Defect-Dependent Nonlinear Absorption in the Lead-Free Double-Perovskite Cs₂AgBiBr₆

Lead-free perovskites have attracted increasing attention because they can address the toxicity and instability problems inherent to lead-halide perovskites. Furthermore, the nonlinear optical (NLO) properties of lead-free perovskites are rarely explored. Herein, we report significant NLO responses and defect-dependent NLO behavior of Cs₂AgBiBr₆. Specifically, a thin film of pristine Cs₂AgBiBr₆ exhibits strong reverse saturable absorption (RSA), while a film of Cs₂AgBiBr₆ with defects (denoted as Cs₂AgBiBr₆(D)) shows saturable absorption (SA). The nonlinear absorption coefficients are ca. 4.0 × 10⁴ cm GW^-1 (515 nm laser excitation) and 2.6 × 10⁴ cm GW^-1 (800 nm laser excitation) for Cs₂AgBiBr₆ and -2.0 × 10⁴ cm GW^-1 (515 nm laser excitation) and -7.1 × 10³ cm GW^-1 (800 nm laser excitation) for Cs₂AgBiBr₆(D). The optical limiting threshold of Cs₂AgBiBr₆ is 8.1 × 10^-4 J cm^-2 (515 nm laser excitation). The samples show excellent long-term performance stability in air. The RSA of pristine Cs₂AgBiBr₆ correlates with excited-state absorption (515 nm laser excitation) and excited-state absorption following two-photon absorption (800 nm laser excitation), while the defects in Cs₂AgBiBr₆(D) strengthen the ground-state depletion and Pauli blocking, resulting in SA.