Relativistic high-harmonic generation from liquid leaf targets at kHz rate

Relativistic high-harmonic generation (RHHG) from plasma mirrors has emerged as a promising approach for generating intense attosecond pulses, as it is predicted to efficiently convert ultra-high peak power laser light into a train of attosecond pulses in the XUV and soft X-ray region [1,2] (although the reported efficiencies so far are ~10^-4 [3,4], with high spatio-temporal quality [5-7]). Until recently, RHHG has primarily been demonstrated using bulk solid disk targets, which provide the necessary plasma density and gradient properties for efficient RHHG. However, such targets require continuous refreshing of the target surface for every laser shot over a finite sized substrate, thereby offering limited shot numbers for applications. The recent development of liquid-leaf targets with optical surface quality constitutes a significant breakthrough for laser-plasma interaction science as it ensures continuous target replenishment at multi-kHz repetition rates [8]. Liquid-leaf targets were successfully used to demonstrate HHG from a plasma mirror driven at 1 kHz repetition rate in the sub relativistic regime and in single-shot mode in the relativistic regime [9].