Site-Selective Transition-Metal Doping Enables Enhanced Nonlinear Optical Absorption in Two-Dimensional Lead-Free Double Perovskites

Achieving high-performance nonlinear optical (NLO) absorption in lead-free perovskites remains a major challenge for advanced photonic applications. Here, we demonstrate that transition-metal doping in two-dimensional lead-free double perovskite (BA)₄AgBiBr₈—where Fe and Cu selectively substitute Bi and Ag sites, respectively—serves as an effective strategy to boost NLO responses. Structural, spectroscopic, and theoretical analyses demonstrate the site-selective doping behavior in (BA)₄AgBiBr₈ and the introduction of distinct defect states within the bandgap, which provide abundant intermediate levels for excited-state absorption under 800 nm femtosecond excitation. These defect-mediated transitions significantly amplify the NLO response, yielding over a twofold increase in the nonlinear absorption coefficient and modulation depths of ∼70–75% at ultralow pulse energy (0.7 nJ). This study establishes site-specific defect engineering as a general approach for designing high-performance, environmentally benign NLO materials.