Resonance fluorescence and laser spectroscopy of three-dimensionally confined excitons in monolayer WSe$_2$
Resonant optical excitation of few-level quantum systems enables coherent quantum control, resonance fluorescence, and direct characterization of dephasing mechanisms. Experimental demonstrations have been achieved in a variety of atomic and solid-state systems. An alternative but intriguing quantum photonic platform is based on single layer transition metal chalcogenide semiconductors, which exhibit a direct band-gap with optically addressable exciton valley-pseudospins in a uniquely two-dimensional form. Here we perform resonance and near-resonance excitation of three-dimensionally confined excitons in monolayer WSe$_2$ to reveal near ideal single photon fluorescence with count rates up to 3 MHz and uncover a weakly-fluorescent exciton state ~ 5 meV blue-shifted from the ground-state exciton. We perform high-resolution photoluminescence excitation spectroscopy of the localized excitons, providing important information to unravel the precise nature of the quantum states. Successful demonstration of resonance fluorescence paves the way to probe the localized exciton coherence. Moreover, these results yield a route for investigations of the spin and valley coherence of confined excitons in two-dimensional semiconductors.