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

ABSTRACT 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) 4 AgBiBr 8 —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) 4 AgBiBr 8 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.