您选择的条件: Shuang Zhang
  • Atmospheric Transmission Algorithm for Pulsed X-rays from High-Altitude Nuclear Detonations Based on Scattering Correction

    分类: 核科学技术 >> 核爆炸工程 提交时间: 2024-02-08

    摘要: In high-altitude nuclear detonations, the proportion of pulsed X-ray energy can exceed 70%, making it a specific monitoring signal for such events. These pulsed X-rays can be captured using a satellite-borne X-ray detector following atmospheric transmission. To quantitatively analyze the effects of different satellite detection altitudes, burst heights, and transmission angles on the physical processes of X-ray transport and energy fluence, we developed an atmospheric transmission algorithm for pulsed X-rays from high-altitude nuclear detonations based on scattering correction. The proposed method is an improvement over the traditional analytical method that only computes direct-transmission X-rays. The traditional analytical method exhibits a maximum relative error of 67.79% compared with the Monte Carlo method. Our improved method reduces this error to within 10% under the same conditions, even reaching 1% in certain scenarios. Moreover, its computation time is 48000 times faster than that of the Monte Carlo method. These results have important theoretical significance and engineering application value for designing satellite-borne nuclear detonation pulsed X-ray detectors, inverting nuclear detonation source terms, and assessing ionospheric effects.

  • A brief review of topological photonics in one, two, and three dimensions

    分类: 光学 >> 量子光学 提交时间: 2023-02-19

    摘要: Topological photonics has attracted increasing attention in recent years due to the unique opportunities it provides to manipulate light in a robust way immune to disorder and defects. Up to now, diverse photonic platforms, rich physical mechanisms and fruitful device applications have been proposed for topological photonics, including one-way waveguide, topological lasing, topological nanocavity, Dirac and Weyl points, Fermi arcs, nodal lines, etc. In this review, we provide an introduction to the field of topological photonics through the lens of topological invariants and bulk-boundary correspondence in one, two, and three dimensions, which may not only offer a unified understanding about the underlying robustness of diverse and distinct topological phenomena of light, but could also inspire further developments by introducing new topological invariants and unconventional bulk-boundary correspondence to the research of topological photonics.

  • Observation of superluminal signaling of terahertz pulses

    分类: 光学 >> 量子光学 提交时间: 2023-02-19

    摘要: Superluminal tunneling of light through a barrier has attracted broad interest in the last several decades. Despite the observation of such phenomena in various systems, it has been under intensive debate whether the transmitted light truly carry the information of the original pulse. Here we report observation of anomalous time response for terahertz electromagnetic pulses passing through thin metal films, with the pulse shape of the transmitted beam faithfully resembling that of the incident beam. A causal theoretical analysis is developed to explain the experiments, though the theory of Special Relativity may confront a challenge in this exceptional circumstance. These findings may facilitate future applications in high-speed optical communication or signal transmission, and may reshape our fundamental understanding about the tunneling of light.

  • Intrinsic Superflat Bands in General Twisted Bilayer Systems

    分类: 光学 >> 量子光学 提交时间: 2023-02-19

    摘要: Twisted bilayer systems with discrete magic angles, such as twisted bilayer graphene featuring moir\'{e} superlattices, provide a versatile platform for exploring novel physical properties. Here, we discover a class of superflat bands in general twisted bilayer systems beyond the low-energy physics of magic-angle twisted counterparts. By considering continuous lattice dislocation, we obtain intrinsic localized states, which are spectrally isolated at lowest and highest energies and spatially centered around the AA stacked region, governed by the macroscopic effective energy potential well. Such localized states exhibit negligible inter-cell coupling and support the formation of superflat bands in a wide and continuous parameter space, which can be mimicked using a twisted bilayer nanophotonic system. Our finding suggests that general twisted bilayer systems can realize continuously tunable superflat bands and the corresponding localized states for various photonic, phononic and mechanical waves.

  • Comparative study of Hermitian and non-Hermitian topological dielectric photonic crystals

    分类: 光学 >> 量子光学 提交时间: 2023-02-19

    摘要: The effects of gain and loss on the band structures of a bulk topological dielectric photonic crystal (PC) with $C_{6v}$ symmetry and the PC-air-PC interface are studied based on first-principle calculation. To illustrate the importance of parity-time (PT) symmetry, three systems are considered, namely the PT-symmetric, PT-asymmetric, and lossy systems. We find that the system with gain and loss distributed in a PT symmetric manner exhibits a phase transition from a PT exact phase to a PT broken phase as the strength of the gain and loss increases, while for the PT-asymmetric and lossy systems, no such phase transition occurs. Furthermore, based on the Wilson loop calculation, the topology of the PT-symmetric system in the PT exact phase is demonstrated to keep unchanged as the Hermitian system. At last, different kinds of edge states in Hermitian systems under the influences of gain and loss are studied and we find that while the eigenfrequencies of nontrivial edge states become complex conjugate pairs, they keep real for the trivial defect states.

  • Photonic nodal lines with quadrupole Berry curvature distribution

    分类: 光学 >> 量子光学 提交时间: 2023-02-19

    摘要: In periodic systems, nodal lines are loops in the three-dimensional momentum space where two bands are degenerate with each other. Nodal lines exhibit rich topological features as they can take various configurations such as rings, links, chains and knots. These line nodes are usually protected by mirror or PT symmetry. Here we propose and demonstrate a novel type of photonic straight nodal lines in a D2d meta-crystal which are protected by roto-inversion time (roto-PT) symmetry. The nodal lines are located at the central axis and hinges of the Brillouin zone. They appear as quadrupole sources of Berry curvature flux and allow for the precise control of the quadrupole strength. Interestingly, there exist topological surface states at all three cutting surfaces, as guaranteed by the pi-quantized Zak phases along all three directions. As frequency changes, the surface state equi-frequency contours evolve from closed to open contours, and become straight lines at a critical transition frequency, at which diffraction-less surface wave propagation are demonstrated, paving way towards development of super-imaging photonic devices.

  • Experimental demonstration of non-adjacent band topology connecting multiple nodal links

    分类: 光学 >> 量子光学 提交时间: 2023-02-19

    摘要: Nodal links are special configurations of band degeneracies in the momentum space, where nodal line branches encircle each other. In PT symmetric systems, nodal lines can be topologically characterized using the eigenvector frame rotations along an encircling loop and the linking structure can be described with non-Abelian frame charges interacting among adjacent bands. In this paper, we present a photonic multiple nodal links system, where non-adjacent band topology is proposed to characterize the hidden relation between nodal lines from non-adjacent band pairs. Through an orthogonal nodal chain, the nodal line from the lower two bands predicts the existence of nodal lines formed between the higher bands. We designed and fabricated a metamaterial, with which the multiple nodal links and non-adjacent band topology are experimentally demonstrated.