Dielectric nano-antennas for strain engineering in atomically thin two-dimensional semiconductors

Atomically thin two-dimensional semiconducting transition metal dichalcogenides (TMDs) can withstand large levels of strain before their irreversible damage occurs. This unique property offers a promising route for control of the optical and electronic properties of TMDs, for instance, by depositing them on nanostructured surfaces, where position-dependent strain can be produced on the nanoscale. Here, we demonstrate strain-induced modifications of the optical properties of mono- and bilayer TMD WSe2 placed on photonic nanoantennas made from gallium phosphide (GaP). Photoluminescence (PL) from the strained areas of the TMD layer is enhanced owing to the efficient coupling with the confined optical mode of the nanoantenna. Thus, by following the shift of the PL peak, we deduce the changes in the strain in WSe2 deposited on the nanoantennas of different radii. In agreement with the presented theory, strain up to approximate to 1.4% observed for WSe2 monolayers. We also estimate that >3% strain is achieved in bilayers, accompanied by the emergence of a direct bandgap in this normally indirect-bandgap semiconductor. At cryogenic temperatures, we find evidence of the exciton confinement in the most strained nanoscale parts of the WSe2 layers, as also predicted by our theoretical model. Our results of direct relevance for both dielectric and plasmonic nanoantennas, show that strain in atomically thin semiconductors can be used as an additional parameter for engineering light matter interaction in nanophotonic devices.