Attenuation of Picosecond Ultrasonic Pulses in a thin Silicon Wafer

We report picosecond ultrasonic measurements of phonon attenuation in a 56 μm thick Si wafer coated with a 25 nm Al film. The measurements are of interest for picosecond ultrasonic imaging schemes, and for investigations of nanoscale heat transport. An ultrafast laser was used to perform the pump-probe experiment. A probe delay stage with a range of several nanoseconds allowed detection of coherent acoustic phonon (picosecond ultrasonic) pulses that had travelled one or two round trips through the Si. By comparing the signals from the first phonon pulse to the second phonon pulse we measured the attenuation of longitudinal acoustic phonons as a function of frequency (30GHz-90GHz) and temperature (100K-300K). We find that the frequency dependence at 100K is reduced compared with the frequency dependence at higher temperatures.