Orbital Hybridization Activating Two‐Photon Absorption in Cu‐Doped BiOI

ABSTRACT Materials exhibiting strong reverse saturable absorption (RSA) characteristics are of critical importance for advanced photonic applications, while developing a convenient approach for high‐performance RSA materials remains a big challenge. In this study, Cu‐doped BiOI was synthesized to investigate the effects of doping duration, copper valence states, orbital hybridization, and band structure modifications on the nonlinear optical absorption behavior of BiOI. Notably, Cu‐doped BiOI demonstrated pronounced RSA behavior under 800 nm laser excitation, with a nonlinear absorption coefficient β eff of (6.8 ± 0.3) × 10 3 cm·GW −1 , whereas the pristine BiOI exhibited negligible nonlinear optical response. Comprehensive structural and spectroscopic analyses revealed that Cu 2 + ions substitute for Bi 3+ sites in the BiOI lattice. The partially filled 3 d orbitals of Cu 2+ hybridize with the O 2 p orbitals, generating new p ‐ d hybridized states near the valence band maximum. This orbital reconstruction modifies the electronic wavefunction character and enhances the dipole matrix elements associated with the two‐photon absorption process, thereby significantly improving the nonlinear optical response. These findings provide a viable strategy for enhancing the nonlinear optical performance of materials with similar electronic structures and offer critical insight into energy band engineering via metal‐ion doping.