Searcharxiv⌕ Search

arXiv subjects

Long Xu

Publications and source records attributed to Long Xu.

53 records · Page 3Linked to original sources

Droplet penetration through an inclined mesh

Droplets with different Weber numbers We impacting meshes at various inclination angles were investigated using high-speed photography. It was found that the droplet mesh penetration can be completely suppressed by inclining the mesh. The phase diagrams in the (We, α) plane determining the expected type of penetration have been determined experimentally for meshes of various structures. It was shown that the Weber numbers for transition between no-penetration and incomplete penetration as well as for transition between incomplete penetration and complete penetration increase monotonically with α. A simple model for predicting the transition thresholds is proposed and is validated by comparisons with experimental results. It is shown that both the inclination angle and the mesh open area fraction determine the type of penetration.

physics.flu-dyn↗

Deep Learning Eliminates Massive Dust Storms from Images of Tianwen-1

Dust storms may remarkably degrade the imaging quality of Martian orbiters and delay the progress of mapping the global topography and geomorphology. To address this issue, this paper presents an approach that reuses the image dehazing knowledge obtained on Earth to resolve the dust-removal problem on Mars. In this approach, we collect remote-sensing images captured by Tianwen-1 and manually select hundreds of clean and dusty images. Inspired by the haze formation process on Earth, we formulate a similar visual degradation process on clean images and synthesize dusty images sharing a similar feature distribution with realistic dusty images. These realistic clean and synthetic dusty image pairs are used to train a deep model that inherently encodes dust irrelevant features and decodes them into dust-free images. Qualitative and quantitative results show that dust storms can be effectively eliminated by the proposed approach, leading to obviously improved topographical and geomorphological details of Mars.

cs.CV↗

Optimal Selective Orientation of Chiral Molecules Using Femtosecond Laser Pulses

We present a comprehensive study of enantioselective orientation of chiral molecules excited by a pair of delayed cross-polarized femtosecond laser pulses. We show that by optimizing the pulses' parameters, a significant (~ 10%) degree of enantioselective orientation can be achieved at zero and at five kelvin rotational temperatures. This study suggests a set of reasonable experimental conditions for inducing and measuring strong enantioselective orientation. The strong enantioselective orientation and the wide availability of the femtosecond laser systems required for the proposed experiments may open new avenues for discriminating and separating molecular enantiomers.

physics.optics↗

Complementary Feature Enhanced Network with Vision Transformer for Image Dehazing

Conventional CNNs-based dehazing models suffer from two essential issues: the dehazing framework (limited in interpretability) and the convolution layers (content-independent and ineffective to learn long-range dependency information). In this paper, firstly, we propose a new complementary feature enhanced framework, in which the complementary features are learned by several complementary subtasks and then together serve to boost the performance of the primary task. One of the prominent advantages of the new framework is that the purposively chosen complementary tasks can focus on learning weakly dependent complementary features, avoiding repetitive and ineffective learning of the networks. We design a new dehazing network based on such a framework. Specifically, we select the intrinsic image decomposition as the complementary tasks, where the reflectance and shading prediction subtasks are used to extract the color-wise and texture-wise complementary features. To effectively aggregate these complementary features, we propose a complementary features selection module (CFSM) to select the more useful features for image dehazing. Furthermore, we introduce a new version of vision transformer block, named Hybrid Local-Global Vision Transformer (HyLoG-ViT), and incorporate it within our dehazing networks. The HyLoG-ViT block consists of the local and the global vision transformer paths used to capture local and global dependencies. As a result, the HyLoG-ViT introduces locality in the networks and captures the global and long-range dependencies. Extensive experiments on homogeneous, non-homogeneous, and nighttime dehazing tasks reveal that the proposed dehazing network can achieve comparable or even better performance than CNNs-based dehazing models.

cs.CV↗

Enantioselective chiral orientation induced by a combination of a long and a short laser pulse

Enantioselective orientation of chiral molecules excited by a shaped picosecond laser pulse and a delayed femtosecond pulse is considered. Using quantum mechanical simulations, we demonstrate a strong field-free enantioselective orientation along the laser propagation direction. In addition, we use a classical model to reproduce the enantioselective orientation. Moreover, the analysis of the corresponding classical system allows understanding the qualitative features of the induced enantioselective orientation. The strong enantioselective orientation may be used for the separation of chiral enantiomers using inhomogeneous electrostatic fields.

physics.optics↗

Three Dimensional Orientation of Complex Molecules Excited by Two-Color Femtosecond Pulses

