Control of the Bright-Dark Exciton Splitting using Lamb Shift in a 2D Semiconductor

We have investigated the exciton fine structure in atomically thin WSe2 -based van der Waals heterostructures where the density of optical modes at the location of the semiconductor monolayer can be tuned. The energy splitting $\Delta$ between the bright and dark exciton has been measured by photoluminescence spectroscopy. We demonstrate that $\Delta$ can be tuned by a few meV, as a result of a significant Lamb shift of the optically active exciton which arises from emission and absorption of virtual photons triggered by the vacuum fluctuations of the electromagnetic field. We also measured strong variations of the bright exciton radiative linewidth, as a result of the Purcell effect. All these experimental results illustrate the strong sensitivity of the excitons to local vacuum field. We found a very good agreement with a model that demonstrates the equivalence, for our system, of a classical electrodynamical transfer matrix formalism and quantum-electrodynamical approach. The bright-dark splitting control demonstrated here should apply to any semiconductor structures.