Experimental study of Relativistic High Harmonic Generation with a PW level short pulse laser system

Surface High harmonic generation (SHHG) arising from the interaction of high and ultra-
high intensity lasers with solid density targets has been extensively studied during recent
years. Besides being a promising source for applications on ultrafast science, the
sensitivity of SHHG mechanisms with respect to changes of pump laser parameters or
target pre-plasma conditions makes the harmonic spectrum a useful diagnostic to gain
insight into the interaction between the laser and the target front surface where, in case of
a solid density target, most of the energy absorption between laser and target takes place.
In the present work, the first measurements of high harmonic spectra on the Draco PW
laser system at HZDR are presented. Most measurements were done using a single
plasma mirror system, achieving a contrast of 10-7 at -1ps for a 5.4x1021W/cm2, 30fs
800nm pulse. XUV spectra were measured for a wide range of different target materials
(plastic, metal, glass), thicknesses; from bulk SiO2 substrates to few tens of nm foils where
relativistic transparency starts to occur; as well as different laser energies (a0~7-50). The
spectral range of the spectrometer (53nm to ~17nm, 15th to 47th harmonic, limited by the
filtering) allowed us to measure changes in SHHG along the spectrum for different
interaction conditions. These measurements were carried in parallel with proton
acceleration experiments, aiming at using the harmonic generation as a gauge for the
laser-plasma interaction at different interaction regimes, and to our knowledge constitute
one of the first measurements of relativistic surface harmonics with a PW short pulse laser.