Quantum pathways of carrier and coherent phonon excitation in bismuth

Quantum pathways inducing coherent lattice dynamics are studied in bismuth (Bi). A crystalline Bi film is excited by femtosecond midinfrared pulses and transient intensity changes on the (111) Bragg reflection are probed by hard x-ray pulses. A fast decrease and coherent oscillations of the diffracted intensity display up to 50% and 10% intensity change, respectively. The oscillation frequency of 2.6 THz is independent of pump intensity. Midinfrared excitation opens different quantum pathways for electron-hole generation, such as field-driven carrier tunneling at the L points, which reduces the crystal symmetry and leads to optical phonon excitation at the X point with the strongest electron-phonon coupling.