Nonperturbative carrier generation and symmetry reduction in bismuth by intense terahertz excitation

The semimetal bismuth is studied by two-dimensional terahertz (2D-THz) spectroscopy in the nonperturbative regime. We observe and separate different components of the nonlinear THz response, including high harmonic generation and pump-probe signals pointing to anisotropic transient carrier distributions and phonon backfolding at reduced symmetry.