Abstract

The experimental and theoretical studies of the reflectivity oscillations of the probe beam from the single-crystal of bismuth excited by fs-laser pulses are presented. The reflectivity was measured by the pump-probe technique with the accuracy 10⁻⁵ and with time-resolution of 35 fs that allowed observing a novel feature - a sharp drop of the reflectivity below the initial value. The reflectivity then increased above the initial value, and then oscillates with the frequency of A_ls phonon in Bi in agreement with earlier findings [1]. Our analysis established the direct link between reflectivity oscillations and atomic vibrations through the electron-phonon coupling rate that is proportional to the phonon's amplitude. The major force driving atomic motion in a laser-excited solid is thermal force proportional to the temperature gradients. It was demonstrated that strain-dependent polarisation [2–4] is a lesser parts of the external field effect on a solid than the thermal force in opaque medium. The proposed theory explains all experimentally observed features of the transient reflectivity.

© 2007 IEEE