Science China Physics, Mechanics & Astronomy最新文献

Interfacial fluid mixing driven by a shock wave is a common phenomenon that occurs frequently in basic science research and in a variety of applications. In this work, shock-tube experiments on the developments of two-dimensional tri-mode interfaces accelerated by a convergent shock wave are performed. Eight sets of different combinations in the value of phase and that of amplitude in a tri-mode perturbed interface are studied to evaluate the effect of mode coupling on the amplitude growths of the basic modes. The qualitative results show that the phase combination obviously affects the interface morphologies and flow features. The alternation of each mode amplitude does not affect the major flow features, such as the number and arrangement of the bubbles and spikes, but affects the local interface features. Depending upon the phase combination, the mode amplitude growth is either promoted or suppressed by mode coupling relative to the single-mode counterpart. By considering the feedbacks from both the first-order and second-order mode couplings, the mode amplitude growth can be qualitatively predicted. Relative to dual-mode interface, mode coupling occurs earlier in tri-mode interface. For the sets of parameters we studied, the effect of initial phase on the amplitude growth is greater and occurs earlier for the mode with low mode number. In addition, the amplitude development of mode with high mode number is more affected by initial amplitudes of basic modes than that of mode with low mode number. The introduction of the third mode affects the amplitude growths of original two modes, but has little effect on the final mixing width growth. Finally, a theoretical model is proposed to predict the amplitude growth of each basic mode.

The dissociative ionization of Ar dimers is investigated in femtosecond laser fields with intensities from 260 to 1020 TW/cm². The three-dimensional momentum and kinetic-energy release of fragmental ions generated from the channels Ar₂²⁺→Ar⁺+Ar⁺, Ar₂³⁺→Ar²⁺+Ar⁺, and Ar₂⁴⁺→Ar²⁺+Ar²⁺ were measured with a cold-target recoil-ion momentum spectrometer. It is shown that the laser intensity significantly modulates the kinetic energies and angular distributions of fragmental ions from dissociative double ionization. Laser-induced charge-transfer following one-site double ionization contributes relatively more to the dissociative double ionization at lower laser intensity. The calculation results of a one-dimensional model based on the WKB approximation suggest that the charge transfer is suppressed at higher laser intensity due to the core polarization effect. In addition, double, triple, and quadruple dissociative ionizations of Ar dimers are accompanied by frustrated-tunneling ionization that increases with the laser intensity.

Bound states in the continuum (BIC) have been widely researched and applied in optics due to their unique electromagnetic response. However, there are still difficulties in predicting and customizing BIC spectra. To address this issue, we design an efficient combined neural network for highly accurate prediction of quasi-bound states in the continuum (q-BIC) spectrum, as well as for the inverse design of the polarization independent enhancement of the Goos-Hänchen (GH) shift. Firstly, we propose a C₄ symmetric metasurface for achieving q-BIC spectrum and providing the condition of enhanced GH shift. By employing a combined neural network, the intensity, position, shape, and phase of q-BIC spectrum with ultra-narrow resonance can be accurately predicted and on-demand customized, even under a small dataset. Besides, we develop a screening algorithm for the q-BIC spectrum to quickly realize the polarization independent enhancement of GH shift. As an application, an ultra-high sensitivity refractive index sensor has been proposed, whose sensitivity can reach 2.31×10⁷ µm/RIU for TM polarization and 1.03·10⁶ µm/RIU for TE polarization. Therefore, this work brings new solutions for quick prediction of q-BIC spectrum and the development of flexible polarization photonic devices.

The continuous progress in N ⁺₂ lasing recently stimulates a great deal of interest in nonlinear and quantum optics of molecular ions, while a complete description of the ionic polarization is still lacking to date. In this work, we are dedicated to constructing the fundamental ionic polarization theory where several ubiquitous strong-field processes including ionization, electronic couplings and molecular alignment jointly determine the spatial arrangement of ions. With the model, the elusive polarization of N ⁺₂ lasing can be well interpreted. Our results show that the different electronic transition rules for strong-field ionization and resonant couplings result in peculiar population distributions of various electronic states of N ⁺₂ in space. Meanwhile, the spatial nonuniformity of population distribution can be aggravated or mitigated during field-free evolutions of coherent molecular rotational wave packets. Furthermore, when a follow-up resonant seed pulse interacts with the prepared ionic system, the anisotropic quantum coherence determining the polarization of subsequent N ⁺₂ lasing can be established. The qualitative agreement between experiments and simulations confirms the validity of the proposed model. The findings provide critical insights into the polarization and radiation mechanisms of molecular ions constructed via ultrafast laser pulses.

Photoionization can generally be decomposed into a photoabsorption process plus a half-scattering process. The streaking time delay observed in the attosecond streak camera, correspondingly, can be decomposed into a photoabsorption time delay and a continuum time delay. We propose a self-consistent method to account for the potential-laser coupling effect and extract the intrinsic absorption time delay, without invoking an implicit assumption of a constant continuum correction made in previous streaking studies. The concept is based on an iterative technique starting from an initial guess of the absorption time delay and using classical electron trajectories to consistently remove the coupling-induced momentum shift to the streak signal. We show that the self-consistent iterative algorithm converges quickly and is robust against different initial guesses. The method is applied to laser-induced ionization of atoms to obtain the absorption time delay in resonant two-photon ionization and to explore possible time delays in above-threshold ionization. The results confirm an absorption time delay linearly dependent on the ionizing pulse duration in resonant two-photon ionization. The method is also shown to perform better than conventional methods and to identify a small negative absorption time delay (less than 2 attoseconds) in above-threshold ionization.

