Pump power dependence of coherent phonons in η-Mo4O11

We studied the pump power dependence of the coherent phonon amplitude in the CDW and the normal states of eta-Mo4O11, which undergoes two CDW phase transitions at T-c1 = 105 K and T-c2 = 35 K. Femtosecond time-resolved pump-probe measurements were conducted at room temperature, 80 K (< T-c1), and 20 K (< T-c2). Unlike Raman spectra, the coherent phonon amplitudes showed a strong pump power dependence below T-c1 and T-c2. We found coherent phonons of which amplitudes were not proportional to the pump power. (C) 1999 Elsevier Science B.V. All rights reserved.