Optical Polarization of Quantum Emitters 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. In this talk, I will discuss our progress on identifying and characterizing isolated defects in the Van der Waals material hexagonal boron nitride (h-BN). First, I will briefly introduce confocal fluorescence measurements of isolated defects in h-BN that exhibit single photon emission when excited by sub-bandgap energy light. The temperature dependence of the zero-phonon, single-photon emission from these defects is well-described by a lattice vibration model that considers coupling to low energy, in-plane acoustic phonons. Next, I will discuss the temperature-independent polarization selection rules of these zero-phonon lines (ZPLs) and compare our findings with the predictions of a Huang-Rhys model involving two electronic states. Our survey, which spans the spectral range ~550-740 nm, reveals that, in disagreement with a two-electronic-level model, the absorption and emission dipoles are often misaligned. We relate the dipole misalignment angle (Δθ) of a ZPL to its energy shift from the excitation energy (ΔE) and find that Δθ≈0° when ΔE is less than the maximum single-phonon energy in h-BN and that 0≦Δθ≦90° when ΔE exceeds the maximum single-phonon energy in h-BN. This observation indicates that a two-level Huang-Rhys model succeeds at describing excitations mediated by one phonon but fails at describing excitations requiring multiple phonons. We propose that excitations requiring multiple phonons are inefficient in h-BN and that ZPLs with large ΔE are 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.