The ionic dynamics induced by strong-field ionization are essential to understand the fundamental physics and chemical reactions. By solving the ionization-coupling equation theoretically, we can simultaneously address strong-field ionization and coupling dynamics in ions. By employing the driving pulse at the wavelength of 1580 nm, we show that the B²Σ ⁺_u state of the strong field ionization created N ⁺₂ could be populated by polarization effect and five-photon resonance but there is no population inversion between X²Σ ⁺_g and B²Σ ⁺_u states for the nitrogen molecular ions aligning along the laser polarization. In addition, both the ultraviolet supercontinuum and the attosecond transient absorption spectroscopy (ATAS) are calculated to illustrate the characteristics of population and coherence. The Stark shift observed from the transient absorption confirms the origin of ultraviolet supercontinuum. Our results show the evolution of the absorption spectral lineshape, varying from Lorentzian to Fano to inverted Lorentzian and back forth and the optical gain is achieved at 394 eV due to the vibrational coherent dynamics. This study offers valuable insights into the strong-field quantum optics of molecular ions.

With a pressing need for high efficiency, low power consumption, and miniaturization of electronics, soft magnetic composites (SMCs) show great potential, especially for applications in key electronic component. However, core loss is still the focused issue for SMCs that hinders their sustainable development and widespread applications. In the present study, high-performance SMCs were fabricated by novel Fe₇₄B₇C₇P₇Si₃Mo₁Cr₁ powders with spherical shape and a fully glassy structure, which were successfully prepared by a gas atomization method. The microstructure and high-frequency magnetic properties of these SMCs were studied in detail. To enhance the soft ferromagnetism, the effects of annealing temperature (T_a) and powder size on their performance were clarified. Increasing T_a up to 703 K not only helps to effectively release internal stress in the powders, but also improves the integrity of the insulation layer structure, which is conducive to decreasing the core loss. In addition, reducing the powder size contributes to the overall performance enhancement. Prepared from the powders with the smallest mean particle size and annealed at 703 K, the SMC exhibits optimum property combination of a stable effective permeability of 26.2 up to 1 MHz, a total core loss of 883 kW m⁻³ (100 kHz, 100 mT), and a DC-Bias performance of 79.3% under 100 Oe field, which is even comparable to those of the most prominent SMCs reported so far. These results are meaningful for potentially stimulating the development and application of new low-loss SMCs.

Ternary GaAs_1−xSb_x nanowires with designable bandgaps and lattice constants show important potential applications in band structure engineering as well as optical and optoelectronic devices. However, large diameters, low aspect ratios and even spontaneous core-shell structures are always found in GaAs_1−xSb_x nanowires, which are hindering their optical and optoelectronic applications. Here, we report the controlled synthesis of ultrathin GaAs_1−xSb_x nanowires on Si (111) substrates by molecular-beam epitaxy. It is found that ultrathin GaAs_1−xSb_x nanowires with a diameter less than 40 nm and an aspect ratio exceeding 35 can be obtained by precisely tuning the Ga flux and the growth temperature. The growth of the ultrathin GaAs_1−xSb_x nanowires with a large-composition-range (0 ⩽ x ⩽ 0.4) are also achieved by directly tuning the antimony flux. Detailed structural studies confirm that these ultrathin nanowires exhibit high crystal-quality, and no spontaneous core-shell nanostructures are observed along the axial and radial directions of the nanowires. As far as we known, it is the first time that homogeneous GaAs_1−xSb_x nanowires are grown by molecular-beam epitaxy. Photoluminescence measurements prove that the ultrathin GaAs_1−xSb_x nanowires have a narrower full width at half maximum of the photoluminescence peak compared with those results reported in the literatures, and their emission wavelengths can be tuned from 850 nm (GaAs) to 1271 nm (GaAs_0.6Sb_0.4). In addition, the optical properties of the ultrathin GaAs_1−xSb_x nanowires can be further improved by using the Al_0.5Ga_0.5As shell as a passivation layer. Our work lays a foundation for the development of high-performance GaAs_1−xSb_x nanowire-based optical and optoelectronic devices.

For the in-memory computation architecture, a ferroelectric semiconductor field-effect transistor (FeSFET) incorporates ferroelectric material into the FET channel to realize logic and memory in a single device. The emerging group III nitride material Al_1−xSc_xN provides an excellent platform to explore FeSFET, as this material has significant electric polarization, ferroelectric switching, and high carrier mobility. However, steps need to be taken to reduce the large band gap of ∼5 eV of Al_1−xSc_xN to improve its transport property for in-memory logic applications. By state-of-the-art first principles analysis, here we predict that alloying a relatively small amount (less than ∼5%) of Sb impurities into Al_1−xSc_xN very effectively reduces the band gap while maintaining excellent ferroelectricity. We show that the co-doped Sb and Sc act cooperatively to give a significant band bowing leading to a small band gap of ∼1.76 eV and a large polarization parameter ∼0.87 C/m², in the quaternary Al_1−xSc_xSb_yN_1−y compounds. The Sb impurity states become more continuous as a result of interactions with Sc and can be used for impurity-mediated transport. Based on the Landau-Khalatnikov model, the Landau parameters and the corresponding ferroelectric hysteresis loops are obtained for the quaternary compounds. These findings indicate that Al_1−xSc_xSb_yN_1−y is an excellent candidate as the channel material of FeSFET.