Small Atomic Displacements Recorded in Bismuth by the Optical Reflectivity of Femtosecond Laser-Pulse Excitations

Abstract : Subtle atomic motion in a Bi crystal excited by a 35 fs-laser pulse has been recovered from the transient reflectivity of an optical probe measured with an accuracy of 10^-5. Analysis shows that a novel effect reported here—an initial negative drop in reflectivity—relates to a delicate coherent displacement of atoms by the polarization force during the pulse. We also show that reflectivity oscillations with a frequency coinciding with that of cold Bi are related to optical phonons excited by the electron temperature gradient through electron-phonon coupling.