Preplasma has an important effect on the interaction between laser and matter, especially in solid targets, which produces different laser absorption mechanisms in sub-relativistic laser states, such as resonance absorption and vacuum heating (the Brunel mechanism). Based on these studies, several interesting results have been obtained recently, such as the Brunel-like mechanism, high harmonic generation, and vacuum electron acceleration. With the gargantuan laser powers projected to be realized in the developing petawatt (PW) facilities ELI in Europe[6–9] and SULF in China, the interaction between lasers and plasmas is poised to occur in the ultrarelativistic state. As is commonly understood, in the quantum electrodynamics (QED)-dominated regime, when an ultraintense laser interacts with a target, γ-rays can be generated by synchrotron radiation arising from high-energy electrons traveling in a strong, background electromagnetic field[12,13]. Previous studies focused on the function of the preplasma before a dense target[14,15], scanning different parameters to obtain the optimal γ-ray source. The other efficient method to generate γ-ray flare is to make accelerated electrons interact with a reflected laser, a method called all-optical Compton backscattering, in which electrons may be accelerated by a wakefield[16,17] or pondermotive force and laser is reflected by a dense plasma mirror. In these cases, underdense gas or nanoparticles are required before the solid targets in their plans. The γ-rays generated in different regimes have lent themselves to many applications, such as dense matter tomography, photonuclear reactions, and laboratory astrophysics. Previous research has shown that an ultraintense laser interacting with a plasma can emit γ-rays in different directions, depending on the plasma density[11,12,23] and the corresponding physical mechanisms at play.
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