PIC modeling of a pump-probe experiment on relativistic induced transparency and laser driven dynamic structures

Relativistic induced transparency (RIT), whereby an intense laser can transmit through an overdense plasma, is critical to understanding intense laser-matter experiments, particularly those using thin targets. This effect is difficult to measure directly (Palaniyappan, et al., Nature Physics 8, 763 (2012) and Bagnoud, et al., Phys. Rev. Lett. 118, 255003 (2017)), especially with femtosecond temporal resolution. Recent theoretical and computational results (Stark, et al., Phys. Rev. Lett. 115, 025002 (2015)) predict a new effect, relativistic induced birefringence, due to anisotropy in electron momentum. Recent pump-probe experiments using the OSU Scarlet laser have measured the ultrafast turn-on and turn-off of RIT as well as relativistic birefringence using liquid crystal targets at intensities up to or exceeding 10²¹ W/cm². We describe 3D particle-in-cell (PIC) simulations that explain these results including the role of dynamic structures that evolve during and after the pump and we also show evidence for a scaling law involving laser intensity and target areal density. This work required a careful separation of pump and probe fields and we describe how this was achieved.