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

Reviews and Highlights	Quantum Science	Molecular and Soft-matter	Ultrafast Nano-optics and Nanophotonics	Mineralogy and Geochemistry

Kyoung-Duck Park, Tao Jiang, Genevieve Clark, Xiaodong Xu, and Markus B. Raschke
Nature Nanotechnology 13, 59 (2018).
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Excitons, Coulomb-bound electron-hole pairs, are elementary photo-excitations in semiconductors that can couple to light through radiative relaxation. In contrast, dark excitons X_D 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 X_D 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 X_D emission at room temperature. Using a monolayer transition metal dichalcogenide (TMD) WSe₂ on a gold substrate, we demonstrate ~6×10⁵-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 ≥2×10³ 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.