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

## TL;DR

This paper demonstrates room-temperature control and enhancement of dark exciton emission in monolayer WSe2 using a nano-optical antenna-tip, achieving significant quantum yield increase via Purcell effect, enabling new quantum opto-electronic applications.

## Contribution

The study introduces a tip-enhanced nano-optical method to modulate dark exciton emission at room temperature with unprecedented Purcell enhancement, overcoming previous limitations of optical control.

## Key findings

- Achieved ~600,000-fold increase in dark exciton photoluminescence quantum yield.
- Demonstrated coupling of nano-optical antenna to dark exciton out-of-plane dipole.
- Realized Purcell factor of ≥2000 in a nano-cavity at room temperature.

## 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$\rm{_D}$ 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$\rm{_D}$ 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$\rm{_D}$ emission even at room temperature. Using monolayer two-dimensional transition metal dichalcogenide (TMD) WSe$\rm{_2}$ 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.