Observation of coherent phonon states in porous silicon films

The dynamics of differential transmission and reflectance spectra of porous silicon films was studied using the femtosecond excitation technique (tau approximate to 50 fs, (h) over bar omega(pump) = 2.34 eV) with supercontinu um probing ((h) over bar omega(probe) = 1.6-3.2 eV) and controlled time del ay with a step of Delta t = 7 fs between the pump and probe pulses. A short -lived region of photoinduced bleaching was observed in the differential tr ansmission spectra at wavelengths shorter than the pump wavelength. The exc itation of coherent phonon states with a spectrum corresponding to nanocrys talline silicon with an admixture of a disordered phase was observed. The r elaxation of electronic excitation was found to slow down in the spectral r egion where the amplitude of excited coherent vibrations was maximal. (C) 2 000 MAIK "Nauka / Interperiodica".