arXiv subjects

Hongtao Wang

Publications and source records attributed to Hongtao Wang.

At least 55 records · Page 3Linked to original sources

arXiv2022

MSSPN: Automatic First Arrival Picking using Multi-Stage Segmentation Picking Network

Picking the first arrival times of prestack gathers is called First Arrival Time (FAT) picking, which is an indispensable step in seismic data processing, and is mainly solved manually in the past. With the current increasing density of seismic data collection, the efficiency of manual picking has been unable to meet the actual needs. Therefore, automatic picking methods have been greatly developed in recent decades, especially those based on deep learning. However, few of the current supervised deep learning-based method can avoid the dependence on labeled samples. Besides, since the gather data is a set of signals which are greatly different from the natural images, it is difficult for the current method to solve the FAT picking problem in case of a low Signal to Noise Ratio (SNR). In this paper, for hard rock seismic gather data, we propose a Multi-Stage Segmentation Pickup Network (MSSPN), which solves the generalization problem across worksites and the picking problem in the case of low SNR. In MSSPN, there are four sub-models to simulate the manually picking processing, which is assumed to four stages from coarse to fine. Experiments on seven field datasets with different qualities show that our MSSPN outperforms benchmarks by a large margin.Particularly, our method can achieve more than 90\% accurate picking across worksites in the case of medium and high SNRs, and even fine-tuned model can achieve 88\% accurate picking of the dataset with low SNR.

cs.CV↗

arXiv2022

Full Geometric Control of Hidden Color Information in Diffraction Gratings under Angled White Light Illumination

Under white light illumination, gratings produce an angular distribution of wavelengths dependent on the diffraction order and geometric parameters. However, previous studies of gratings are limited to at least one geometric parameter (height, periodicity, orientation, angle of incidence) kept constant. Here, we vary all geometric parameters in the gratings using a versatile nanofabrication technique, two-photon polymerization lithography, to encode hidden color information through 2 design approaches. The first approach hides color information by decoupling the effects of grating height and periodicity under normal and oblique incidence. The second approach hides multiple sets of color information by arranging gratings in sectors around semi-circular pixels. Different images are revealed with negligible crosstalk under oblique incidence and varying sample rotation angles. Our analysis shows that an angular separation >= 10° between adjacent sectors is required to suppress crosstalk. This work has potential applications in information storage and security watermarks.

physics.optics↗

arXiv2022

Colorful Optical Vortices with White Light Illumination

The orbital angular momentum (OAM) of light holds great promise for applications in optical communication, super-resolution imaging, and high-dimensional quantum computing. However, the spatio-temporal coherence of the light source has been essential for generating OAM beams, as incoherent ambient light would result in polychromatic and obscured OAM beams in the visible spectrum. Here, we extend the applications of OAM to ambient lighting conditions. By miniaturizing spiral phase plates and integrating them with structural color filters, we achieve spatio-temporal coherence using only an incoherent white light source. These optical elements act as building blocks that encode both color and OAM information in the form of colorful optical vortices. Thus, pairs of transparent substrates that contain matching positions of these vortices constitute a reciprocal optical lock and key system. Due to the multiple helical eigenstates of OAM, the pairwise coupling can be further extended to form a one-to-many matching and validation scheme. Generating and decoding colorful optical vortices with broadband white light could find potential applications in anti-counterfeiting, optical metrology, high-capacity optical encryption, and on-chip 3D photonic devices.

