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

Isolated point defects in wide bandgap semiconductors are single photon sources with applications in quantum optics, precision sensing, and quantum information processing technologies. Here we investigate the polarization selection rules of zero-phonon lines (ZPLs) from isolated defects in h-BN and compare our findings with the predictions of a two-level Huang-Rhys model. Our survey, which spans the spectral range ~550-740 nm, reveals that, in disagreement with the two-level model, the absorption and emission dipoles are often misaligned. We relate the dipole misalignment angle (△θ) of a ZPL to its energy shift from the excitation energy (△E) and find that △θ ≈ 0° when △E corresponds to an allowed h-BN phonon frequency and that 0° ≤ △θ ≤ 90° when △E exceeds the maximum allowed h-BN phonon frequency. Consequently, a two-level Huang-Rhys model succeeds at describing one-phonon processes but fails at describing excitations requiring multiple phonons. We propose that direct excitations requiring multiple phonons are inefficient in h-BN and that these ZPLs are excited indirectly via an intermediate electronic state. This hypothesis is corroborated by polarization that indicate a single ZPL may be excited via multiple mechanisms.