Direct Measurement of Anharmonic Decay Channels of a Coherent Phonon

TL;DR

This paper directly measures how a coherent phonon in bismuth decays anharmonically into other phonons via three-phonon scattering, revealing the underlying decay channels and coupling constants.

Contribution

It provides the first direct experimental observation of anharmonic decay channels of a coherent phonon using ultrafast diffuse x-ray scattering.

Findings

Observation of coherent oscillations in target phonon modes.

Measurement of anharmonic coupling constants.

Confirmation of decay channels via three-phonon scattering.

Abstract

We observe anharmonic decay of the photoexcited coherent A1g phonon in bismuth to points in the Brillouin zone where conservation of momentum and energy are satisfied for three-phonon scattering. The decay of a coherent phonon can be understood as a parametric resonance process whereby the atomic displacement periodically modulates the frequency of a broad continuum of modes. This results in energy transfer through resonant squeezing of the target modes. Using ultrafast diffuse x-ray scattering, we observe build up of coherent oscillations in the target modes driven by this parametric resonance over a wide range of the Brillouin zone. We compare the extracted anharmonic coupling constant to first principles calculations for a representative decay channel.