  Quantum emitters based on atomic defects in layered hexagonal Boron Nitride
(hBN) have emerged as promising solid state 'artificial atoms' with atom-like
photophysical and quantum optoelectronic properties. Similar to other atom-like
emitters, defect-phonon coupling in hBN governs the characteristic
single-photon emission and provides an opportunity to investigate the atomic
and electronic structure of emitters as well as the coupling of their spin- and
charge-dependent electronic states to phonons. Here, we investigate these
questions using photoluminescence excitation (PLE) experiments at T=4K on
single photon emitters in multilayer hBN grown by chemical vapor deposition. By
scanning up to 250 meV from the zero phonon line (ZPL), we can precisely
measure the emitter's coupling efficiency to different phonon modes. Our
results show that excitation mediated by the absorption of one in-plane optical
phonon increases the emitter absorption probability ten-fold compared to that
mediated by acoustic or out-of-plane optical phonons. We compare these
measurements against theoretical predictions by first-principles
density-functional theory of four defect candidates, for which we calculate
prevalent charge states and their spin-dependent coupling to bulk and local
phonon modes. Our work illuminates the phonon-coupled dynamics in hBN quantum
emitters at cryogenic temperature, with implications more generally for
mesoscopic quantum emitter systems in 2D materials and represents possible
applications in solid-state quantum technologies.
