Radiative control of dark excitons at room temperature by nano-optical antenna-tip Purcell effect

By: Kyoung-Duck Park, Tao Jiang, Genevieve Clark, Xiaodong Xu, and Markus B. Raschke

Nature Nanotechnology (2017), in press

Excitons, Coulomb-bound electron-hole pairs, are elementary photo-excitations in semiconductors that can couple to light through radiative relaxation. In contrast, dark excitons XD show anti-parallel spin polarization with generally forbidden radiative emission. Because of their long lifetimes, these dark excitons are appealing candidates for quantum computing and optoelectronics. However, optical read-out and control of XD states has remained challenging due to their decoupling from light.  Here, we present a tip-enhanced nano-optical approach to induce, switch, and programmably modulate the XD emission at room temperature. Using a monolayer transition metal dichalcogenide (TMD) WSe2 on a gold substrate, we demonstrate ~6x105-fold enhancement in dark exciton photoluminescence quantum yield achieved through coupling of the antenna-tip to the dark exciton out-of-plane optical dipole moment, with large Purcell factor of ≥2x103 of the tip-sample nano-cavity. Our approach provides a new and facile way to harness excitonic properties in low-dimensional semiconductors offering new strategies for quantum optoelectronics.