Optical generation of a broadband acoustic frequency comb in the 100 GHz-regime

Acoustic properties in the GHz frequency range are not very well understood for most semiconductors although intrinsic attenuation and scattering at interfaces of acoustic waves is of great interest for both fundamental and applied science. Additionally adhesion properties of very thin films are both hard to control and to evaluate. A thin silicon membrane with an aluminum transducer on top is used to gain knowledge of these crucial properties. The two-layer system is investigated by asynchronous optical sampling, a pump-probe technique employing 2 fs lasers [1]. The pump pulse generates strain fronts in the membrane, which are propagating and thereby defining a pulse. The Si/air-interface and the Al/air interface act as mirros and reflect the acoustic pulse. As a result an acoustic cavity is formed.