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 (τ≈50 fs, ?ω_pump=2.34 eV) with supercontinuum probing (?ω_probe=1.6–3.2 eV) and controlled time delay with a step of Δt=7 fs between the pump and probe pulses. A short-lived region of photoinduced bleaching was observed in the differential transmission spectra at wavelengths shorter than the pump wavelength. The excitation of coherent phonon states with a spectrum corresponding to nanocrystalline silicon with an admixture of a disordered phase was observed. The relaxation of electronic excitation was found to slow down in the spectral region where the amplitude of excited coherent vibrations was maximal.