Ultrashort laser-pulses can be used to excite and detect very high frequency acoustic wave typically between a few GHz up to a few THz. The so-called picosecond acoustic technique has reached this way an unexplored frequency range by using femtosecond laser pulses in a pump and probe scheme. Such frequency falls typically in the hundreds of gigahertz range, a range which suits very well the acoustic vibrations of nanoscale objects. Then a femtosecond laser and a pump-probe setup offer a unique way of studying the impact of size reduction on elastic properties and phonon confinement.
In this work, we study the acoustical response of ultra-thin silicon membranes in the time-domain using various experimental conditions. Free standing crystalline silicon membranes were prepared from SOI wafers. The membrane thickness is comprised between 30 and 100 nm. We report on the excitation of longitudinal acoustic modes and discussed about both the excitation and detection mechanisms. We especially focus on the remarkable efficiency of such a time-resolved technique is due to the simultaneous confinement of acoustic and optic energy within such nanoscale objects.