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

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.

Figure 1

Comparison between the experimental [solid (dark) line] and theoretical [dashed (red) line] behavior of the transient reflectivity: (a) excitation flux $2.7\mathrm{mJ}/{\mathrm{cm}}^2$ and (b) excitation flux $6.7\mathrm{mJ}/{\mathrm{cm}}^2$.