Intense mid-infrared laser pulse generated via flying-mirror red-shifting in near-critical-density plasmas

Relativistic femtosecond mid-infrared pulses can be generated efficiently by laser interaction with near-critical-density plasmas. It is found theoretically and numerically that the radiation pressure of a circularly polarized laser pulse first compresses the plasma electrons to form a dense flying mirror with a relativistic high speed. The pulse reflected by the mirror is red-shifted to the mid-infrared range. Full three-dimensional simulations demonstrate that the central wavelength of the mid-infrared pulse is tunable from 3 $\rm{\mu m}$ to 14 $\rm{\mu m}$ and the laser energy conversion efficiency can reach as high as 13%. With a 0.5-10 PW incident laser pulse, the generated mid-infrared pulse reaches a peak power of 10-180 TW, which could be interesting for various applications in ultrafast and high-field sciences.