Volume 75, Issue 1

A potential method to produce isolated attosecond pulses (IAPs) by using low-intensity chirped-UV combined field has been investigated. The results can be separated into three parts. First, by properly introducing the mid-chirp or down-chirp of the low-intensity laser field, the harmonic cutoff can be extended and achieve the referenced value, which is produced from the high-intensity referenced field. Moreover, the spectral continuum is contributed by a single harmonic emission peak, which is beneficial to produce IAPs. However, the harmonic yield is very low due to the lower driven laser intensity. Second, by properly adding a UV pulse, the harmonic yield can be enhanced and achieve the referenced value due to the UV resonance ionisation. The intensity of the combined field is lower than that of the referenced field, which reduces the experimental requirements for producing high-intensity spectral continuum. Third, with the introduction of the positive or negative inhomogeneous effect of the mid-chirped combined field or down-chirped combined field, respectively, the similar harmonic cutoff and harmonic yield can also be obtained but with a much lower driven laser intensity. Finally, by superposing the harmonics on the spectral continuum, the IAPs with the durations of sub-38 as can be obtained.

Multistability analysis has received intensive attention in recently, however, its control in systems with more than two coexisting attractors are still to be discovered. This paper reports numerically the multistability control of five disconnected attractors in a self-excited simplified hyperchaotic canonical Chua’s oscillator (hereafter referred to as SHCCO) using a linear augmentation scheme. Such a method is appropriate in the case where system parameters are inaccessible. The five distinct attractors are uncovered through the combination of hysteresis and parallel bifurcation techniques. The effectiveness of the applied control scheme is revealed through the nonlinear dynamical tools including bifurcation diagrams, Lyapunov’s exponent spectrum, phase portraits and a cross section basin of attractions. The results of such numerical investigations revealed that the asymmetric pair of chaotic and periodic attractors which were coexisting with the symmetric periodic one in the SHCCO are progressively annihilated as the coupling parameter is increasing. Monostability is achieved in the system through three main crises. First, the two asymmetric periodic attractors are annihilated through an interior crisis after which only three attractors survive in the system. Then, comes a boundary crisis which leads to the disappearance of the symmetric attractor in the system. Finally, through a symmetry restoring crisis, a unique symmetric attractor is obtained for higher values of the control parameter and the system is now monostable.

We propose a novel equation of state (EoS) which explains the evolutionary history of a flat Friedmann–Lemaitre–Robertson–Walker universe. The uniqueness of this EoS lies in the fact that it incorporates the Lambert W function in a special fashion. It is explicitly demonstrated that with observationally relevant values of the unknown parameters ϑ 1 and ϑ 2 , all the evolutionary phases of the universe can be reproduced. Moreover, it also shows that the initial singularity is unavoidable and asserts that the late-time acceleration of the universe would continue forever.

Infinite-dimensional space of axisymmetric exact solutions to the Navier–Stokes equations with time-dependent viscosity ν(t)$\nu(t)$ is constructed. Inner transformations of the exact solutions are defined that produce an infinite sequence of new solutions from each known one. The solutions are analytic in the whole space ℝ 3 and are described by elementary functions. The bifurcations of the instantaneous (for t=t0$t={t_{0}}$) phase portraits of the viscous fluid flows are studied for the new exact solutions. Backlund transforms between the axisymmetric Helmholtz equation and a linear case of the Grad–Shafranov equation are derived.

This article investigates boundary layer mechanism of a two-phase nanofluid over a thin liquid film of power-law fluid. We take the coupled interface dynamics between the thin liquid film and nanofluid into consideration, where the thermal conductivity and dynamic viscosity are assumed to be linear functions of nanoparticle concentration. The influence of Brownian motion and thermophoresis of the nanofluid is also considered. Numerical results are carried out by employing similarity transformation and bvp4c technique. The heat and mass transfer in the flow boundary layer are analysed by relevant parameters with the assistance of graphs. The results show that heat conduction decreases significantly with the increase of rheological properties parameter and tensile velocity ratio. Rheological properties parameter, tensile velocity ratio, Brownian motion parameter and thermophoresis parameter play important roles in mass transfer.

Composition–gradient electrodes have been proved to have excellent electrochemical performances. The effect of irreversible electrochemical reaction on diffusion and diffusion-induced stresses in such electrodes is investigated. The diffusion equation and mechanical equation considering the electrochemical reaction and the heterogeneous factors are derived, and the distributions of lithium (Li) concentration and stresses in a spherical electrode are obtained under potentiostatic charging. The results indicate that the electrochemical reaction will slow down the diffusion process and enhance the stresses of the electrode. However, the heterogeneous factors of the electrochemical reaction could provide a positive effect on the stresses, which means that the heterogeneous factor is conducive to avoid capacity fading of the battery. The results suggest that the composition–gradient electrodes could be designed to have a decreasing transition form of forward reaction rate and an increasing transition form of the difference of atomic volume between reactants and the reaction product.

We proposed a new method for designing graded index lens using liquid crystal infiltration into annular photonic crystals. Applying an external nonuniform voltage in the transverse direction perpendicular to the direction of light propagation yields different orientation of liquid crystal molecules inside the photonic crystal unit cells. As a result, a gradient refractive index was modulated. We numerically investigate focusing properties of the designed graded index structure using plane-wave expansion and finite-difference time-domain methods. The gradient refractive index profile was adjusted by varying the nonuniform voltage excitations, which consequently altered the focal distance of the graded index structure. A wide tuning range of 1856 nm was achieved for focal distance by the proposed graded index structure. This feature can be implemented for planning a flat lens with tunable focal distance based on electro-optic effect. These achievements may have future applications in some optical devices such as near-field imaging and scanning.

Layered crystal Bi 2 O 2 Te has recently been found to have high electron mobility and excellent thermoelectric properties for technical applications; however, its other properties are not well studied yet. In this work, the electronic, elastic and optical properties of Bi 2 O 2 Te are systematically studied using the density functional method. The results indicate that Bi 2 O 2 Te is a narrow band gap semiconductor. The gap is formed by the Te 5p orbital at the top of the valence band and the Bi 6p orbital at the bottom of the conduction band. There are both ionic and covalent interactions within the Bi–O layers, and these layers are linked by the ionic Bi–Te bonds forming the crystal. Bi 2 O 2 Te is mechanically stable but anisotropic. It is easy to fracture along the c axis under shear stress. Its shear modulus is far smaller than its bulk modulus, so shape deformation is easier to occur than pure volume change. Its melting point is predicted to be 1284.0 K based on an empirical formula. The calculated refractive index at zero frequency reveals that Bi 2 O 2 Te is a negative uniaxial crystal with a birefringence of 0.51, making it a potential tuning material for optical application. The characteristics and origins of other optical properties are also discussed.

A nonreciprocal transmission structure is designed using a one-dimension multilayer medium, which consists of two asymmetric structure filled with the electric–gyrotropic dispersive media. The total transmission coefficients have been deduced using the transfer matrix method. Numerical results further provided evidence for the occurrence of the nonreciprocal surface electromagnetic waves. These states are affected by the thickness of layers, incident angles, and the externally applied magnetic fields. Given that the electric–gyrotropic media are inherently dispersive, our investigations will contribute to the practical application of nonreciprocal structures.