Relativistic birefringence and polarization-dependent reflectivity in femtosecond pump-probe systems

At relativistic laser intensities, plasmas exhibit higher transparency due to the relativistic motion of the electrons. In cases where the relativistic electron momenta is anisotropic, the plasma surface exhibits polarizer-like, polarization-dependent reflectivity. Additionally, the preferred orientation of electron momenta results in collective volumetric birefringence. Understanding the dependence of polarization on relativistic transparency and birefringence is key to developing a fundamental understanding of relativistic laser-plasma interactions for potential applications. Using one- and two-dimensional particle-in-cell simulations, we show how this electron momenta anisotropy can be driven by femtosecond pulses from a pump beam to dynamically control the polarization properties of a probe beam in underdense plasmas. We investigate how the surface reflectivity and bulk birefringence can be uniquely identified under varying plasma densities and relativistic laser intensities.