The multiple effects of gradient coupling on network synchronization

Recent studies have shown that synchronizability of complex networks can be significantly improved by asymmetric couplings, and increase of coupling gradient is always in favor of network synchronization. Here we argue and demonstrate that, for typical complex networks, there usually exists an optimal coupling gradient under which the maximum network synchronizability is achieved. After this optimal value, increase of coupling gradient could deteriorate synchronization. We attribute the suppression of network synchronization at large gradient to the phenomenon of network breaking, and find that, in comparing with sparsely connected homogeneous networks, densely connected heterogeneous networks have the superiority of adopting large gradient. The findings are supported by indirect simulations of eigenvalue analysis and direct simulations of coupled nonidentical oscillator networks.Comment: 4 pages, 4 figure

Achieving relativistically intense X-rays from structured plasma lens

Focusing of high-power X-rays is still a great challenge and the intensity of X-ray attained in existing focusing schemes is still far below the relativistic threshold. Here, we propose that solid density plasma lens can potentially focus X-ray lasers at very high power levels. The interaction of high-power X-ray laser with solid-density plasmas is systematically studied. It is theoretically shown that there exists a certain range of wavelengths for X-ray lasers that can be focused in solid-density plasmas when the input power and plasma density are determined. To avoid the essential laser-plasma instabilities and obtain high-gain intensity amplification for X-ray, we design concave structured plasma lens. Particle-in-cell simulation results show that such regime can effectively avoid the instabilities and focus X-ray of micrometer-sized spot and multi-TW power, and thus lead to the generation of relativistic intensity X-ray. The parameters of the concave structures and the effects of quantum electrodynamics are also discussed and it indicates that our scheme is quite robust. We further demonstrate that the relativistic X-ray laser interacting with thin-foil leads to high-quality attosecond electron bunches

Relativistic Vlasov code development for high energy density plasmas

A newly developed relativistic Vlasov code is introduced. The governing Vlasov-Maxwell equation system is solved numerically in one-dimensional space and three-dimensional momentum space. Spherical coordinate system is adopted to characterize the momentum variables for its potential advantage on reducing computational cost. The resulting Vlasov equation is split into two advection equations with respect to position and momentum, respectively. They are solved with a conservative finite volume scheme, together with techniques suppressing numerical oscillations at sharp interfaces. Relativistic longitudinal plasma oscillations are investigated for different plasma temperatures and wave numbers. Results from code simulations are in good agreement with the existing theories

Attosecond Transient Absorption Below the Excited States

In this study, the attosecond transient absorption (ATA) spectrum below the excited states of the helium atom was investigated by numerically solving the fully three-dimensional time-dependent Schrödinger equation. Under single-active electron approximation, the helium atom was illuminated by a combined field comprising of extreme ultraviolet (XUV) and delayed infrared (IR) fields. The response function demonstrates that the absorption near the central frequency (ωX) of the XUV field is periodically modulated during the overlapping between the XUV and IR pulses. Using the time-dependent perturbation, the absorption near ωX is attributed to the wavepacket excited by the XUV pulse. The wave function oscillating at the frequency of the XUV pulse was obtained. Furthermore, the chirp-dependent absorption spectrum near ωX potentially provides an all-optical method for characterizing the attosecond pulse duration. Finally, these results can extend to other systems, such as solids or liquids, indicating a potential for application in photonic devices, and they may be meaningful for quantum manipulation

Electron reflux dynamics in relativistically transparent plasma

The electron re-injection effect has revealed the great probability rate of photon generation due to the head-on collision between relativistic electrons and laser. We study the electron re-injection dynamics when the ultra-intense laser irradiates the near-critical-density plasma and successfully controls the photon radiation by means of the transversely tailored plasma. Starting from the relativistic corrected ponderomotive force, the critical strength of the laser field required by the refluxing effect is theoretically obtained. Then, the theoretical description of the wavefront formed by electron refluxing is given via plugging in the difference in the transverse phase velocity of the plasma wave. Simulation results display a curved surface of the refluxing electrons, which are in good agreement with the calculation results stemming from the physics model. The re-built phase space of the refluxing electrons illustrates that they gain energy mainly from the longitudinal electrostatic field on the re-injection path. Despite the energy of the refluxing electron being relatively low, it could radiate more photons via more efficient non-linear Compton scattering than the electron being accelerated in the positive direction. Furthermore, we employ a transverse density profile in the plasma and successfully achieve control of the electron re-injection effect and the properties of the resultant photons as well. Simulation results exhibit that overcritical electron beams are successively re-injected from the plasma density peaks. These backward electrons emit photons along the two maximal plasma densities as they collide with the laser pulse. Although the quality of the photons is not improved, their spatial distribution is changed, which is a big step toward manipulating light sources

Structural Stability, Thermodynamic and Elastic Properties of Cubic Zr0.5Nb0.5 Alloy under High Pressure and High Temperature

Structural stability, sound velocities, elasticity, and thermodynamic properties of cubic Zr0.5Nb0.5 alloy have been investigated at high pressure and high temperature by first-principles density functional calculations combined with the quasi-harmonic Debye model. A pronounced pressure-induced shear wave velocity stiffening in Zr0.5Nb0.5 alloy is observed at pressures above ~11 GPa, owing to its structural instability under high pressure, whose anomalous behavior is also observed in the end members of Zr-Nb alloys for Zr at ~13 GPa and for Nb at ~6 GPa upon compression, respectively. In addition, high-pressure elasticity and elastic-correlated properties of cubic Zr0.5Nb0.5 are reported, as compared with previous studies on Zr-Nb alloys with different compositions. A comprehensive study of the thermodynamic properties of cubic Zr0.5Nb0.5, such as heat capacity (Cv), thermal expansion coefficients (α), and Debye temperature (ΘD), are also predicted at pressures and temperatures up to 30 GPa and 1500 K using the quasi-harmonic Debye model

Polarization Splitting at Visible Wavelengths with the Rutile TiO₂ Ridge Waveguide

On-chip polarization control is in high demand for novel integrated photonic applications such as polarization division multiplexing and quantum communications. However, due to the sensitive scaling of the device dimension with wavelength and the visible-light absorption properties, traditional passive silicon photonic devices with asymmetric waveguide structures cannot achieve polarization control at visible wavelengths. In this paper, a new polarization-splitting mechanism based on energy distributions of the fundamental polarized modes in the r-TiO2 ridge waveguide is investigated. The bending loss for different bending radii and the optical coupling properties of the fundamental modes in different r-TiO2 ridge waveguide configurations are analyzed. In particular, a polarization splitter with a high extinction ratio operating at visible wavelengths based on directional couplers (DCs) in the r-TiO2 ridge waveguide is proposed. Polarization-selective filters based on micro-ring resonators (MRRs) with resonances of only TE or TM polarizations are designed and operated. Our results show that polarization-splitters for visible wavelengths with a high extinction ratio in DC or MRR configurations can be achieved with a simple r-TiO2 ridge waveguide structure