Nonreciprocal Metasurface with Space-Time Phase Modulation

Creating materials with time-variant properties is critical for breaking reciprocity that imposes fundamental limitations to wave propagation. However, it is challenging to realize efficient and ultrafast temporal modulation in a photonic system. Here, leveraging both spatial and temporal phase manipulation offered by an ultrathin nonlinear metasurface, we experimentally demonstrated nonreciprocal light reflection at wavelengths around 860 nm. The metasurface, with traveling-wave modulation upon nonlinear Kerr building blocks, creates spatial phase gradient and multi-terahertz temporal phase wobbling, which leads to unidirectional photonic transitions in both momentum and energy spaces. We observed completely asymmetric reflections in forward and backward light propagations within a sub-wavelength interaction length of 150 nm. Our approach pointed out a potential means for creating miniaturized and integratable nonreciprocal optical components.Comment: 25 pages, 5 figure

Iris si iv line profiles: An indication for the plasmoid instability during small-scale magnetic reconnection on the sun

Our understanding of the process of fast reconnection has undergone a dramatic change in the last 10 years driven, in part, by the availability of high-resolution numerical simulations that have consistently demonstrated the break-up of current sheets into magnetic islands, with reconnection rates that become independent of Lundquist number, challenging the belief that fast magnetic reconnection in flares proceeds via the Petschek mechanism that invokes pairs of slow-mode shocks connected to a compact diffusion region. The reconnection sites are too small to be resolved with images but these reconnection mechanisms, Petschek and the plasmoid instability, have reconnection sites with very different density and velocity structures and so can be distinguished by high-resolution line-profiles observations. Using IRIS spectroscopic observations we obtain a survey of typical line profiles produced by small-scale events thought to be reconnection sites on the Sun. Slit-jaw images are used to investigate the plasma heating and re-configuration at the sites. A sample of 15 events from two active regions is presented. The line profiles are complex with bright cores and broad wings extending to over 300 km/s. The profiles can be reproduced with the multiple magnetic islands and acceleration sites that characterise the plasmoid instability but not by bi-directional jets that characterise the Petschek mechanism. This result suggests that if these small-scale events are reconnection sites, then fast reconnection proceeds via the plasmoid instability, rather than the Petschek mechanism during small-scale reconnection on the Sun.Comment: 10 pages, 18 Figures, to be published in Ap

Molding Free-Space Light with Guided-Wave-Driven Metasurfaces

Metasurfaces with unparalleled controllability of light have shown great potential to revolutionize conventional optics. However, they mainly work with free-space light input, which makes it difficult for full on-chip integration. On the other hand, integrated photonics enables densely packed devices but has limited free-space light controllability. Here, we show that judiciously designed guided-wave-driven metasurfaces can mold guided waves into arbitrary free-space modes to achieve complex free-space functions, such as beam steering and focusing, with ultrasmall footprints and potentially no diffraction loss. Based on the same concept together with broken inversion symmetry induced by metasurfaces, we also realized direct orbital angular momentum (OAM) lasing from a micro-ring resonator. Our study works towards complete control of light across integrated photonics and free-space platforms, and paves new exciting ways for creating multifunctional photonic integrated devices with agile access to free space which could enable a plethora of applications in communications, remote sensing, displays, and etc.Comment: 37 pages, 5 figure

Identification of open crack of beam using model based method

This research aims at identifying the position and depth of the open transverse crack of the beam using the model based method. The stiffness matrix of the cracked beam element and the basic principle of the model based method are introduced. It is discussed to estimate the generalized displacement of all nodes of the beam by the measured displacements of a few degrees of freedom. The relative change rate of the equivalent external load between the intact and cracked elements is compared with that of mode shape, nature frequency and displacement amplitude between the intact and cracked beam. The position and depth of the crack are identified by the model based method in two cases. In first case, the measured displacement is assumed not to include noise. The identification results based on the actual displacement and rotation of all nodes are compared with the results using the estimated generalized displacement. In second case, the measured displacement includes noise and the generalized displacement of all nodes is estimated by the displacement of two measurement points. The simulation results shown there is no error to identify the position, the relative depth identification error of the crack with 1 μm depth is 2.34 % without noise, and the relative depth identification error of the crack with 200 μm depth could be down to about 5 % with the energy signal to noise ratio being about 7.00 before denoising

