Quantifying Spin Defect Density in hBN via Raman and Photoluminescence Analysis

<jats:title>Abstract</jats:title> <jats:p> Negatively charged boron vacancies () in hexagonal boron nitride (hBN) are emerging as promising solid‐state spin qubits due to their optical accessibility, structural simplicity, and compatibility with photonic platforms. However, quantifying the density of such defects in thin hBN flakes has remained elusive, limiting progress in device integration and reproducibility. Here, an all‐optical method is presented to quantify defect density in hBN by correlating Raman and photoluminescence (PL) signatures with irradiation fluence. Two defect‐induced Raman modes, D1 and D2, are identified and assigned them to vibrational modes of using polarization‐resolved Raman measurements and density functional theory (DFT) calculations. By adapting a numerical model originally developed for graphene, an empirical relationship linking Raman (D1, <jats:italic>E</jats:italic> <jats:sub>2g</jats:sub> ) and PL intensities is established to absolute defect densities. This method is universally applicable across various irradiation types and uniquely suited for thin flakes, where conventional techniques fail. The approach enables accurate, direct, and non‐destructive quantification of spin defect densities down to 10 <jats:sup>15</jats:sup>  defects/cm <jats:sup>3</jats:sup> , offering a powerful tool for optimizing and benchmarking hBN for quantum optical applications. </jats:p>