Fig. 3

Nonlinear absorption-induced PL in perovskite NCs. a Photoluminescence intensity of CsPbBr₃ NCs as a function of the below-band-gap excitation wavelength. The PL intensity is normalized by the laser fluence (see Supplementary Fig. 2). The dashed line is a guide to the eye. The inset shows the normalized photoluminescence spectrum for excitation wavelengths varying from 870 to 700 nm (specifically: 870, 850, 820, 800, 790, 770, 750, 720 and 700 nm). b Integrated photoluminescence intensity as a function of the laser excitation intensity for excitation wavelengths λ₄ (700 nm), λ₃ (790 nm) and λ₂ (1030 nm). The solid lines represent the respective fit with power functions, from which we can deduce the order of the process: 4 for λ₄ excitation, 3 for λ₃ excitation and 2 for λ₂ excitation. c Order of the absorption processes as a function of the below-band-gap excitation wavelength