Optical absorption and emission mechanisms of single defects in hexagonal boron nitride

TL;DR

This study examines the polarization properties of defect-related zero-phonon lines in hexagonal boron nitride, revealing limitations of the two-level model and suggesting indirect excitation mechanisms for certain optical transitions.

Contribution

It demonstrates that a simple two-level configuration coordinate model cannot fully explain the excitation mechanisms of ZPLs in h-BN, proposing the involvement of intermediate states for multi-phonon processes.

Findings

Dipole misalignment correlates with Stokes shift.

Two-level model explains single-phonon excitation but not multi-phonon processes.

Multi-phonon excitations are likely mediated by intermediate states.

Abstract

We investigate the polarization selection rules of sharp zero-phonon lines (ZPLs) from isolated defects in hexagonal boron nitride (h-BN) and compare our findings with the predictions of a configuration coordinate model involving two electronic states. Our survey, which spans the spectral range ~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 (${\Delta}{\theta}$) to the ZPL Stokes shift (${\Delta}E$) and find that ${\Delta}{\theta}\sim 0{\deg}$ when ${\Delta}E$ corresponds to an allowed h-BN phonon frequency and that $0{\deg}\leq{\Delta}{\theta}\leq 90{\deg}$ when ${\Delta}E$ exceeds the maximum allowed h-BN phonon frequency. Consequently, a two-level configuration coordinate 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 h-BN 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 h-BN.