exceptional properties including high photostability, extremely long charge diffusion length, and thermal stability. The perovskites have become emerging high-performance materials for various optoelectronic applications such as solar cells, [1][2][3] light emitting diodes, [4,5] photodetectors, [6,7] and lasers. [8,9] The halide perovskites also exhibit strong nonlinear optical (NLO) responses, thus opening up possibilities for optoelectronic device applications. [10][11][12][13][14] For instance, organic-inorganic hybrid perovskites with large two-and three-photon absorption (2PA, 3PA) properties [14][15][16] can be utilized in applications such as bioimaging, [17,18] optical limiting, [19] optical sensor protection, [20] and optical data storage. [21] Nonlinear absorption properties of single crystal, nanocrystal, and filmed perovskites have been intensively studied. [10][11][12][13][14][15][16][22][23][24] Recently, lead halide perovskites such as MAPbI 3 (MA = CH 3 NH 3 ) and MAPbBr 3 have shown higher multiphoton absorption (MPA) probability as the excitation wavelength increases in the IR region. [10][11][12][13][14][22][23][24] Johnson et al. [25] reported that the observation of third-order nonlinearities and decay kinetics in MAPbBr 3 and MAPbBr 2 I with different halogen composition. They showed that the third-order nonlinearities of these materials are larger than those of conventional semiconductors. The photoluminescence of MAPbBr 3 single crystal has also been observed in the green region (522-550 nm) with 1044 nm femtosecond laser excitation due to 2PA. [26] Similar nonlinear optical properties were reported in inorganic perovskites, including CsPbI 3 and CsPbBr 3 . [22,27,28] Another type of perovskite, formamidinium lead iodide (FAPbI 3 ), has a low bandgap (1.47 eV) and greater photostability and thermal stability [29,30] than MAPbI 3 . However, FAPbI 3 has a weak structural stability due to humidity, which prevents it from real-life applications. [31] Recently, it has been demonstrated that the mixture of Cs and MAPbI 3 , CsMAPbI 3 , can lead to a stable solar cell with power conversion efficiency of ≈8%. [32] In addition, the mixture of Cs in FAPbI 3 , such as FA 0.9 Cs 0.1 PbI 3 , showed an improved thermal stability and results in high power conversion efficiency of ≈16.5%. [33] By using this material, Yi et al. have demonstrated power conversion efficiency up to 18% with a halide mixer I/Br. [34] The malleable association The versatility of perovskites through chemical composition modification is critical for enhancing performance of a range of potential applications such as light emitters and integrated photovoltaics. However, a systematic study on the nonlinear optical (NLO) responses of these perovskites is still lacking. Here, for the first time, the NLO properties of triple cation mixed halide perovskite (TCMHP) films, (Cs 0.06 FA 0.79 MA 0.15 )Pb (I 0.85 Br 0.15 ) 3 , using Z-scan and time-resolved spectroscopy, are investigated. It is found that TCMHP film shows strong saturable abs...