We study the excitation of asymmetric-top (including chiral) molecules by two-color femtosecond laser pulses. In the cases of non-chiral asymmetric-top molecules excited by an orthogonally polarized two-color pulse, we demonstrate, classically and quantum mechanically, three-dimensional orientation. For chiral molecules, we show that the orientation induced by a cross-polarized two-color pulse is enantioselective along the laser propagation direction, namely, the two enantiomers are oriented in opposite directions. On the short time scale, the classical and quantum simulations give results that are in excellent agreement, whereas on the longer time scale, the enantioselective orientation exhibits quantum beats. These observations are qualitatively explained by analyzing the interaction potential between the two-color pulse and molecular (hyper-)polarizability. The prospects for utilizing the long-lasting orientation for measuring and using the enantioselective orientation for separating the individual enantiomers are discussed.

physics.optics↗

Long-Lasting Orientation of Symmetric-top Molecules Excited by Two-Color Femtosecond Pulses

Impulsive orientation of symmetric-top molecules excited by two-color femtosecond pulses is considered. In addition to the well-known transient orientation appearing immediately after the pulse and then reemerging periodically due to quantum revivals, we report the phenomenon of field-free long-lasting orientation. Long-lasting means that the time averaged orientation remains non-zero until destroyed by other physical effects, e.g. intermolecular collisions. The effect is caused by the combined action of the field-polarizability and field-hyperpolarizability interactions. The dependence of degree of long-lasting orientation on temperature and pulse's parameters is considered. The effect can be measured by means of second (or higher-order) harmonic generation, and may be used to control the deflection of molecules traveling through inhomogeneous electrostatic fields.

physics.optics↗

Enantioselective Orientation of Chiral Molecules Induced by Terahertz Pulses with Twisted Polarization

Chirality and chiral molecules are key elements in modern chemical and biochemical industries. Individual addressing, and the eventual separation of chiral enantiomers has been and still is an important elusive task in molecular physics and chemistry, and a variety of methods has been introduced over the years to achieve this goal. Here, we theoretically demonstrate that a pair of cross-polarized THz pulses interacting with chiral molecules through their permanent dipole moments induces an enantioselective orientation of these molecules. This orientation persists for a long time, exceeding the duration of the THz pulses by several orders of magnitude, and its dependency on temperature and pulses' parameters is investigated. The persistent orientation may enhance the deflection of the molecules in inhomogeneous electromagnetic fields, potentially leading to viable separation techniques.

physics.optics↗

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↗

Long-lasting Molecular Orientation Induced by a Single Terahertz Pulse

Control of molecular rotations by laser fields is an active area of research focusing on the alignment/orientation of otherwise isotropic molecular samples. Oriented molecules are useful in many applications, such as molecular orbital tomography, femtosecond imaging of molecular structure, dynamics and chemical reaction control. In this work, we present a novel, previously unreported phenomenon appearing in thermal ensembles of non-linear polar molecules excited by a single THz pulse. We find that the induced orientation lasts long after the excitation pulse is over. In the case of symmetric-top molecules, the ensemble averaged orientation remains indefinitely constant after the pulse, whereas in the case of asymmetric-top molecules the orientation persists for a long time after the end of the pulse. We discuss the underlying mechanism, study its non-monotonous temperature dependence, and show that there exists an optimal temperature for the maximal residual orientation. The persistent orientation implies long lasting macroscopic dipole moment, which may be probed by second (or higher order) harmonic generation.

physics.chem-ph↗

Solar Image Deconvolution by Generative Adversarial Network

With Aperture synthesis (AS) technique, a number of small antennas can assemble to form a large telescope which spatial resolution is determined by the distance of two farthest antennas instead of the diameter of a single-dish antenna. Different from direct imaging system, an AS telescope captures the Fourier coefficients of a spatial object, and then implement inverse Fourier transform to reconstruct the spatial image. Due to the limited number of antennas, the Fourier coefficients are extremely sparse in practice, resulting in a very blurry image. To remove/reduce blur, "CLEAN" deconvolution was widely used in the literature. However, it was initially designed for point source. For extended source, like the sun, its efficiency is unsatisfied. In this study, a deep neural network, referring to Generative Adversarial Network (GAN), is proposed for solar image deconvolution. The experimental results demonstrate that the proposed model is markedly better than traditional CLEAN on solar images.

astro-ph.SR↗

Image Quality Assessment for Omnidirectional Cross-reference Stitching

Along with the development of virtual reality (VR), omnidirectional images play an important role in producing multimedia content with immersive experience. However, despite various existing approaches for omnidirectional image stitching, how to quantitatively assess the quality of stitched images is still insufficiently explored. To address this problem, we establish a novel omnidirectional image dataset containing stitched images as well as dual-fisheye images captured from standard quarters of 0$^\circ$, 90$^\circ$, 180$^\circ$ and 270$^\circ$. In this manner, when evaluating the quality of an image stitched from a pair of fisheye images (e.g., 0$^\circ$ and 180$^\circ$), the other pair of fisheye images (e.g., 90$^\circ$ and 270$^\circ$) can be used as the cross-reference to provide ground-truth observations of the stitching regions. Based on this dataset, we further benchmark six widely used stitching models with seven evaluation metrics for IQA. To the best of our knowledge, it is the first dataset that focuses on assessing the stitching quality of omnidirectional images.

