Overdriven Regime Research Articles

Enhanced XUV Harmonics Generation with an Intense Laser Field in the Overdriven Regime

High-order harmonic generation with high photon flux has been a challenging task in strong-field physics. According to the high-order harmonic generation process, the essential requirements for achieving efficient harmonic radiations inside a gas medium are the improvement of the induced atomic dipole moment amplitude of the single-atom response in the microscopic and the phase matching of the high harmonics in the macroscopic medium. In this work, we demonstrated a feasible approach to enhance the extreme-ultraviolet harmonics in the plateau region by increasing the intensity of the driving laser while keeping the laser energy constant. The simulation results showed that by increasing the laser intensity to the overdriven regime, the average extreme-ultraviolet harmonics yield in the plateau region is approximately twice as high as that obtained optimally in the conventional loose focusing geometry scheme by utilizing a relatively low-intensity driving laser with the same laser energy. The quantitative analysis of the harmonics generation process in the macroscopic medium and the phase matching revealed that the observed enhancement in harmonics can be attributed to the amplification of the induced atomic dipole moment amplitude of the single-atom response in the high-intensity driving laser and the favorable transient phase matching in the overdriven regime. Furthermore, the investigation of the driving laser indicated that the favorable transient phase matching is caused by the spatiotemporal reshaping of the driving laser in the overdriven regime.

A coarse-grain reactive model of RDX: Molecular resolution at the μm scale.

Predictive models for the thermal, chemical, and mechanical response of high explosives at extreme conditions are important for investigating their performance and safety. We introduce a particle-based, reactive model of 1,3,5-trinitro-1,3,5-triazinane (RDX) with molecular resolution utilizing generalized energy-conserving dissipative particle dynamics with reactions. The model is parameterized with respect to the data from atomistic molecular dynamics simulations as well as from quantum mechanical calculations, thus bridging atomic processes to the mesoscales, including microstructures and defects. It accurately captures the response of RDX under a range of thermal loading conditions compared to atomistic simulations. In addition, the Hugoniot response of the CG model in the overdriven regime reasonably matches atomistic simulations and experiments. Exploiting the model's high computational efficiency, we investigate mesoscale systems involving millions of molecules and characterize size-dependent criticality of hotspots in RDX. The combination of accuracy and computational efficiency of our reactive model provides a tool for investigation of mesoscale phenomena, such as the role of microstructures and defects in the shock-to-deflagration transition, through particle-based simulation.

Efficient soft x-ray high-order harmonic generation via dual-pulse driving lasers in the overdriven regime

We theoretically investigate the high-order harmonic generation (HHG) driven by dual-pulse lasers. The first pulse of 800 nm can create plasma in the high-density gas target. Then, the second pulse of 1800 nm is spatiotemporally reshaped by the plasma and generates soft x-ray high-order harmonics in the overdriven regime. The simulation results show that adding the first pulse can promote the phase matching of HHG in the cutoff region. As a result, a cutoff-extended HHG is achieved, which is one order of magnitude more intense than that in one pulse scheme reported previously. In dual-pulse driving lasers, the harmonic yield and the cutoff photon energy can be optimized effectively by increasing the gas pressure inside the water window region.

Phase-Matching for Generation of Isolated Attosecond XUV and Soft-X-Ray Pulses with Few-Cycle Drivers

Isolated attosecond pulses (IAPs) produced through laser-driven high-harmonic generation (HHG) hold promise for unprecedented insight into biological processes via attosecond x-ray diffraction with tabletop sources. However, efficient scaling of HHG towards x-ray energies has been hampered by ionization-induced plasma generation impeding the coherent buildup of high-harmonic radiation. Recently, it has been shown that these limitations can be overcome in the so-called 'overdriven regime' where ionization loss and plasma dispersion strongly modify the driving laser pulse over small distances, albeit without demonstrating IAPs. Here, we report on experiments comparing the generation of IAPs in argon and neon at 80 eV via attosecond streaking measurements. Contrasting our experimental results with numerical simulations, we conclude that IAPs in argon are generated through ionization-induced transient phase-matching gating effective over distances on the order of 100 $\mu$m. We show that the decay of the intensity and blue-shift due to plasma defocussing are crucial for allowing phase-matching close to the XUV cutoff at high plasma densities. We perform simulations for different gases and wavelengths and show that the mechanism is important for the phase-matching of long-wavelength, tightly-focused laser beams in high-pressure gas targets, which are currently being employed for scaling isolated attosecond pulse generation to x-ray photon energies.

