Ultrafast time-resolved spectroscopy of the spin-Peierls compound CuGeO3

Coherent phonons and electronic response of the spin-Peierls compound CuGeO3 are investigated at room temperature using time-resolved transmission measurement. Four Ag modes are observed strongly in the E-parallel b polarization, consistent with the Raman spectra. Coherent lattice oscillation with the highest frequency of 860 cm-1 is kicked on with an initial phase of -(0.5±0.2)π radian and decays with a time constant of about 0.9 ps, indicating the ground-state phonon is induced by the process of impulsive stimulated Raman scattering (ISRS). The decay-time constants are rather long compared to the values expected from the line width of the stationary Raman spectrum. In addition, instantaneous photo-induced absorption is observed in the E-parallel b polarization at delay-time zero, whose temporal width is close to that of the cross-correlation between pump and probe, attributed to the two-photon absorption of the pump and the probe photons. The intensity of the negative peak-like response at delay-time zero increases in the E-parallel c polarization, and is ascribed to a cascade excitation via the Zhang-Rice (ZR)-like exciton. This absorption decays rapidly within 100 fs, indicating that the ZR excitons experience extremely fast relaxation after photoexcitation