Nonlinear absorption and temperature-dependent fluorescence of perovskite FAPbBr nanocrystal.

Recent progress in solar cell and light-emitting devices makes halide perovskite a research hot spot in optics. In this Letter, the nonlinear absorption and fluorescence properties of FAPbBr nanocrystal, one typical organometallic halide perovskite, have been investigated via Z-scan measurements and a density-dependent photoluminescence (PL) spectrum. The FAPbBr nanocrystal exhibits nonlinear absorption under the excitation of 800 nm, whose photon energy is below the bandgap of FAPbBr. The significant absorption is experimentally confirmed to be induced by two-photon absorption (TPA), and the TPA coefficient is measured to be ∼0.0042  cm/GW. Moreover, the PL induced by TPA in FAPbBr nanocrystal shows different temperature-dependent behaviors in the range of 90 to 350 K. The peaks of the PL spectrum remain nearly constant at 100-160  K, with a very shallow trough at around 150 K, while a linear blue shift (0.496 meV/K) of the spectrum is observed when temperature is above 160 K. These temperature-dependent fluorescence behaviors can be ascribed to the structural phase transition at about 150 K and the contribution of thermal expansion. Moreover, the exciton binding energy around 160 meV and the optical phonon energy of 15.3 meV are also extracted from the temperature-dependent PL data.