physics.optics↗

arXiv2022

Tri-Functional Metasurface for Phase, Amplitude, and Luminescence Control

In optical anti-counterfeiting, several distinct optically variable devices (OVDs) are often concurrently employed to compensate for the insufficient security level of constituent OVDs. Alternatively, metasurfaces that exhibit multiple optical responses effectively combine multiple OVDs into one, thus significantly enhancing their security and hindering fraudulent replication. This work demonstrates the simultaneous control of three separate optical responses, i.e., phase, amplitude, and luminescence, using anisotropic gap-plasmon metasurfaces. Due to the incorporated geometric anisotropy, the designed structure exhibits distinct responses under x- and y-polarized light, revealing either a color image, or a holographic projection in the far field. Furthermore, inserting upconversion nanoparticles (UCNPs) into the dielectric gaps of the structures, the designed metasurface is able to generate a third luminescent image upon illumination with the near-infrared light. The stochastic distribution of the UCNPs constitutes a unique fingerprint, achieving a physically unclonable function (PUF) layer. Crucially, our triple-mode metasurface requires only readily attainable equipment such as a macro-lens/camera and a laser pointer to read most of the channels, thus paving the way towards highly secure and easy-to-authenticate metasurface-driven OVDs (mOVDs).

physics.optics↗

arXiv2022

Overcoming Van der Waals Forces in reconfigurable nanostructures

Reconfigurable metamaterials require constituent nanostructures to demonstrate switching of shapes with external stimuli. For generality, such nanostructures would touch and stick to other surfaces in one of its configurations. Yet, a longstanding challenge is in overcoming this stiction caused by Van der Waals forces, which impedes shape recovery. Here, we introduce a stiff yet self-recovering material system based on acrylic acid, and tested it in high-aspect ratio structures, where recovery is weak. This designer material has a storage modulus of ~5.2 GPa at room temperature and ~90 MPa in the rubbery state at 150 Celsius, an order of magnitude higher than previous reports. A high-resolution resin for two-photon lithography was developed based on this polymer system, enabling 3D printing of nanopillars with diameters of ~400 nm and aspect ratio as high as ~10. Experimentally, we observed self-recovery as collapsed and touching structures overcome stiction to stand back up. We developed a theoretical model to explain the recoverability of these sub-micron structures. Reconfigurable structural colour prints and holograms were demonstrated, indicating potential applications of the material system as a shape memory polymer suitable for sub-micron reconfigurable metamaterials.

physics.app-ph↗

arXiv2022

Reconfiguring colours of single relief structures by directional stretching

Colour changes can be achieved by straining photonic crystals or gratings embedded in stretchable materials. However, the multiple repeat units and the need for a volumetric assembly of nanostructures limit the density of information content. Inspired by surface reliefs on oracle bones and music records as means of information archival, here we endow surface-relief elastomers with multiple sets of information that are accessible by mechanical straining along in-plane axes. Distinct from Bragg diffraction effects from periodic structures, we report trenches that generate colour due to variations in trench depth, enabling individual trench segments to support a single colour. Using 3D printed cuboids, we replicated trenches of varying geometric parameters in elastomers. These parameters determine the initial colour (or lack thereof), the response to capillary forces, and the appearance when strained along or across the trenches. Strain induces modulation in trench depth or the opening and closure of a trench, resulting in surface reliefs with up to six distinct states, and an initially featureless surface that reveals two distinct images when stretched along different axes. The highly reversible structural colours are promising in optical data archival, anti-counterfeiting, and strain-sensing applications.

physics.optics↗

arXiv2022

High-order Photonic Cavity Modes Enabled 3D Structural Color

It remains a challenge to directly print three-dimensional arbitrary shapes that exhibit structural colors at the micrometer scale. Woodpile photonic crystals (WPCs) fabricated via two-photon lithography (TPL) are promising as building blocks to produce 3D geometries that generate structural colors due to their ability to exhibit either omnidirectional or anisotropic photonic stopbands. However, existing approaches have focused on achieving structural colors when illuminating WPCs from the top, which necessitates print resolutions beyond the limit of commercial TPL and/or post-processing techniques. Here, we devised a new strategy to support high-order photonic cavity modes upon side-illumination on WPCs that surprisingly generate large reflectance peaks in the visible spectrum. Based on that, we demonstrate one-step printing of 3D photonic structural colors without requiring post-processing or subwavelength features. Vivid colors with reflectance peaks exhibiting a full width at half maximum of ~25 nm, a maximum reflectance of 50%, gamut of ~85% of sRGB, and large viewing angles, were achieved. In addition, we also demonstrated voxel-level manipulation and control of colors in arbitrary-shaped 3D objects constituted with WPCs as unit cells, which has great potential for applications in dynamic color displays, colorimetric sensing, anti-counterfeiting, and light-matter interaction platforms.