Real-World Image Super Resolution via Unsupervised Bi-directional Cycle Domain Transfer Learning based Generative Adversarial Network

Deep Convolutional Neural Networks (DCNNs) have exhibited impressive performance on image super-resolution tasks. However, these deep learning-based super-resolution methods perform poorly in real-world super-resolution tasks, where the paired high-resolution and low-resolution images are unavailable and the low-resolution images are degraded by complicated and unknown kernels. To break these limitations, we propose the Unsupervised Bi-directional Cycle Domain Transfer Learning-based Generative Adversarial Network (UBCDTL-GAN), which consists of an Unsupervised Bi-directional Cycle Domain Transfer Network (UBCDTN) and the Semantic Encoder guided Super Resolution Network (SESRN). First, the UBCDTN is able to produce an approximated real-like LR image through transferring the LR image from an artificially degraded domain to the real-world LR image domain. Second, the SESRN has the ability to super-resolve the approximated real-like LR image to a photo-realistic HR image. Extensive experiments on unpaired real-world image benchmark datasets demonstrate that the proposed method achieves superior performance compared to state-of-the-art methods.Comment: 12 pages, 5 figures,3 tables. This work is submitted to IEEE Transactions on Systems, Man, and Cybernetics: Systems (2022). It's under review by IEEE Transactions on Systems, Man, and Cybernetics: Systems for no

Simultaneous Ni Doping at Atom Scale in Ceria and Assembling into Well-Defined Lotuslike Structure for Enhanced Catalytic Performance

Oxide materials with redox capability have attracted worldwide attentions in many applications. Introducing defects into crystal lattice is an effective method to modify and optimize redox capability of oxides as well as their catalytic performance. However, the relationship between intrinsic characteristics of defects and properties of oxides has been rarely reported. Herein, we report a facile strategy to introduce defects by doping a small amount of Ni atoms (∼1.8 at. %) into ceria lattice at atomic level through the effect of microstructure of crystal on the redox property of ceria. Amazingly, a small amount of single Ni atom-doped ceria has formed a homogeneous solid solution with uniform lotuslike morphology. It performs an outstanding catalytic performance of a reduced T50 of CO oxidation at 230 °C, which is 135 °C lower than that of pure CeO2 (365 °C). This is largely attributed to defects such as lattice distortion, crystal defects and elastic strain induced by Ni dopants. The DFT calculation has revealed that the electron density distribution of oxygen ions near Ni dopant, the reduced formation energy of oxygen vacancy originated from local chemical effect caused by local distortion after Ni doping. These differences have a great effect on increasing the concentration of oxygen vacancies and enhancing the migration of lattice oxygen from bulk to a surface which is closely related to optimized redox properties. As a result, oxygen storage capacity and the associated catalytic reactivity has been largely increased. We have clearly demonstrated the change of crystal lattice and the charge distribution effectively modify its chemical and physical properties at the atomic scale

Life estimation of the beam with normal distribution parameters and subjected to cyclic load

This research aims at estimating the life of the beam with normal distribution parameters and subjected to cyclic load. It is tested by Monte-Carlo simulation that the generalized displacement and velocity are normally distributed when the coefficient of variation (CV) of the random parameter is small (generally CV≤ 0.01). The random perturbation method is employed to estimate the mean and variance of the generalized displacement and velocity. The random dynamic stress and its derivative with respect to the time t of the beam is formulated according to the shape function of beam element and the stress equation in a Euler-Bernoulli beam. Their mean, variance and correlation coefficient are given using the first-order approximation in a Taylor series. Based on Palmgren-Miner rule, the expected cumulative damage equation is given and is used to estimate the life where the random dynamic stress is non-stationary and follows the normal distribution with the nonzero mean at any time t. The presented method could also estimate the life of other structure or component which has several normal random parameters, is subjected to cyclic load and obeys the linear dynamics and elastic theory when the random parameter’s CV is small