian2003/CANOE.

cs.LG↗

arXiv2025

Improving Deep Learning for Accelerated MRI With Data Filtering

Deep neural networks achieve state-of-the-art results for accelerated MRI reconstruction. Most research on deep learning based imaging focuses on improving neural network architectures trained and evaluated on fixed and homogeneous training and evaluation data. In this work, we investigate data curation strategies for improving MRI reconstruction. We assemble a large dataset of raw k-space data from 18 public sources consisting of 1.1M images and construct a diverse evaluation set comprising 48 test sets, capturing variations in anatomy, contrast, number of coils, and other key factors. We propose and study different data filtering strategies to enhance performance of current state-of-the-art neural networks for accelerated MRI reconstruction. Our experiments show that filtering the training data leads to consistent, albeit modest, performance gains. These performance gains are robust across different training set sizes and accelerations, and we find that filtering is particularly beneficial when the proportion of in-distribution data in the unfiltered training set is low.

eess.IV↗

arXiv2025

HS-Mamba: Full-Field Interaction Multi-Groups Mamba for Hyperspectral Image Classification

Hyperspectral image (HSI) classification has been one of the hot topics in remote sensing fields. Recently, the Mamba architecture based on selective state-space models (S6) has demonstrated great advantages in long sequence modeling. However, the unique properties of hyperspectral data, such as high dimensionality and feature inlining, pose challenges to the application of Mamba to HSI classification. To compensate for these shortcomings, we propose an full-field interaction multi-groups Mamba framework (HS-Mamba), which adopts a strategy different from pixel-patch based or whole-image based, but combines the advantages of both. The patches cut from the whole image are sent to multi-groups Mamba, combined with positional information to perceive local inline features in the spatial and spectral domains, and the whole image is sent to a lightweight attention module to enhance the global feature representation ability. Specifically, HS-Mamba consists of a dual-channel spatial-spectral encoder (DCSS-encoder) module and a lightweight global inline attention (LGI-Att) branch. The DCSS-encoder module uses multiple groups of Mamba to decouple and model the local features of dual-channel sequences with non-overlapping patches. The LGI-Att branch uses a lightweight compressed and extended attention module to perceive the global features of the spatial and spectral domains of the unsegmented whole image. By fusing local and global features, high-precision classification of hyperspectral images is achieved. Extensive experiments demonstrate the superiority of the proposed HS-Mamba, outperforming state-of-the-art methods on four benchmark HSI datasets.

cs.CV↗

arXiv2024

Ultrafast Kapitza-Dirac effect

Similar to the optical diffraction of light passing through a material grating, the Kapitza-Dirac effect occurs when an electron is diffracted by a standing light wave. In its original description the effect is time-independent. In the present work, we extend the Kapitza-Dirac concept to the time domain. By tracking the spatiotemporal evolution of a pulsed electron wave packet diffracted by a femtosecond (10 15 second) standing wave pulse in a pump-probe scheme, we observe so far unseen time-dependent diffraction patterns. The fringe spacing in the observed pattern differs from that generated by the conventional Kapitza-Dirac effect, moreover it decreases as the pump-probe delay time increases. By exploiting this time-resolved diffraction scheme, we gather access to the time evolution of the previously inaccessible phase properties of a free electron.

physics.optics↗

arXiv2023

Robustness of Deep Learning for Accelerated MRI: Benefits of Diverse Training Data

Deep learning based methods for image reconstruction are state-of-the-art for a variety of imaging tasks. However, neural networks often perform worse if the training data differs significantly from the data they are applied to. For example, a model trained for accelerated magnetic resonance imaging (MRI) on one scanner performs worse on another scanner. In this work, we investigate the impact of the training data on a model's performance and robustness for accelerated MRI. We find that models trained on the combination of various data distributions, such as those obtained from different MRI scanners and anatomies, exhibit robustness equal or superior to models trained on the best single distribution for a specific target distribution. Thus training on such diverse data tends to improve robustness. Furthermore, training on such a diverse dataset does not compromise in-distribution performance, i.e., a model trained on diverse data yields in-distribution performance at least as good as models trained on the more narrow individual distributions. Our results suggest that training a model for imaging on a variety of distributions tends to yield a more effective and robust model than maintaining separate models for individual distributions.

eess.IV↗

arXiv2022

Image-to-Image MLP-mixer for Image Reconstruction

Neural networks are highly effective tools for image reconstruction problems such as denoising and compressive sensing. To date, neural networks for image reconstruction are almost exclusively convolutional. The most popular architecture is the U-Net, a convolutional network with a multi-resolution architecture. In this work, we show that a simple network based on the multi-layer perceptron (MLP)-mixer enables state-of-the art image reconstruction performance without convolutions and without a multi-resolution architecture, provided that the training set and the size of the network are moderately large. Similar to the original MLP-mixer, the image-to-image MLP-mixer is based exclusively on MLPs operating on linearly-transformed image patches. Contrary to the original MLP-mixer, we incorporate structure by retaining the relative positions of the image patches. This imposes an inductive bias towards natural images which enables the image-to-image MLP-mixer to learn to denoise images based on fewer examples than the original MLP-mixer. Moreover, the image-to-image MLP-mixer requires fewer parameters to achieve the same denoising performance than the U-Net and its parameters scale linearly in the image resolution instead of quadratically as for the original MLP-mixer. If trained on a moderate amount of examples for denoising, the image-to-image MLP-mixer outperforms the U-Net by a slight margin. It also outperforms the vision transformer tailored for image reconstruction and classical un-trained methods such as BM3D, making it a very effective tool for image reconstruction problems.