physics.optics↗

arXiv2022

The Variability of the Narrow-line Seyfert 1 Galaxies from the PanSTARRS's View

By means of the data sets from the Pan-STARRAS1 survey, we have systematically examined the relationship between the variability characteristics and the physical parameters of the largest NLS1 galaxy sample up to now. The results are summarized as follows: (1). We find significant anti-correlations between variability amplitude and absolute magnitude in g, r, i, z and y bands, which are consistent with the results in previous works. (2) The correlations between the variability amplitude in optical band and many physical parameters (e.g., λL(5100 Å), black hole mass, Eddington ratio, R4570 and R5007) are investigated. The results show the variability amplitude is significantly anti-correlated with L(5100 Å), MBH, Eddington ratio and R4570, but positively correlated with R5007. The relation could be explained by the simple standard accretion disk model. (3) We further investigate the relationship between optical variability and radio luminosity/radio-loudness. The results present weak positive correlation in g and r bands, but insignificant correlation in i, z and y bands. The large error of the approximate fraction of the host galaxy in i, z and y bands may lead to insignificant correlations.

astro-ph.GA↗

arXiv2022

Learning Free Gait Transition for Quadruped Robots via Phase-Guided Controller

Gaits and transitions are key components in legged locomotion. For legged robots, describing and reproducing gaits as well as transitions remain longstanding challenges. Reinforcement learning has become a powerful tool to formulate controllers for legged robots. Learning multiple gaits and transitions, nevertheless, is related to the multi-task learning problems. In this work, we present a novel framework for training a simple control policy for a quadruped robot to locomote in various gaits. Four independent phases are used as the interface between the gait generator and the control policy, which characterizes the movement of four feet. Guided by the phases, the quadruped robot is able to locomote according to the generated gaits, such as walk, trot, pacing and bounding, and to make transitions among those gaits. More general phases can be used to generate complex gaits, such as mixed rhythmic dancing. With the control policy, the Black Panther robot, a medium-dog-sized quadruped robot, can perform all learned motor skills while following the velocity commands smoothly and robustly in natural environment.

cs.RO↗

arXiv2021

Tunable Mie Resonances in the Visible Spectrum

Dielectric optical nanoantennas play an important role in color displays, metasurface holograms, and wavefront shaping applications. They usually exploit Mie resonances as supported on nanostructures with high refractive index, such as Si and TiO2. However, these resonances normally cannot be tuned. Although phase change materials, such as the germanium-antimony-tellurium alloys and post transition metal oxides, such as ITO, have been used to tune optical antennas in the near infrared spectrum, tunable dielectric antennae in the visible spectrum remain to be demonstrated. In this paper, we designed and experimentally demonstrated tunable dielectric nanoantenna arrays with Mie resonances in the visible spectrum, exploiting phase transitions in wide-bandgap Sb2S3 nano-resonators. In the amorphous state, Mie resonances in these Sb2S3 nanostructures give rise to a strong structural color in reflection mode. Thermal annealing induced crystallization and laser induced amorphization of the Sb2S3 resonators allow the color to be tuned reversibly. We believe these tunable Sb2S3 nanoantennae arrays will enable a wide variety of tunable nanophotonic applications, such as high-resolution color displays, holographic displays, and miniature LiDAR systems.

