Coherent Phonon Excitation by Ultrafast Pulse Trains

Time resolved reflectivity of bismuth thin film evaporated on a silicon substrate is measured by an 80 femtosecond (fs) laser at a center wavelength of 800 nm. The reflectivity data reveal that coherent optical phonons (A_1g) near 2.9 THz (1 THz = 10¹² Hz) are excited by the 80 fs laser pulses. Analyses of the reflectivity data reveal key parameters related to electron and phonon dynamics, including phonon excitation and de-phasing and electron-phonon energy coupling. It is also found that the phonon frequency peaks are red-shifted and broadened at higher laser fluences. We also show that the phonon oscillation can be manipulated by specifically designed ultrafast pulse trains with different pulse separation times.