Rate Equation Model of Boron Vacancy Photophysics
Andrew Beling
University of Maryland, College Park, Maryland, USA
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The negatively charged boron vacancy (VB–) is amongst the most studied defects in solid-state materials for quantum sensing and remains the only defect in hexagonal boron nitride (hBN) with a definitively known chemical structure. Despite this, many questions remain unanswered regarding the photophysics, transition rates, and photoionization behavior of VB–. Resolution of these discrepancies is critical to explaining phenomena such as the observed high ODMR contrast, low brightness, and potential for photonic enhancement. Several efforts to measure the four intersystem crossing transition rates by fitting to a rate equation model have yielded conflicting results, which also disagree with DFT (Density Functional Theory) predictions by several orders of magnitude. Here we describe efforts to probe fast VB– photophysics with a nanosecond rise-time laser, which can produce more accurate measurements of parameters than AOM (acousto-optic modulator) pulses. We fit time-resolved PL data from thermal and polarized spin distributions of VB– to a 9-level energy model to quantify the power dependence of the optical pumping rate and ground-state polarization. We demonstrate power dependent charge-state conversion, and comment on the degree of agreement between the data and the model. Finally, we investigate the effect of the radiative decay rate and optical pumping rise time on both the absolute brightness and PL shape predicted by the model. These results not only place constraints on several transition rates but also clarify open questions in the VB– literature.
Email: abeling@umd.edu