Highly directional and coherent emission from dark excitons enabled by bound states in the continuum

TL;DR

This paper demonstrates how bound states in the continuum (BICs) can enable highly directional, coherent emission from dark excitons in monolayer tungsten diselenide, overcoming their optical inaccessibility and opening new avenues for quantum photonics.

Contribution

The study introduces a novel approach using BICs to manipulate dark excitons, achieving giant enhancement and directional emission at room temperature.

Findings

Giant enhancement factor of ~3,100 for dark excitons.

Highly directional emission with a divergence angle of 7 degrees.

Coherent emission confirmed at room temperature.

Abstract

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.