A liquid crystal plasma mirror "fuse" for petawatt-class lasers to protect against back reflections

Petawatt-class lasers employ optics which are expensive and have very long lead-times to replace. Due to the dynamical plasma formed from high-density targets, back reflections risk damaging these optics even in non-normal incidence geometries. We present results on the use of a liquid crystal (LC) based plasma mirror (PM) that acts as a "fuse" to prevent back reflections by expanding below the critical density before reflections from the target arrive. This work was performed at ELI-NP using the recently commissioned High Power Laser System (HPLS). The experiment used a F/3.7 focusing optic and an LC PM. A Linear Slide Target Inserter (LSTI) was used to form in situ films of 20-2000 nm thickness. A pump-probe setup was used to measure the lifetime and reflectivity of the plasma mirror on picosecond timescales ranging before and after the plasma mirror formation. Results indicate the PM reflectivity lifetime is on the order of 10 ps for 20 nm films, showing the feasibility of a LC PM fuse. Measurements of suppression of the back reflection itself are presented.