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 (|uq|2) 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.
Henighan, T., Trigo, M., Chollet, M., Clark, J. N., Fahy, S., Glownia, J. M., … Reis, D. A. (2016). Control of two-phonon correlations and the mechanism of high-wavevector phonon generation by ultrafast light pulses. Physical Review B, 94(2). https://doi.org/10.1103/PhysRevB.94.020302