Pump-Probe Measurement of Relativistic Transparency and Relativistic Birefringence *

Relativistic induced transparency (RIT) is the transmission of a laser through an overdense plasma due to the relativistic mass increase of the laser heated plasma electrons. Direct measurements of RIT are difficult and uncommon, limiting comparison to theory. Recent theoretical findings (Stark, et al., Phys. Rev. Lett. 115, 025002 (2015)) predict a new effect, relativistic induced birefringence due to anisotropy in the electron momentum, offering additional motivation for new experiments. We present results of a pump-probe study of RIT carried out at the Scarlet Laser Facility at Ohio State. A pump with peak intensity >10²¹ W/cm² was incident on ~23 nm thick liquid crystal targets while a probe was incident from the other side. The transmission of the pump and probe (polarization resolved) was measured over a delay range of about 15 ps. We measured the ultrafast turn-on and turn-off of RIT. The return of the system to opacity resolves the ambiguity of whether RIT was achieved or if there was simply a decrease in plasma density due to target heating and transport. Finally, an average rotation of the probe polarization ellipticity angle of ∼7.8^o at zero delay was measured for the highest pump intensities, serving as strong experimental evidence of relativistic birefringence.