"Control of two-phonon correlations and the mechanism of high-wavevector phonon generation by ultrafast light pulses"

T. Henighan: M. Trigo, M. Chollet, J. N. Clark, S. Fahy, J. M. Glownia, M. P. Jiang, M. Kozina, H. Liu, S. Song, D. Zhu, and D. A. Reis; Physical Review B , 07/20/16.

Additional Authors: M. Trigo, M. Chollet, J. N. Clark, S. Fahy, J. M. Glownia, M. P. Jiang, M. Kozina, H. Liu, S. Song, D. Zhu, and D. A. Reis

Abstract:

Impulsive optical excitation can generate both coherent and squeezed phonons through first- and second-order Raman-like processes. The expectation value of the phonon displacement ⟨uq⟩  oscillates at the phonon mode frequency for the coherent state but remains zero for a pure squeezed state. In contrast, both show oscillations  in ⟨∣uq2⟩ at  twice the phonon mode frequency. Therefore it can be difficult to distinguish them in a second-order measurement of the displacements as is typical in x-ray diffuse scattering. Here we demonstrate a simple method to distinguish the generation mechanism by measurement of the diffuse scattering following double-impulsive excitation. We find in the case of Ge and GaAs that the generation of large wavevector phonons spanning the Brillouin zone is dominated by a second-order process.