Unveiling interlayer exciton formation dynamics in transition metal dichalcogenide heterostructures with femtosecond optical spectroscopy.

In type-II transition metal dichalcogenide heterostructures (TMD HS) electrons and holes reside in different monolayers with the potential for formation of interlayer excitons (IeX). These IeX states are long-lived, opening a unique platform for fundamental studies and future applications. However, to date, little is known regarding the microscopic picture of IeX formation. In this contribution, we exploit ultrafast transient transmission spectroscopy to study the processes governing the formation of IeX in a large-area $MoSe_{2}/WSe_{2}$ HS. We simultaneously track the dynamics of both intralayer exciton (IaX) and IeX transitions following resonant excitation of lowest-energy $MoSe_{2}$ A exciton at $1.57 {eV}$. The signal attributed to the IeX transition shows a rise time on the picosecond timescale, which is much longer than the formation signal of $WSe_{2}$ IaX with a build-up time of about $100 {fs}$, unambiguously demonstrating that different physical processes are responsible for the occurrence of these signals. We attribute the delayed formation of the IeX signal to the cooling of hot IeX. Our study provides a new insight into the ultrafast processes in TMD HS.