cs.CV↗

arXiv2022

Femtosecond rotational dynamics of D$_2$ molecules in superfluid helium nanodroplets

Rotational dynamics of D$_2$ molecules inside helium nanodroplets is induced by a moderately intense femtosecond (fs) pump pulse and measured as a function of time by recording the yield of HeD$^+$ ions, created through strong-field dissociative ionization with a delayed fs probe pulse. The yield oscillates with a period of 185 fs, reflecting field-free rotational wave packet dynamics, and the oscillation persists for more than 500 periods. Within the experimental uncertainty, the rotational constant BHe of the in-droplet D$_2$ molecule, determined by Fourier analysis, is the same as Bgas for an isolated D$_2$ molecule. Our observations show that the D$_2$ molecules inside helium nanodroplets essentially rotate as free D$_2$ molecules.

physics.atm-clus↗

arXiv2021

Magnetic-field effect in high-order above-threshold ionization

We experimentally and theoretically investigate the influence of the magnetic component of an electromagnetic field on high-order above-threshold ionization of xenon atoms driven by ultrashort femtosecond laser pulses. The nondipole shift of the electron momentum distribution along the light-propagation direction for high energy electrons beyond the classical cutoff is found to be vastly different from that below the cutoff. A V-shape structure in the momentum dependence of the nondipole shift above the cutoff is identified for the first time. With the help of classical and quantum-orbit analysis, we show that large-angle rescattering of the electrons strongly alters the partitioning of the photon momentum between electron and ion. The sensitivity of the observed nondipole shift to the electronic structure of the target atom is confirmed by three-dimensional time-dependent Schr\"odinger equation simulations for different model potentials.

physics.atom-ph↗

arXiv2021

Photoelectron energy peaks shift against the radiation pressure in strong field ionization

The photoelectric effect describes the ejection of an electron upon absorption of one or several photons. The kinetic energy of this electron is determined by the photon energy reduced by the binding energy of the electron and, if strong laser fields are involved, by the ponderomotive potential in addition. It has therefore been widely taken for granted that for atoms and molecules the photoelectron energy does not depend on the electron's emission direction but theoretical studies have questioned this since 1990. Here we provide experimental evidence, that the energies of photoelectrons emitted against the light-propagation direction are shifted towards higher values while those electrons that are emitted along the light-propagation direction are shifted to lower values. We attribute the energy shift to a nondipole contribution from the interaction of the moving electrons with the incident photons.

physics.atom-ph↗

arXiv2020

Echoes in Unidirectionally Rotating Molecules

Abstract We report the experimental observation of molecular unidirectional rotation (UDR) echoes, and analyze their origin and behavior both classically and quantum mechanically. The molecules are excited by two time-delayed polarization-twisted ultrashort laser pulses and the echoes are measured by exploding the molecules and reconstructing their spatial orientation from the detected recoil ions momenta. Unlike alignment echoes which are induced by linearly polarized pulses, here the axial symmetry is broken by the twisted polarization, giving rise to molecular unidirectional rotation. We find that the rotation sense of the echo is governed by the twisting sense of the second pulse even when its intensity is much weaker than the intensity of the first pulse. In our theoretical study, we rely on classical phase space analysis and on three-dimensional quantum simulations of the laser-driven molecular dynamics. Both approaches nicely reproduce the experimental results. Echoes in general, and the unique UDR echoes in particular, provide new tools for studies of relaxation processes in dense molecular gases.

physics.optics↗

arXiv2019

Echo in a Single Molecule

Echo is a ubiquitous phenomenon found in many physical systems, ranging from spins in magnetic fields to particle beams in hadron accelerators. It is typically observed in inhomogeneously broadened ensembles of nonlinear objects, and is used to eliminate the effects of environmental-induced dephasing, enabling observation of proper, inherent object properties. Here, we report experimental observation of quantum wave packet echoes in a single isolated molecule. In contrast to conventional echoes, here the entire dephasing-rephasing cycle occurs within a single molecule without any inhomogeneous spread of molecular properties, or any interaction with the environment. In our experiments, we use a short laser pulse to impulsively excite a vibrational wave packet in an anharmonic molecular potential, and observe its oscillations and eventual dispersion with time. A second delayed pulsed excitation is applied, giving rise to an echo: a partial recovery of the initial coherent wavepacket. The vibrational dynamics of single molecules is visualized by time-delayed probe pulse dissociating them one at a time. Two mechanisms for the echo formation are discussed: ac Stark-induced molecular potential shaking and creation of depletion-induced "hole" in the nuclear spatial distribution. Interplay between the optically induced echoes and quantum revivals of the vibrational wave packets is observed and theoretically analyzed. The single molecule wave packet echoes may lead to the development of new tools for probing ultrafast intramolecular processes in various molecules.

