Defect-Dependent Nonlinear Absorption in the Lead-Free Double-Perovskite CsAgBiBr.

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 CsAgBiBr. Specifically, a thin film of pristine CsAgBiBr exhibits strong reverse saturable absorption (RSA), while a film of CsAgBiBr with defects (denoted as CsAgBiBr(D)) shows saturable absorption (SA). The nonlinear absorption coefficients are ca. 4.0 × 10 cm GW (515 nm laser excitation) and 2.6 × 10 cm GW (800 nm laser excitation) for CsAgBiBr and -2.0 × 10 cm GW (515 nm laser excitation) and -7.1 × 10 cm GW (800 nm laser excitation) for CsAgBiBr(D). The optical limiting threshold of CsAgBiBr is 8.1 × 10 J cm (515 nm laser excitation). The samples show excellent long-term performance stability in air. The RSA of pristine CsAgBiBr 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 CsAgBiBr(D) strengthen the ground-state depletion and Pauli blocking, resulting in SA.