  Precise control over the electronic and optical properties of defect centers
in solid-state materials is necessary for their applications as quantum
sensors, transducers, memories, and emitters. In this study, we demonstrate,
from first principles, how to tune these properties via the formation of defect
polaritons. Specifically, we investigate three defect types -- CHB, CB-CB, and
CB-VN -- in monolayer hexagonal boron nitride (hBN). The lowest-lying
electronic excitation of these systems is coupled to an optical cavity where we
explore the strong light-matter coupling regime. For all defect systems, we
show that the polaritonic splitting that shifts the absorption energy of the
lower polariton is much higher than can be expected from a Jaynes-Cummings
interaction. In addition, we find that the absorption intensity of the lower
polariton increases by several orders of magnitude, suggesting a possible route
toward overcoming phonon-limited single photon emission from defect centers.
Finally, we find that initially localized electronic transition densities can
become delocalized across the entire material under strong light-matter
coupling. These findings are a result of an effective continuum of electronic
transitions near the lowest-lying electronic transition for both pristine hBN
and hBN with defect centers that dramatically enhances the strength of the
light-matter interaction. We expect our findings to spur experimental
investigations of strong light-matter coupling between defect centers and
cavity photons for applications in quantum technologies.
