Measurement of Shorter-Than-Skin-Depth Acoustic Pulses in a Metal Film Via Transient Reflectivity

The detection of ultrashort laser-generated acoustic pulses at a metal surface and the reconstruction of the acoustic strain profile are investigated. A 2 ps-long acoustic pulse generated in an SrRuO[subscript 3] layer propagates through an adjacent gold layer and is detected at its surface by a reflected probe pulse. We show that the intricate shape of the transient reflectivity waveform and the ability to resolve acoustic pulses shorter than the optical skin depth are controlled by a single parameter, which is determined by the ratio of the real and imaginary parts of the photoelastic constant of the material. We also demonstrate a Fourier transform-based algorithm that can be used to extract acoustic strain profiles from transient reflectivity measurements.United States. Dept. of Energy (Grant DE-FG0200ER15087)Pays de la Loire (France)United States. Army Research Office (Grant W911NF-10-1-0362)National Science Foundation (U.S.). Graduate Research Fellowship ProgramUnited States. Air Force Office of Scientific Research (Grant FA9550-12-1-0342