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

Abstract

Excitons, Coulomb-bound electron and hole pairs, are elementary photo-excitations in semiconductors, that can couple directly to light through radiative relaxation. In contrast to these bright excitons, dark excitons X${}_{\mathrm{D}}$ with anti-parallel electron spin polarization exist, with generally forbidden radiative emission. Because of their associated long lifetimes, these dark excitons are appealing candidates for quantum computing and opto-electronic devices. However, optical read-out and control of X${}_{\mathrm{D}}$ states have remained a major challenge due to their decoupling from light. Here, we present a novel tip-enhanced nano-optical approach to precisely switch and programmably modulate the X${}_{\mathrm{D}}$ emission even at room temperature. Using monolayer two-dimensional transition metal dichalcogenide (TMD) WSe${}_2$ on a gold film as model system, we demonstrate $\sim 6\times 10^5$-fold enhancement in dark exciton photoluminescence quantum yield. This is achieved by the unique coupling of the nano-optical antenna-tip to the dark exciton \textit{out-of-plane} optical dipole moment, with an extraordinary Purcell factor of $\ge 2\times 10^3$ of the tip-sample nano-cavity. Compared to the necessity of cryogenic temperatures and high magnetic fields in conventional approaches, our work provides a new way to harness excitonic properties in low-dimensional semiconductors and new strategies for quantum opto-electronic devices.

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• Sub-diffraction-resolved spatial distribution of emitting excitons in STM-induced luminescence of 2D semiconductors via Richardson-Lucy deconvolution
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