Ultrafast broadband spectroscopy of crystalline bismuth

Femtosecond spectroscopy in the wavelength range 0.4 — 2.3 μm has been used to probe ultrafast electronic and lattice processes in bismuth. The photoresponse of a bismuth crystal is shown to comprise components with relaxation times of 1 ps, 7 ps, and ~1 ns. The electron—hole and electron—phonon interaction strengths in bismuth are found to depend significantly on the wave vector in the Г—T direction of the Brillouin zone. Comparison of the spectral dependences of the amplitudes of coherent E_g and A_1g phonons and the corresponding dependences of the Raman scattering cross sections indicates that these phonon modes differ in generation mechanism. The generation of coherent A_1g phonons is mainly due to displacement of the equilibrium position of atoms in the crystal lattice in a nonequilibrium state. This process differs fundamentally from resonance Raman scattering responsible for the coherent excitation of low-symmetry phonon modes.