Intense single attosecond pulse generation from near-critical-density plasmas irradiated by a few-cycle laser pulse

Coherent synchrotron emission (CSE) from relativistic near-critical-density (NCD) plasmas irradiated by a few-cycle laser pulse is investigated theoretically and numerically. Due to the unique and larger laser-plasma interaction region in relativistic NCD plasmas, compared to those in solid targets, not only the required stringent conditions for CSE on laser and target are relaxed but also the radiation intensities are enhanced by two orders of magnitude. Moreover, it is found that a single attosecond pulse can also be easily obtained in the transmitted direction through CSE in this regime. Its energy conversion efficiencies from laser to emission can reach 10−3–10−2, which is more than one order of magnitude larger than those of attosecond trains from solids. Two-dimensional particle-in-cell simulations show that an intense single pulse at a peak intensity of ∼1019 W/cm2 and duration of ∼98 as in the transmitted direction is produced by the drive laser at an intensity of I0 = 8.6 × 1020 W/cm2.