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Exploring Laser Interaction with Matters in the Quantum Electrodynamics Regime

Periodic Reporting for period 1 - ELIQED (Exploring Laser Interaction with Matters in the Quantum Electrodynamics Regime)

Reporting period: 2017-09-01 to 2019-08-31

Based on our KLAPS code, we developed a 3D QED-PIC code including photon generation and pair creation. The new code was benchmarked and applied in our following investigations. Our research motivation is mainly based on the development of ultra-short and ultra-intense laser technology and new physics appearing in such laser interactions with matter. Extremely high power laser pulses at 10 - 100 PW levels are available or will be available in near future, e.g. the European ELI system, the OMEGA EP-OPAL system in USA, two 100-PW laser facilities in Shanghai and Mianyang China, etc. With such a laser pulse, laser plasma interactions are entering a QED-dominant regime accompanied with generation of abundant photons and pairs. We found that the new QED effects will significantly affect the classic pictures in the laser plasma physics, such as laser propagation and absorption in plasma. This push the development of the classic laser-plasma physics. On the other hand, we also found that high-energy, ultra-bright, femtosecond-duration gamma-rays can be driven via the QED effects. Such gamma-rays can be applied in nuclear photonics, radiotherapy, and laboratory astrophysics.
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.
Novel gamma-ray sources. Our scheme could provide table-top gamma-ray sources much compacter than the conventional large-scale X-ray free electron lasers and synchrotron radiation facilities, which usually at the level of kilometers. The compacter gamma-ray sources are much cheaper than the large scale facilities. Besides the scale and economic advantages, our gamma-rays sources could provide gamma-rays or X-rays with very unique photon energy bands which is not available in the X-ray free electron lasers and synchrotron radiation facilities. Even though high-quality X and gamma-rays with photon energy below mega-electron-volt (MeV) are available from large scale X-ray free electron lasers and synchrotron radiation facilities, it remains a great challenge to generate bright gamma-rays over ten MeV. In our work, we proposed the scheme to efficiently generate gamma-rays of hundreds of MeV from sub-micrometer wires irradiated by petawatt lasers, where electron accelerating and wiggling are achieved simultaneously. The wiggling is caused by the quasistatic electric and magnetic fields induced around the wire surface, and these are so high that even quantum electrodynamics (QED) effects become significant for gamma-ray generation, although the driving lasers are only at the petawatt level. Our full three-dimensional simulations show that directional, ultra-bright gamma-rays are generated with the photon energies between 5 and 500 MeV within 10 femtosecond duration. The brilliance is the second only to X-ray free electron lasers, while the photon energy is 3 orders of magnitude higher than the latter. Such high-energy, ultra-bright, femtosecond-duration gamma-rays are highly demanded in broad applications ranging from laboratory astrophysics, emerging nuclear photonics, photon-photon colliders, fine measurement of atomic nuclei, to radiotherapy.

Novel Terahertz-ray sources. We found strong liquid terahertz (THz) sources, which could be more easily to apply than the well-known two-color air THz source, which has been studied most broadly. Our scheme provides the possibility of achieving a THz yield as high as that of a typical the two-color laser air THz source (as high as 0.01%), yet via a single-color laser driven in our scheme. Indeed, this scheme could result in a more compact solution due to absence of the relatively long focal length necessary to give rise to long plasma filaments. In addition, our work reports on a timely and very interesting topic, i.e. the THz generation from a liquid state source. THz pulse generation from water film was observed first very recently in 2017. In this work, we reported on their observation of THz pulse generation in water column having sub-mm diameter. We thoroughly investigated this phenomenon experimentally and explained it by and theoretic model and the results of PIC simulations.
Figure 1. Three-dimensional isosurfaces of the laser intensity and the slices at 50 laser cycles
Figure 12. Schematic of our experiments. The inset illustrates the geometry of the laser interaction
Figure 15. THz field scaling, polarization, and angular distribution.
Figure 11.Number of electrons with energies. Left: 0.6 um wire. Right: 0.3um wires
Figure 13. THz field waveform from (A) water column (B) water film and (C) air
Figure 14. THz strength dependence on deviation xL between the laser axis and water- column center.
Figure 6. Schematic of the wire scheme. (A) Schematic (B) chart of photon energy and brilliance
Figure 7. (A) laser field and (B) photon density (C) angular distributions and (D) energy spectra
Figure 8. (A) electrostatic and (C) magnetostatic fields, (B) electron density (D) current densities
Figure 9. Trace of typical electrons from the 0.6 micrometer (A, C) and 0.3 micrometer wires (B, D)
Figure 2. Electron (left) and ion (right) density distributions at 10 laser cycles
Figure 3. Evolution of the laser amplitude peak when the channel depth is taken as 1836 depths
Figure 10. Angular distributions of gamma-rays (A),(B). (C),(D) Conversion efficiency. (E) Spectra
Figure 4. Three-dimensional isosurfaces of the laser intensity and the slices at 30 and 390 cylces
Figure 5. Evolution of the laser amplitude peak. (a) Different amplitudes (b) depths (c)laser powers