Fröhlich-Type Polarons in isotopically enriched Hexagonal Boron Nitride

Exciton–phonon coupling governs the optical response of hexagonal boron nitride (hBN), yet direct experimental quantification has been lacking. Here we investigate isotopically enriched hBN (¹⁰B) using low temperature cathodoluminescence, focusing on the indirect exciton at 5.955 eV and its longitudinal optical phonon replica detuned by approximately 178 meV. From this pair we extract a Fröhlich coupling constant of 0.352, an exciton energy of 315 meV, and a polaron radius larger than the lattice constant, consistent with large polaron formation. The exciton scattering time (~58 fs) corresponds to a homogeneous linewidth of approximately 11.3 meV, compared with the measured 32 meV linewidth dominated by inhomogeneous broadening. These results provide direct quantitative insight into exciton–phonon interactions in isotopically engineered hBN, establishing a foundation for tailoring its optical response in quantum photonics.