Optical Absorption and Emission Mechanisms of Single Defects in Hexagonal Boron Nitride

We investigate the polarization selection rules of sharp zero-phonon lines (ZPLs) from isolated defects in hexagonal boron nitride (HBN) and compare our findings with the predictions of a Huang-Rhys model involving two electronic states. Our survey, which spans the spectral range similar to 550-740 nm, reveals that, in disagreement with a two-level model, the absorption and emission dipoles are often misaligned. We relate the dipole misalignment angle (Delta theta) of a ZPL to its energy shift from the excitation energy (Delta E) and find that Delta theta approximate to 0 degrees when Delta E corresponds to an allowed HBN phonon frequency and that 0 degrees <= Delta theta <= 90 degrees when Delta E exceeds the maximum allowed HBN phonon frequency. Consequently, a two-level Huang-Rhys model succeeds at describing excitations mediated by the creation of one optical phonon but fails at describing excitations that require the creation of multiple phonons. We propose that direct excitations requiring the creation of multiple phonons are inefficient due to the low Huang-Rhys factors in HBN and that these ZPLs are instead excited indirectly via an intermediate electronic state. This hypothesis is corroborated by polarization measurements of an individual ZPL excited with two distinct wavelengths that indicate a single ZPL may be excited by multiple mechanisms. These findings provide new insight on the nature of the optical cycle of novel defect-based single-photon sources in HBN.