Lead halide perovskites have been widely explored in the field of photovoltaics, light-emitting diodes, and lasers due to their outstanding linear and nonlinear optical (NLO) properties. But, the presence of lead toxicity and low chemical stability remain serious concerns. Lead-free double perovskite with excellent optical properties and chemical stability could be an alternative. However, proper examination of the NLO properties of such a material is crucial to identify their utility for future nonlinear device applications. Herein, we have made use of femtosecond (fs) Z-scan technique to explore the NLO properties of Cs₂AgIn_0.9Bi_0.1Cl₆ nanocrystals (NCs). Our measurements suggest that under nonresonant fs excitation, perovskite NCs exhibit strong two-photon absorption (TPA). The observed saturation of TPA at high light intensities has been explained by a customized model. Furthermore, we have demonstrated a change in the nonlinear refractive index of the NCs under varying input intensities. The strong TPA absorption of lead-free double perovskite NCs could be used for Kerr nonlinearity-based nonlinear applications such as optical shutters for picosecond lasers.

中文翻译：

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飞秒激发下无铅双钙钛矿纳米晶体中双光子吸收的三阶非线性光学特性和饱和度

卤化铅钙钛矿由于其出色的线性和非线性光学 (NLO) 特性，在光伏、发光二极管和激光器领域得到了广泛的探索。但是，铅毒性和低化学稳定性的存在仍然是严重的问题。具有优异光学性能和化学稳定性的无铅双钙钛矿可能是一种替代方案。然而，正确检查这种材料的 NLO 特性对于确定它们在未来非线性设备应用中的效用至关重要。在此，我们利用飞秒 (fs) Z 扫描技术来探索 Cs ₂ AgIn _0.9 Bi _0.1 Cl ₆的非线性光学特性纳米晶体 (NC)。我们的测量表明，在非共振 fs 激发下，钙钛矿 NC 表现出强烈的双光子吸收 (TPA)。在高光强度下观察到的 TPA 饱和已由定制模型解释。此外，我们已经证明了在不同输入强度下 NC 的非线性折射率会发生变化。无铅双钙钛矿 NC 的强 TPA 吸收可用于基于克尔非线性的非线性应用，例如皮秒激光器的光闸。