Probing ultrafast dynamics of solid-density plasma generated by high-contrast intense laser pulses

We present ultrafast dynamics of solid-density plasma created by high-contrast (picosecond contrast ∼10-9), high-intensity (∼4 × 1018 W/cm2) laser pulses using time-resolved pump-probe Doppler spectrometry. Experiments show a rapid rise in blue-shift at early time delay (2-4.3 ps) followed by a rapid fall (4.3-8.3 ps) and then a slow rise in blue-shift at later time delays (>8.3 ps). Simulations show that the early-time observations, specifically the absence of any red-shifting of the reflected probe, can only be reproduced if the front surface is unperturbed by the laser pre-pulse at the moment that the high intensity pulse arrives. A flexible diagnostic which is capable of diagnosing the presence of low-levels of pre-plasma formation would be useful for potential applications in laser-produced proton and ion production, such as cancer therapy and security imaging

Tracking ultrafast dynamics of intense shock generation and breakout at target rear

We report upon the picosecond plasma dynamics at the rear surface of a thin aluminium foil (of either 5.5 um or 12 um thickness) excited by high contrast (picosecond intensity contrast of 10^10), 800 nm, femtosecond pulses at an intensity of 3 x 10^19 W/cm2. We employ ultrafast pump-probe reflectometry using a second harmonic probe (400 nm) interacting with the rear surface of the target. A rise in the probe reflectivity 30 picoseconds after the pump pulse interaction reveals the breakout of a shock wave at the target rear surface which reflects the 400 nm probe pulse. Simulations using the ZEPHYROS hybrid particle-in-cell code were performed to understand the heating of the target under the influence of the high intensity laser pulse, and the temperature profile was then passed to the radiation-hydrodynamics simulation code HYADES in order to model the shock wave propagation in the target. A good agreement was found between the calculations and experimental results

Quasisteady state interpulse plasmas

The generation of quasisteady state plasmas in the power off phase, by short pulses [pulse duration (Τp) - 0.5-1.2 µ s] of intense (60-100 kW) microwaves in the X band (9.45 GHz) is observed experimentally. The steady state is sustained from a few to tens of microseconds and depends upon the ionization processes in the interpulse phase and the characteristic diffusion length. The results are explained by a model, which considers the electron acceleration effects by the large amplitude of the field, the energy losses, and the characteristic electromagnetic field decay time. The effects of wave frequency, microwave power density, and particle diffusion on the steady state are investigated. A striking difference with conventional afterglows of pulsed discharges is pointed out

Observation of ex-situ microstructure relaxation of non-conventional misorientations post femtosecond laser shock exposure in cp-Ti

The effect of shock passage in commercially pure titanium (cp-Ti) with minimal macroscopic plastic strain was investigated in the present study. For the first time, ex-situ residual strain relaxation in a shocked material was recorded post-shock exposure. Femtosecond laser was used to produce a shock in cp-Ti. Post-shock investigation was carried using EBSD, TEM imaging, and TEM OIM. Uniform but low misorientations were observed inside the grains. Lamella like structures and nano grain pockets were present in the material. Observed misorientations of the remnant nanograins correspond to the multiple twinned region of twins with low twinning shear. However, a large fraction of misorientation pertains to the double twinned structure. Lattice misorientation build-up was absent in the microstructure, in spite of a large number of defects observed. This is confirmed by the GND component analysis, indicating that the macroscopic plastic strain is minimal. The lattice displays alternating curvature which cancels out over long distances, which indicates the presence of SSDs. Ex-situ strain relaxation was observed in the TEM sample resulting in bending of the TEM lamella about the triple point junction of grain. This resulted in the reverting back of defects to the parent grain, which is similar to defect annihilation on shock unloading as reported in previous MD simulations. A schematic spanning the microstructural changes before and after shock passage, and showing the effect of further strain relaxation on the microstructure is shown. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Spectral and intensity control of high energy terahertz radiation from bulk liquids

International audienceHigh power, broadband terahertz (THz) radiation from liquids, excited by intense, femtosecond 800 nm laser pulses has been demonstrated recently, overturning the long held belief that liquids would not give such emission due to their absorption in the THz region. Given the widespread interest in the use of THz radiation for several applications and the energy and bandwidth limitations of existing sources, liquids are expected to attract great attention in the future. While the emission at tens of microjoules from liquids is very promising, it is important to explore whether control of the THz flux and spectrum is achievable by manipulating the laser or liquid parameters. In this paper we present results on manipulating the spectrum of THz radiation from liquids by chirping the input laser pulse and optimizing the THz output energy by laser chirp as well as optimizing the focal position. We demonstrate tunability by varying the chirp of the laser pulse and show that the THz emission predominantly comes from the region near the end of the liquid path, consistent with the absorption of THz radiation by liquids. This control gives us an opportunity to tune the THz radiation to suit experimental needs. We present simulations that support the results

Highly efficient broadband terahertz generation from ultrashort laser filamentation in liquids

Developing simple and efficient table-top sources of intense terahertz radiation is an ongoing pursuit. Here, Dey et al. demonstrate broadband terahertz generation from laser filamentation in liquids with an order of magnitude higher energy than from conventional two-color filamentation in air

Mapping the Damping Dynamics of Mega-Ampere Electron Pulses Inside a Solid

We report the lifetime of intense-laser ($2 \times 10^{19}$ W/cm$^2$) generated relativistic electron pulses in solids by measuring the time evolution of their Cherenkov emission. Using a picosecond resolution optical Kerr gating technique, we demonstrate that the electrons remain relativistic as long as 50 picoseconds—more than 1000 times longer than the incident light pulse. Numerical simulations of the propagation of relativistic electrons and the emitted Cherenkov radiation with Monte Carlo geant4 package reproduce the striking experimental findings