Long-lived coherent acoustic waves generated by femtosecond light pulses

We report on photoexcitation of coherent longitudinal acoustic phonons in single-crystal cuprate thin films on epitaxialy matched substrates. The photoinduced reflectance oscillations are unusually long lived; in some samples we could easily resolve them for hundreds of periods. We studied the effect of varying a number of parameters, including the film doping level, thickness, and temperature, as well as the pump and probe beam wavelength, power, polarization, and incidence angle. We account quantitatively for the oscillation period, dispersion, phase, and decay of amplitude with time.