physics.optics↗

arXiv2021

Photonic Bound-States-in-the-Continuum Observed with an Electron Nanoprobe

Bound-states-in-the-continuum (BIC)is a wave-mechanical concept that generates resonances with vanishing spectral linewidths. It has many practical applications in Optics, such as narrow-band filters, mirror-less lasing, and nonlinear harmonic generation. As true BIC optical modes non-radiative and confined to the near field of nanostructures, they cannot be excited using propagating light. As a result, their direct experimental observation has been elusive. Rather than using light, we demonstrate probing BIC modes on arrays of silicon nanoantennas using a focused beam of electrons in a tranmission electron microscope. By combining cathodoluminescence (CL) and monochromated electron energy-loss spectroscopy (EELS) with controlled nanofabrication, we provide direct experimental evidence of "true" BIC modes, and demonstrate a BIC mode in the visible spectrum at 720 nm. The ability to observe and quantify these guided resonances with a spatial precision more than two orders of magnitude higher than previous far-field measurements allows the probing of individual elements in the nano-antenna arrays. The high-resolution experimental results are supported by numerical simulations as well as multipolar decomposition analysis, allowing us to demonstrate that the coherent interaction length of the quasi-BIC resonance requires at least 6 neighboring antenna elements, achieving over 60 times higher emissivity than for unpatterned silicon.

physics.optics↗

arXiv2021

High-resolution light field prints by nanoscale 3D printing

A light field print (LFP) displays three-dimensional (3D) information to the naked-eye observer under ambient white light illumination. Changing perspectives of a 3D image are seen by the observer from varying angles. However, LFPs appear pixelated due to limited resolution and misalignment between their lenses and colour pixels. A promising solution to create high-resolution LFPs is through the use of advanced nanofabrication techniques. Here, we use two-photon polymerization lithography as a one-step nanoscale 3D printer to directly fabricate LFPs out of transparent resin. This approach produces simultaneously high spatial resolution (29 - 45 μm) and high angular resolution (~ 1.6 °) images with smooth motion parallax across 15 {\times} 15 views. Notably, the smallest colour pixel consists of only a single nanopillar (~ 300 nm diameter). Our LFP signifies a step towards hyper-realistic 3D images that can be applied in print media and security tags for high-value goods.

physics.optics↗

arXiv2020

From the Fermi blazar sequence to the relation between Fermi blazars and gamma-ray Narrow-line Seyfert 1 Galaxies

We use the third catalog of blazars detected by Fermi/LAT (3LAC) and gamma-ray Narrow-line Seyfert 1 Galaxies (gamma-NLSy1s) to study the blazar sequence and relationship between them. Our results are as follows: (i) There is a weak anti-correlation between synchrotron peak frequency and peak luminosity for both Fermi blazars and gamma-NLSy1s, which supports the blazar sequence. However, after Doppler correction, the inverse correlation disappeared, which suggests that anti-correlation between synchrotron peak frequency and peak luminosity is affected by the beaming effect. (ii) There is a significant anti-correlation between jet kinetic power and synchrotron peak frequency for both Fermi blazars and gamma-NLSy1s, which suggests that the gamma-NLSy1s could fit well into the original blazar sequence. (iii) According to previous work, the relationship between synchrotron peak frequency and synchrotron curvature can be explained by statistical or stochastic acceleration mechanisms. There are significant correlations between synchrotron peak frequency and synchrotron curvature for whole sample, Fermi blazars and BL Lacs, respectively. The slopes of the correlation are consistent with statistical acceleration. For FSRQs, LBLs, IBLs, HBLs, and gamma-NLS1s, we also find a significant correlation, but in these cases the slopes can not be explained by previous theoretical models. (iv) The slope of relation between synchrotron peak frequency and synchrotron curvature in gamma-NLS1s is large than that of FSRQs and BL Lacs. This result may imply that the cooling dominates over the acceleration process for FSRQs and BL Lacs, while gamma-NLS1s is the opposite.

astro-ph.HE↗

arXiv2020

Painting with Hue, Saturation, and Brightness Control by Nanoscale 3D Printing

Varying only the in-plane or out-of-plane dimensions of nanostructures produces a wide range of colourful elements in metasurfaces and thin films. However, achieving shades of grey and control of colour saturation remains challenging. Here, we introduce a hybrid approach to colour generation based on the tuning of nanostructure geometry in all three dimensions. Through two-photon polymerization lithography, we systematically investigated colour generation from the simple single nanopillar geometry made of low-refractive-index material; realizing grayscale and full colour palettes with control of hue, saturation, brightness through tuning of height, diameter, and periodicity of nanopillars. Arbitrary colourful and grayscale images were painted by mapping desired prints to precisely controllable parameters during 3D printing. We extend our understanding of the scattering properties of the low-refractive-index nanopillar to demonstrate grayscale inversion and colour desaturation, with steganography at the level of single nanopillars.