Optimal Spatial Separation of High-Order Harmonics from Infrared Driving Lasers with an Annular Beam in the Overdriven Regime

We study high-order harmonic generation (HHG) with an annularly shaped high-power, high-repetition-rate laser. We investigate how the spatial separation (or divergence) of the incident laser and the generated high-order harmonics depends on the gas pressure when the laser intensity is in the overdriven regime. It is shown that, when the cone size of the incident annular beam is relatively large, the resulting harmonics are highly collimated along the propagation axis and are spatially separated from the incident beam. Under the overdriven condition, dynamic phase matching favors HHG emissions from short-trajectory electrons that are efficiently built up in the second half of the gas medium. We verify that using an annular beam is a simple and effective method for separating the intense incident infrared laser from the generated high harmonics. Such high-flux and low-divergence light pulses are expected to be powerful sources in the laboratory for ultrafast probing of electron dynamics.

Shock compression of Cu x Zr100−x metallic glasses from molecular dynamics simulations

The shock response of Cu x Zr100−x (x = 30, 50 and 70) metallic glasses (MGs) is characterized using large-scale molecular dynamics simulations. A wide range of piston velocities U p = 0.125–2.5 km/s are simulated corresponding to shock pressures from 3 to 130 GPa. Independent of composition, the metallic glasses exhibit the following shock wave propagation regimes: (1) single elastic shock wave for U p < 0.25 km/s, (2) split elastic and plastic shock waves for 0.25 < U p < 0.75 km/s and (3) overdriven plastic shock wave with a narrow elastic precursor for U p > 0.75 km/s. Within the split wave and overdriven regimes, the amplitude of the elastic precursor increases with increasing shock intensity, thereby indicating a pressure-dependent yield criterion. Hugoniot states are strongly dependent on the Cu content of the MG with Cu70Zr30 exhibiting a much higher resistance to plastic deformation than either Cu50Zr50 or Cu30Zr70.

Shock compression modeling of metallic single crystals: comparison of finite difference, steady wave, and analytical solutions

The shock response of metallic single crystals can be captured using a micro-mechanical description of the thermoelastic–viscoplastic material response; however, using a such a description within the context of traditional numerical methods may introduce a physical artifacts. Advantages and disadvantages of complex material descriptions, in particular the viscoplastic response, must be framed within approximations introduced by numerical methods. Three methods of modeling the shock response of metallic single crystals are summarized: finite difference simulations, steady wave simulations, and algebraic solutions of the Rankine–Hugoniot jump conditions. For the former two numerical techniques, a dislocation density based framework describes the rate- and temperature-dependent shear strength on each slip system. For the latter analytical technique, a simple (two-parameter) rate- and temperature-independent linear hardening description is necessarily invoked to enable simultaneous solution of the governing equations. For all models, the same nonlinear thermoelastic energy potential incorporating elastic constants of up to order 3 is applied. Solutions are compared for plate impact of highly symmetric orientations (all three methods) and low symmetry orientations (numerical methods only) of aluminum single crystals shocked to 5 GPa (weak shock regime) and 25 GPa (overdriven regime). For weak shocks, results of the two numerical methods are very similar, regardless of crystallographic orientation. For strong shocks, artificial viscosity affects the finite difference solution, and effects of transverse waves for the lower symmetry orientations not captured by the steady wave method become important. The analytical solution, which can only be applied to highly symmetric orientations, provides reasonable accuracy with regards to prediction of most variables in the final shocked state but, by construction, does not provide insight into the shock structure afforded by the numerical methods.