How to distinguish squeezed and coherent phonons in femtosecond x-ray diffuse scattering

TL;DR

This paper presents a method to distinguish squeezed from coherent phonons in femtosecond x-ray diffuse scattering by using double impulsive excitation, revealing squeezed phonons in several materials and confirming previous theoretical predictions.

Contribution

The study introduces a simple experimental approach to differentiate squeezed and coherent phonons, demonstrating its effectiveness across multiple materials.

Findings

Femtosecond optical excitation generates squeezed phonons in Ge, GaAs, and InSb.

The method successfully distinguishes squeezed from coherent phonons using double excitation.

Results confirm the mechanism of squeezed phonon generation as previously suggested.

Abstract

Impulsive optical excitation can generate both coherent and squeezed phonons. The expectation value of the phonon displacement $<u_q>$ oscillates at the mode frequency for the coherent state but remains zero for a pure squeezed state. In contrast, both show oscillations in $<|u_q|^2>$ at twice the mode frequency. Therefore it can be difficult to distinguish them in a second-order measurement of the displacements, such as in first-order x-ray diffuse scattering. Here we demonstrate a simple method to distinguish squeezed from coherent atomic motion by measurement of the diffuse scattering following double impulsive excitation. We find that femtosecond optical excitation generates squeezed phonons spanning the Brillouin zone in Ge, GaAs and InSb. Our results confirm the mechanism suggested in [Nature Physics 9, 790 (2013)].