cs.CV↗

Multi-Channel Interference in Resonance-Like Enhancement of High-Order Above-Threshold Ionization

The intensity-dependent resonance-like enhancement phenomenon in high-order above-threshold ionization spectrum is a typical quantum effect for atoms or molecules in the intense laser field, which has not been well understood. The calculations of TDSE are in remarkable agreement with the experimental data, but they can not clarify the contributions of the bound states. The semi-classical approach of strong field approximation, in which no excited states are involved, can obtain the similar phenomenon, but the laser intensities of enhanced regions predicted by SFA are inconsistent with the results of TDSE. In this letter, a new fully quantum model is established from TDSE with not any excited states. Two types of enhanced structures, unimodal and multimodal structures, are found in the results of TDSE and model. Besides, the calculations of model reproduce the key features of the results of TDSE. It shows that the excited states are not the key factor in the resonance-like enhancements in our calculated system, since there are not any excited states in our model. Based on the calculations of our model, we show that such resonance-like enhancements are caused by the constructive interference of different momentum transfer channels. Last, the Fano-like lineshapes are also discussed for the features of multi-channel interference.

physics.atom-ph↗

Understanding tunneling ionization of atoms in laser fields with the essence of multiphoton absorption

The elaborate energy and momentum spectra of ionized electrons from atoms in laser fields suggest that the ionization dynamics described by tunneling theory should be modified. Although many efforts have been done within semiclassical models, there are few discussions describing multiphoton absorption process with quantum framework. In this letter, by comparing the results obtained with the time-dependent Schrodinger equation (TDSE) and Keldysh-Faisal-Reiss (KFR) theory, we have studied the nonperturbative effects of ionization dynamics beyond KFR theory. The difference in momentum spectra between multiphoton and tunneling regimes is understood in a unified picture with virtual multiphoton absorption processes. For the multiphoton regime, the momentum spectra can be obtained by coherent interference of each periodic contribution. However, the interference of multiphoton absorption peaks will result in the complex structure of virtual multiphoton bands in the tunneling regime. It is shown that the virtual spectra will be almost continuous in the tunneling regime instead of the discrete levels in the multiphoton regime. Finally, with a model combining TDSE and KFR theory, we have tried to understand the different effects of virtual multiphoton processes on ionization dynamics.

physics.atom-ph↗

Frequency-resolved photon-electronic spectroscopy for excited state population detection

Atomic excitation to excited states in strong laser field is the key to high-order harmonic generation below ionization threshold, yet remains unclear mainly due to the lack of proper detection methods. We propose a frequency-resolved photon-electron spectroscopy technique to reconstruct population of excited states with the second delayed laser pulse. The technique utilizes Fourier transformation to separate ionization from different excited states to different positions on the spectrum. With the advantage of separation, we provide a scheme to reconstruct populations on different excited states after the first pulse. The scheme is validated by high-precision population reconstruction of helium and hydrogen atoms.

physics.atom-ph↗

Visual Attention on the Sun: What Do Existing Models Actually Predict?

Visual attention prediction is a classic problem that seems to be well addressed in the deep learning era. One compelling concern, however, gradually arise along with the rapidly growing performance scores over existing visual attention datasets: do existing deep models really capture the inherent mechanism of human visual attention? To address this concern, this paper proposes a new dataset, named VASUN, that records the free-viewing human attention on solar images. Different from previous datasets, images in VASUN contain many irregular visual patterns that existing deep models have never seen. By benchmarking existing models on VASUN, we find the performances of many state-of-the-art deep models drop remarkably, while many classic shallow models perform impressively. From these results, we find that the significant performance advance of existing deep attention models may come from their capabilities of memorizing and predicting the occurrence of some specific visual patterns other than learning the inherent mechanism of human visual attention. In addition, we also train several baseline models on VASUN to demonstrate the feasibility and key issues of predicting visual attention on the sun. These baseline models, together with the proposed dataset, can be used to revisit the problem of visual attention prediction from a novel perspective that are complementary to existing ones.

cs.CV↗

Image Quality Assessment Guided Deep Neural Networks Training

For many computer vision problems, the deep neural networks are trained and validated based on the assumption that the input images are pristine (i.e., artifact-free). However, digital images are subject to a wide range of distortions in real application scenarios, while the practical issues regarding image quality in high level visual information understanding have been largely ignored. In this paper, in view of the fact that most widely deployed deep learning models are susceptible to various image distortions, the distorted images are involved for data augmentation in the deep neural network training process to learn a reliable model for practical applications. In particular, an image quality assessment based label smoothing method, which aims at regularizing the label distribution of training images, is further proposed to tune the objective functions in learning the neural network. Experimental results show that the proposed method is effective in dealing with both low and high quality images in the typical image classification task.

cs.CV↗