Cubic boron nitride (cBN) is a wide-bandgap polymorph of boron nitride whose optical response remains only partially understood due to the coexistence of indirect electronic transitions and strong exciton-phonon coupling. Using first-principles many-body perturbation theory, we investigate the optical properties of cBN by combining GW quasiparticle corrections with Bethe-Salpeter equation calculations of excitonic effects. Phonon-assisted absorption and emission processes are explicitly included through the exciton-phonon coupling formalism. We find that phonon-mediated optical transitions provide a dominant contribution to both absorption and luminescence spectra, partially reconciling the discrepancy between the theoretical optical gap (≃ 11 eV) and experimental emission around 6-7 eV. Our results demonstrate the importance of including exciton-phonon interactions for the correct interpretation of experimental spectra, offering new insights into light emission in wide-bandgap materials.
@article{arxiv.2602.18259,
title = {Phonon assisted light absorption and emission in cubic-Boron Nitride},
author = {Ashwin Pillai and Elena Cannuccia and Aurelien Manchon and Fulvio Paleari and Claudio Attaccalite},
journal= {arXiv preprint arXiv:2602.18259},
year = {2026}
}