Impact ionization dynamics in small band-gap 2D materials from a coherent phonon mechanism

TL;DR

This paper theoretically investigates impact ionization and carrier multiplication in small band-gap 2D materials induced by ultrafast optical excitation and coherent phonons, highlighting their signatures in time-resolved photoemission.

Contribution

It introduces a model linking impact ionization, coherent phonons, and band dynamics, providing insights into ultrafast carrier multiplication in 2D materials.

Findings

Impact ionization significantly affects ultrafast electronic response.

Band shape influences carrier multiplication efficiency.

Signatures of carrier multiplication are identifiable in photoemission data.

Abstract

We study theoretically the role of carrier multiplication due to impact ionization after an ultrafast optical excitation in a model system of a quasi-two dimensional material with a small band gap. As a mechanism for the photo-induced band gap narrowing we use coherent phonons, which mimics the quenching of an insulator phase. We discuss the importance of impact ionization in the ultrafast response, and investigate the interplay between carrier and band dynamics. Our model allows us to compare with recent experiments and identify signatures of carrier multiplication in typical electronic distribution curves as measured by time-resolved photoemission spectroscopy. In particular we investigate the influence of the shape of the bands on the carrier multiplication and the respective contributions of band and carrier dynamics to electronic distribution curves.