Femtosecond reflectivity study of photoacoustic responses in bismuth thin films

We report on generation and observation of photoacoustic pulses in bismuth thin films using femtosecond reflectivity spectroscopy. Upon photoexcitation of bismuth thin films with a femtosecond laser pulse, hot carriers are created in the optical penetration depth and diffuse into the interior. They scatter off the interface effectively and raise the lattice temperature generating a picosecond acoustic phonon pulse which propagates towards the free surface and modulates the reflectivity. By monitoring the reflectivity change at the surface in real time, we investigate photoacoustic responses as a function of excitation fluence and film thickness.