Time-dependent screening explains the ultrafast excitonic signal rise in 2D semiconductors

TL;DR

This study uses first-principles real-time simulations to explain the ultrafast rise of excitonic signals in 2D semiconductors, attributing it to carrier relaxation and screening effects rather than exciton formation.

Contribution

It introduces a physical model showing how carrier dynamics and screening cause the delayed transient reflection signal in 2D materials.

Findings

Delay in signal rise depends on pump laser energy.

Carrier relaxation towards K valleys enhances screening.

Band gap renormalization dominates over excitonic effects.

Abstract

We calculate the time evolution of the transient reflection signal in an MoS$_2$ monolayer on a SiO$_2$/Si substrate using first-principles out-of-equilibrium real-time methods. Our simulations provide a simple and intuitive physical picture for the delayed, yet ultrafast, evolution of the signal whose rise time depends on the excess energy of the pump laser: at laser energies above the A- and B-exciton, the pump pulse excites electrons and holes far away from the K valleys in the first Brillouin zone. Electron-phonon and hole-phonon scattering lead to a gradual relaxation of the carriers towards small $\textit{Active Excitonic Regions}$ around K, enhancing the dielectric screening. The accompanying time-dependent band gap renormalization dominates over Pauli blocking and the excitonic binding energy renormalization. This explains the delayed buildup of the transient reflection signal of the probe pulse, in excellent agreement with recent experimental data. Our results show that the observed delay is not a unique signature of an exciton formation process but rather caused by coordinated carrier dynamics and its influence on the screening.