Low-energy carrier dynamics in graphene and other 2D materials

Phonons in graphene and interexcitonic transition in transition metal dichalcogenides are examples for low-energy excitations in 2D materials. Free-electron lasers such as FELBE deliver tunable short mid-infrared pulses that are ideally suited to study the carrier dynamics in 2D materials in the energy range of these low-energy excitations. We present results on the electron dynamics in graphene and the exciton dynamics in MoSe₂.

I. Introduction

Novel atomically thin 2D materials can feature insulating, semiconducting, or semimetallic properties. The band structure makes many of these materials particularly interesting for optical studies. For example, the gapless material graphene is characterized by a constant absorption of 2.3 % and many monolayer transition metal dichalcogenides (TMDs) are direct semiconductors while their multilayer and bulk counterparts are indirect materials. Another basic property is a fairly strong Coulomb interaction that leads to a high excitonic binding energy in TMDs. Using the free-electron laser FELBE at Dresden-Rossendorf we investigate the electron population and polarization dynamics in graphene and the excitonic dynamics in MoSe₂.