Wang Gang, Liu Tanghao, Wang Bingzhe, Gu Hao, Wei Qi, Zhang Zhipeng, He Jun, Li Mingjie, Xing Guichuan
Joint Key Laboratory of the Ministry of Education, Institute of Applied Physics and Materials Engineering, University of Macau, Avenida da Universidade, Taipa, Macao, SAR 999078, China.
Department of Physics, Hong Kong Baptist University, 224 Waterloo Road, Kowloon, Hong Kong SAR, 999077, China.
Nat Commun. 2022 Nov 14;13(1):6935. doi: 10.1038/s41467-022-34705-8.
Controlling the high-power laser transmittance is built on the diverse manipulation of multiple nonlinear absorption (NLA) processes in the nonlinear optical (NLO) materials. According to standard saturable absorption (SA) and reverse saturable absorption (RSA) model adapted for traditional semiconductor materials, the coexistence of SA and RSA will result in SA induced transparency at low laser intensity, yet switch to RSA with pump fluence increasing. Here, we observed, in contrast, an unusual RSA to SA conversion in quasi-two-dimensional (2D) perovskite film with a low threshold around 2.6 GW cm. With ultrafast transient absorption (TA) spectra measurement, such abnormal NLA is attributed to the competition between excitonic absorption enhancement and non-thermalized carrier induced bleaching. TA singularity from non-thermalized "Fermi Sea" is observed in quasi-2D perovskite film, indicating an ultrafast carrier thermalization within 100 fs. Moreover, the comparative study between the 2D and 3D perovskites uncovers the crucial role of hot-carrier effect to tune the NLA response. The ultrafast carrier cooling of quasi-2D perovskite is pointed out as an important factor to realize such abnormal NLA conversion process. These results provide fresh insights into the NLA mechanisms in low-dimensional perovskites, which may pave a promising way to diversify the NLO material applications.
控制高功率激光透过率基于对非线性光学（NLO）材料中多种非线性吸收（NLA）过程的多样化操控。根据适用于传统半导体材料的标准饱和吸收（SA）和反饱和吸收（RSA）模型，SA和RSA共存会在低激光强度下导致SA诱导的透明性，但随着泵浦通量增加会切换到RSA。在此，我们观察到，与之相反，在阈值约为2.6 GW/cm²的准二维（2D）钙钛矿薄膜中存在一种不寻常的从RSA到SA的转换。通过超快瞬态吸收（TA）光谱测量，这种异常的NLA归因于激子吸收增强与非热载流子诱导漂白之间的竞争。在准二维钙钛矿薄膜中观察到来自非热“费米海”的TA奇点，表明在100 fs内载流子超快热化。此外，二维和三维钙钛矿之间的对比研究揭示了热载流子效应在调节NLA响应中的关键作用。准二维钙钛矿的超快载流子冷却被指出是实现这种异常NLA转换过程的一个重要因素。这些结果为低维钙钛矿中的NLA机制提供了新的见解，这可能为拓展NLO材料应用铺平一条有前景的道路。
