Narrowband generation of ultrafast acoustic and thermal transients in thin films for enhanced detectability

A technique for the narrowband generation of ultrafast acoustic and thermal transients in thin films is demonstrated; this technique allows for enhanced detectability of these transients. The approach pursued uses a reduced-bandwidth, optical pulse train for excitation that is constructed from a series of time-delayed pulses derived from a single-laser pulse. The underlying physical limitations of this approach are considered in order to assess conditions under which successful bandwidth reduction can be realized. Results in an aluminum thin film on a tungsten-carbide substrate show successful generation and detection of a narrowband acoustic signal centered at 32.34 GHz.