Excitation of Coherent Phonons in Crystalline Bi: Theory for Driving Atomic Vibrations by Femtosecond Pulses
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1016-CC06-03
Excitation of Coherent Phonons in Crystalline Bi: Theory for Driving Atomic Vibrations by Femtosecond Pulses Davide Boschetto1, Eugene G. Gamaly2, Andrei V. Rode2, Thomas Garl1, and Antoine Rousse1 1 Laboratoire d'Optique AppliquÈe, ENSTA, Chemin de la HuniËre, Palaiseau, 91761, France 2 Laser Physics Centre, Research School of Physical Sciences and Engineering, The Australian National University, Canberra, ACT 0200, Australia ABSTRACT In this paper we present experimental and theoretical studies of reflectivity oscillations of an optical probe beam reflected from a single-crystal of bismuth excited by 35 fs laser pulses at deposited energy density above the melting temperature. Coherent and incoherent lattice dynamics as well as electrons dynamics were investigated starting from the reflectivity changes, measured with high accuracy ∆R/R < 10-5. The complex behaviour of the reflectivity could not be explained in the light of the existing theories. Therefore, we developed a new theory, starting from the very basic principles of laser-matter interaction, which shows good agreement with experimental results. We establish a direct dependence of the transient reflectivity on atomic motions driven by electron temperature gradient through electron-phonon coupling.
INTRODUCTION Coherent optical phonons has received a lot of attention with the advent of femtosecond lasers, as the pump-probe technique provides a means to follow the lattice dynamics after the excitation with sub-picosecond resolution. Optical phonons can be seen as an elementary atomic movement, and therefore their knowledge is a key point to understand the structural dynamics. In particular, exciting a selected phonon mode will pave the way to control phase transitions. Possible applications are phase transitions like the insulator-to-metal transitions, transitions to superconductivity, and many others. Since the early 1980s a number of experiments have been performed which measured the transient reflectivity and transmission of solids using pump-probe techniques with time resolution around 100 fs [1-13]. The reflectivity of a probe beam as a function of delay following the pump pulse has been measured for a variety of materials, both opaque and transparent, including insulators, semimetals, and semiconductors. The most salient feature of those measurements was reflectivity oscillations at frequencies corresponding to the optical phonon modes of the material. A number of mechanisms have been proposed to explain those oscillations, including the most frequently cited displacive excitation of coherent phonons (DECP) [2] and impulsive stimulated Raman scattering (ISRS) [3,4]. However, a general theory explaining excitation of atomic vibrations in transparent and opaque media and their connection to reflectivity oscillations is still absent, to the best of our knowledge. We show that the mechanism of coherent phonon excitation is related to the interplay of several types of forces, which are expressed through the ìfield+matterî stress tensor. Those
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