Ultrafast carrier dynamics and symmetry reduction in bismuth by nonperturbative optical excitation in the terahertz range

The semimetal bismuth displays a multifaceted nonlinear terahertz response, which is studied by two-dimensional terahertz (2D-THz) spectroscopy. Nonperturbative excitation drives intra- and interband electron transitions close to the narrow band gaps at the L points of the Brillouin zone, giving rise to pump-probe signals and high-harmonic generation. Preferential excitation in two of the six L valleys generates an anisotropic carrier distribution across the Brillouin zone, causing a hexagonal azimuthal angular dependence of the pump-probe signal. The concomitant symmetry reduction allows excitation of coherent phonon wavepackets along back-folded phonon coordinates.