  A double-edged sword in two-dimensional material science and technology is an
optically forbidden dark exciton. On the one hand, it is fascinating for
condensed matter physics, quantum information processing, and optoelectronics
due to its long lifetime. On the other hand, it is notorious for being
optically inaccessible from both excitation and detection standpoints. Here, we
provide an efficient and low-loss solution to the dilemma by reintroducing
photonics bound states in the continuum (BICs) to manipulate dark excitons in
the momentum space. In a monolayer tungsten diselenide under normal incidence,
we observed a giant enhancement with an enhancement factor of ~3,100 for dark
excitons enabled by transverse magnetic BICs with intrinsic out-of-plane
electric fields. By further employing widely tunable Friedrich-Wintgen BICs, we
demonstrated highly directional emission from the dark excitons with a
divergence angle of merely 7 degrees. We found that the directional emission is
coherent at room temperature, unambiguously shown in polarization analyses and
interference measurements. Therefore, the BICs reintroduced as a momentum-space
photonic environment could be an intriguing platform to reshape and redefine
light-matter interactions in nearby quantum materials, such as low-dimensional
materials, otherwise challenging or even impossible to achieve.
