Abstract

In the last ten years, plasma mirrors[1] have often been used to improve the temporal contrast of high-intensity short-pulse lasers, so that deleterious pre-ionization effects in laser-matter interaction experiments are avoided. Indeed, the use of an anti-reflection-coated mirror close to a focus in the laser beam allows one to gain more than 2 orders of magnitude in temporal contrast per reflecting surface. For petawatt-class lasers that are very complicated machines, this is one of the few solutions that one has at hand to reach the temporal contrast required by many experiments. Much work has been published on the temporal characterization of plasma mirror setups at low energy and short pulse durations[2]; however the impact of the plasma mirror on the focal spot quality of high-energy sub-picosecond lasers has been much less studied. We propose and demonstrate an experimental setup capable of handling many 10’s of Joules, allowing for the direct characterization of the focal spot of a petawatt-class laser after a plasma mirror. On the one hand we observed that the focal spot shape of the laser is qualitatively not affected by the mirror, even at high working intensities. On the other hand the Strehl ratio of the beam is largely reduced at high intensities because of scattering on the expending plasma. Together with the measurement of the mirror reflectivity, we could define the optimal working condition of the mirror.

© 2013 IEEE