We have performed the project well and achieved the following main results.
1)Laser pulses at multi-petawatts are available recently, offering opportunities to study laser-matter interactions at unprecedented laser intensities. To achieve such intensities and apply them effectively, it is essential to tightly focus laser pulses and guide them over certain distances. However, due to electron cavitation by the ponderomotive force with such lasers, both the linear and nonlinear guiding effects with electrons disappear. In our work, we show that ion response can cause effective guiding of such pulses even if full electron cavitation occurs. The conditions for the required ion-density distribution and laser power are derived and verified by three-dimensional particle-in-cell simulations.
2) High-quality gamma-rays below MeV photon energy are available from large-scale synchrotron radiation facilities, but it remains a great challenge to generate bright gamma-ray beams in the MeV-GeV range. We propose a scheme to efficiently generate such beams from sub-micron wires irradiated by petawatt lasers, where electron acceleration and wiggling are combined simultaneously. Our full-scale 3D simulations show that directional gamma-rays with 100,000-fold higher brilliance and thousand-fold higher photon energy can be generated compared to synchrotron radiation facilities. In addition, the photon yield efficiency approaches 10% - 100,000-fold higher than betatron radiation and Compton scattering based on laser-wakefield acceleration.
3) In addition, we present a novel scheme to overcome these limitations and efficiently produce collimated ultra-bright beams of gamma-rays with photon energies tunable up to GeV-levels. This is achieved by focusing a multi-petawatt laser pulse available recently into a two-stage wakefield accelerator. The high-intensity laser enables the generation of a tens-nC multi-GeV electron beam with high density in the first stage. Subsequently, the beam is sent to the second radiator stage of the relatively high-density plasma with the laser pulse, where high-energy photons are emitted when the energetic beam electrons interact with the high quasi-static electromagnetic fields induced in this stage. Our full-scale 3D simulations demonstrate that more than 1012 gamma-ray photons are produced with efficiency in excess of 10% for photons above 1 MeV.
4) Achieving table-top terahertz (THz) sources with high field strength and broad bandwidth is an outstanding issue in THz science. Such sources can find applications in material research, biomedical imaging, non-destructive detectio, and THz-field interaction with matter. Previous studies have demonstrated THz generation from solids and gases via different mechanisms. However, THz generation from liquid, in
particular water, has long been considered impossible because of its strong absorption of THz radiation. Terahertz radiation from liquid water was observed in 2017 for the first time, but the mechanism remains unclear and the yield efficiency is low. In our work, we show experimentally that the efficiency can be enhanced by three orders of magnitude when a water line is adopted. The field strength approaches MV/cm even with a mJ laser pulse. We propose a laser-ponderomotive-force-induced current model to explain the mechanism, which is supported by particle-in-cell simulations.
