Photoinduced modulation of the excitonic resonance via coupling with coherent phonons in a layered semiconductor

S Mor,P Franceschini,D Sangalli,V F Agekyan,A Molina-Sánchez,V Gosetti,S Achilli,S Pagliara,C Giannetti,L Sangaletti

doi:10.1103/physrevresearch.3.043175

Abstract

The coupling of excitons with atomic vibrations plays a pivotal role on the nonequilibrium optical properties of layered semiconductors. However, addressing the dynamical interaction between excitons and phonons represents a hard task both experimentally and theoretically. By means of time-resolved broadband optical reflectivity combined with state-of-the-art ab-initio calculations of a bismuth triiodide single crystal, we unravel the universal spectral fingerprints of exciton--phonon coupling in layered semiconductors. Furthermore, we microscopically relate a photoinduced coherent energy modulation of the excitonic resonance to coherent optical phonons, thereby tracking the extent of the photoinduced atomic displacement in real-space. Our findings represent a step forward on the road to coherent manipulation of the excitonic properties on ultrafast timescales.

Highlights

Electron-phonon coupling is among the fundamental interactions in condensed matter which govern the nonequilibrium optoelectronic properties of materials by, for instance, guiding the relaxation dynamics of quasiparticles [1,2,3,4] and assisting nonthermally driven electronic phase transitions [5,6,7]
By means of time-resolved broadband optical reflectivity combined with ab initio calculations of a bismuth tri-iodide single crystal, we set the spectral fingerprints for the optical detection of exciton-phonon coupling in layered semiconductors
All these findings demonstrate that tracking the energy modulation of the excitonic resonance induced by coherent phonons enables to trace atomic displacements along the direction of the phonon mode

Summary

INTRODUCTION

Electron-phonon coupling is among the fundamental interactions in condensed matter which govern the nonequilibrium optoelectronic properties of materials by, for instance, guiding the relaxation dynamics of quasiparticles [1,2,3,4] and assisting nonthermally driven electronic phase transitions [5,6,7]. This method further enables an estimate of atomic displacements with exceptionally high spatial resolution on the subpicometer scale [24] In this Article, by combining time-resolved broadband optical reflectivity measurements with state-of-the-art spinorial ab initio BSE calculations [25], we unveil the spectral fingerprints of exciton-phonon coupling in a representative van-der-Waals layered semiconductor, the bismuth tri-iodide (BiI3) single crystal. Transient absorption of spin-coated thin films of BiI3 observed a combined dynamics of coherent optical phonons and excitons upon ultrafast photoexcitation [33] All these aspects make BiI3 an ideal platform for exploring experimentally and theoretically the photophysics of excitonphonon coupling in a rather straightforward manner compared to more complex systems [15,20].

TRANSIENT REFLECTIVITY DATA AND AB INITIO SIMULATIONS

TRACKING THE PHONON-MEDIATED MODULATION OF THE EXCITONIC RESONANCE

CONCLUSION