physics.optics↗

arXiv2020

Structural Multi-Colour Invisible Inks with Submicron 4D Printing of Shape Memory Polymers

Four-dimensional (4D) printing of shape memory polymer (SMP) imparts time responsive properties to 3D structures. Here, we explore 4D printing of a SMP in the submicron length scale, extending its applications to nanophononics. We report a new SMP photoresist based on Vero Clear achieving print features at a resolution of ~300 nm half pitch using two-photon polymerization lithography (TPL). Prints consisting of grids with size-tunable multi-colours enabled the study of shape memory effects to achieve large visual shifts through nanoscale structure deformation. As the nanostructures are flattened, the colours and printed information become invisible. Remarkably, the shape memory effect recovers the original surface morphology of the nanostructures along with its structural colour within seconds of heating above its glass transition temperature. The high-resolution printing and excellent reversibility in both microtopography and optical properties promises a platform for temperature-sensitive labels, information hiding for anti-counterfeiting, and tunable photonic devices.

physics.app-ph↗

arXiv2020

Defense of Word-level Adversarial Attacks via Random Substitution Encoding

The adversarial attacks against deep neural networks on computer vision tasks have spawned many new technologies that help protect models from avoiding false predictions. Recently, word-level adversarial attacks on deep models of Natural Language Processing (NLP) tasks have also demonstrated strong power, e.g., fooling a sentiment classification neural network to make wrong decisions. Unfortunately, few previous literatures have discussed the defense of such word-level synonym substitution based attacks since they are hard to be perceived and detected. In this paper, we shed light on this problem and propose a novel defense framework called Random Substitution Encoding (RSE), which introduces a random substitution encoder into the training process of original neural networks. Extensive experiments on text classification tasks demonstrate the effectiveness of our framework on defense of word-level adversarial attacks, under various base and attack models.

cs.CL↗

arXiv2020

Direct Observation of Room-Temperature Dislocation Plasticity in Diamond

It is well known that diamond does not deform plastically at room temperature and usually fails in catastrophic brittle fracture. Here we demonstrate room-temperature dislocation plasticity in sub-micrometer sized diamond pillars by in-situ mechanical testing in the transmission electron microscope. We document in unprecedented details of spatio-temporal features of the dislocations introduced by the confinement-free compression, including dislocation generation and propagation. Atom-resolved observations with tomographic reconstructions show unequivocally that mixed-type dislocations with Burgers vectors of 1/2<110> are activated in the non-close-packed {001} planes of diamond under uniaxial compression of <111> and <110> directions, respectively, while being activated in the {111} planes under the <100> directional loading, indicating orientation-dependent dislocation plasticity. These results provide new insights into the mechanical behavior of diamond and stimulate reconsideration of the basic deformation mechanism in diamond as well as in other brittle covalent crystals at low temperatures.

cond-mat.mtrl-sci↗

arXiv2020

Dislocation Slip or Phase Transformation Lead to Room-Temperature Plasticity in Diamond: Comment on Plastic Deformation of Single-Crystal Diamond Nanopillars

Despite decades of extensive research on mechanical properties of diamond, much remains to be understood in term of plastic deformation mechanisms due to the poor deformability at room temperature. In a recent work in Advanced Materials, it was claimed that room-temperature plasticity occurred in <001>-oriented single-crystal diamond nanopillars based on observation of unrecovered deformation inside scanning electron microscope. The plastic deformation was suggested to be mediated by a phase transition from sp3 carbon to an O8-carbon phase by molecular dynamics simulations. By comparison, our in-situ transmission electron microscopy study reveals that the room-temperature plasticity can be carried out by dislocation slip in both <100> and <111>-oriented diamond nanopillars. The brittle-to-ductile transition is highly dependent on the stress state. We note that the surface structure may play a significant role in the deformation mechanisms as the incipient plasticity always occurs from the surface region in nanoscale diamonds.

cond-mat.mtrl-sci↗