physics.chem-ph↗

arXiv2018

Field-free Three-Dimensional Orientation of Asymmetric-Top Molecules

Alignment and orientation of molecules by intense, ultrashort laser fields are crucial for a variety of applications in physics and chemistry. These include control of high harmonics generation, molecular orbitals tomography, control of molecular photoionization and dissociation processes, production of "molecular movies" with the help of X-ray free-electron laser sources and ultrafast electron diffraction of relativistic electrons. While the dynamics of laser-induced molecular alignment has been extensively studied and demonstrated, molecular orientation is a much more challenging task, especially for asymmetric-top molecules. Here we report the first experimental demonstration of a field-free, all-optical three-dimensional orientation of asymmetric-top molecules by means of phase-locked cross-polarized two-color laser pulses. In addition to the conventional integrated orientation factor, we report the differential degree of orientation which is not amenable to optical measurements, but is readily accessible in our angle-resolved imaging technique. Our scheme applies to a wide class of asymmetric molecules and opens new ways towards controlling their orientation, eventually leading to direct imaging of structure of gas-phase molecules using advanced free electron laser beams with extremely high spatiotemporal resolution.

physics.chem-ph↗

arXiv2016

Spatiotemporal steering of photoelectron emission in multiphoton above-threshold ionization

We experimentally demonstrate spatiotemporal steering of photoelectron emission in multiphoton above-threshold single ionization of atoms exposed to a phase-controlled orthogonally polarized two-color (OTC) laser pulse. Spatial and energy resolved photoelectron angular distributions are measured as a function of the laser phase, allowing us to look into the fine structures and emission dynamics. The slow and fast photoelectrons, distinguished by the energy larger or smaller than 2Up with Up being the ponderomotive energy of a free electron in the laser field, have distinct spatiotemporal dependences of the laser waveform. The phase-of-phase of the slow electron oscillates as functions of both the energy and emission direction, however, the fast electron present rather flat phase structure which merely depends on its emission direction. Three-dimensional generalized quantum trajectory Monte Carlo simulations are performed to explore the sub-cycle dynamics of the electron emission in the phase-controlled OTC pulse.

physics.optics↗

arXiv2016

Disentangling the role of laser coupling in directional breaking of molecules

The directional control of molecular dissociation with the laser electric field waveform is a paradigm and was demonstrated for a variety of molecules. In most cases, the directional control occurs via a dissociative ionization pathway. The role of laser-induced coupling of electronic states in the dissociating ion versus selective ionization of oriented neutral molecules, however, could not be distinguished for even small heteronuclear molecules such as CO. Here, we introduce a technique, using elliptically polarized pump and linearly polarized two-color probe pulses that unambiguously distinguishes the roles of laser-induced state coupling and selective ionization. The measured photoelectron momentum distributions governed by the light polarizations allow us to coincidently identify the ionization and dissociation from the pump and probe pulses. Directional dissociation of CO+ as a function of the relative phase of the linearly polarized two-color pulse is observed for both parallel and orthogonally oriented molecules. We find that the laser-induced coupling of various electronic states of CO+ plays an important role for the observed directional bond breaking, which is verified by quantum calculations.

physics.optics↗

arXiv2016

Molecular echoes in space and time

Mountain echoes are a well-known phenomenon, where an impulse excitation is mirrored by the rocks to generate a replica of the original stimulus, often with reverberating recurrences. For spin echoes in magnetic resonance and photon echoes in atomic and molecular systems the role of the mirror is played by a second, time delayed pulse which is able to reverse the ow of time and recreate the original event. Recently, laser-induced rotational alignment and orientation echoes were introduced for molecular gases, and discussed in terms of rotational-phase-space filamentation. Here we present, for the first time, a direct spatiotemporal analysis of various molecular alignment echoes by means of coincidence Coulomb explosion imaging. We observe hitherto unreported spatially rotated echoes, that depend on the polarization direction of the pump pulses, and find surprising imaginary echoes at negative times.

physics.optics↗

arXiv2014

Visualizing molecular unidirectional rotation

A molecule can be optically accelerated to rotate unidirectionally at a frequency of a few terahertzes which is many orders higher than the classical mechanical rotor. Such a photon-induced ultrafast molecular unidirectional rotation has been well explored as a controllable spin of the molecular nuclear wave packet. Although it has been observed for more than 10 years, a complete imaging of the unidirectional rotating nuclear wave packet is still missing, which is essentially the cornerstone of all the exploring applications. Here, for the first time, we experimentally visualize the time-dependent evolution of the double-pulse excited molecular unidirectional rotation by Coulomb explosion imaging the rotational nuclear wave packet. Our results reveal comprehensive details undiscovered in pioneering measurements, which exhibits as a joint of the quantum revival of the impulsively aligned rotational wave packet and its unidirectional rotation following the angular momentum conservation. The numerical simulations well reproduce the experimental observations and intuitively revivify the thoroughgoing evolution of the rotational wave packet.

physics.optics↗