Strongly coupled coherent phonons in single-layer MoS$_2$

TL;DR

This paper demonstrates the generation and detection of coherent phonons in single-layer MoS$_2$, revealing strong exciton-phonon coupling and resonant enhancement effects, with implications for ultrafast optical control of excitonic states.

Contribution

It introduces a broadband transient absorption setup with high temporal resolution to study vibrational coherences in 1L-MoS$_2$, revealing strong exciton-phonon interactions and a Raman-like excitation process.

Findings

Enhanced CP amplitude resonant with C exciton peak

Strong coupling between A'_{1} phonons and C excitons confirmed by ab initio calculations

CP generation follows the spectral profile of Raman susceptibility tensor

Abstract

We present a transient absorption setup combining broadband detection over the visible-UV range with high temporal resolution ($\sim$20fs) which is ideally suited to trigger and detect vibrational coherences in different classes of materials. We generate and detect coherent phonons (CPs) in single layer (1L) MoS$_2$, as a representative semiconducting 1L-transition metal dichalcogenide (TMD), where the confined dynamical interaction between excitons and phonons is unexplored. The coherent oscillatory motion of the out-of-plane $A'_{1}$ phonons, triggered by the ultrashort laser pulses, dynamically modulates the excitonic resonances on a timescale of few tens fs. We observe an enhancement by almost two orders of magnitude of the CP amplitude when detected in resonance with the C exciton peak, combined with a resonant enhancement of CP generation efficiency. Ab initio calculations of the change in 1L-MoS$_2$ band structure induced by the $A'_{1}$ phonon displacement confirm a strong coupling with the C exciton. The resonant behavior of the CP amplitude follows the same spectral profile of the calculated Raman susceptibility tensor. This demonstrates that CP excitation in 1L-MoS$_2$ can be described as a Raman-like scattering process. These results explain the CP generation process in 1L-TMDs, paving the way for coherent all-optical control of excitons in layered materials in the THz frequency range.