Defect‐Mediated Inelastic Light Scattering by Infrared Phonons in Hexagonal Boron Nitride

Phonons in hexagonal boron nitride (hBN) possess exceptionally strong oscillator strengths, giving rise to hyperbolic phonon polaritons and enabling robust phonon‐mediated nonlinear interactions. Defect states in hBN are known to interact strongly with lattice vibrations, yet the role of defects in enabling inelastic optical scattering through otherwise Raman‐inactive, infrared (IR)‐active phonon modes remains largely unexplored. Here, we demonstrate a highly efficient inelastic phonon scattering mechanism in which the general selection rules are violated, leading to a down‐converted laser emission mediated by IR‐active phonons in defect‐engineered hBN, via stimulated Raman scattering. We further show that the phonon landscape of hBN can be tailored through substrate choice, enabling multiple inelastic scattering processes and sequential down‐conversion of the excitation laser frequency. These results establish a general strategy for nonlinear defect–phonon scattering and tailoring the nonlinear‐optical selection rules in hBN.