One of the paradigm-shifting phenomena triggered in laser-plasma interactions
at relativistic intensities is the so-called relativistic transparency. As the
electrons become heated by the laser to relativistic energies, the plasma
becomes transparent to the laser light even though the plasma density is
sufficiently high to reflect the laser pulse in the non-relativistic case. This
paper highlights the impact that relativistic transparency can have on
laser-matter interactions by focusing on a collective phenomenon that is
associated with the onset of relativistic transparency: plasma birefringence in
thermally anisotropic relativistic plasmas. The optical properties of such a
system become dependent on the polarization of light, and this can serve as the
basis for plasma-based optical devices or novel diagnostic capabilities