Nonlinear Absorption and Low-threshold Two-photon Pumped Amplified Stimulated Emission from FAPbBr₃ Nanocrystals

Formamidinium lead bromide（FAPbBr₃）nanocrystals（NCs）have been considered to be a good optoelectronic material due to their pure green emission，excellent stability and superior carrier transport characteristics. However，two-photon pumped amplified spontaneous emission（ASE）and the corresponding nonlinear optical properties of FAPbBr₃ NCs are scarcely revealed. Herein，we synthesized colloidal FAPbBr₃ NCs with different sizes by changing the molar ratio of FABr/PbBr₂ in the precursor solution，using ligand assisted precipitation（LARP）technology at room temperature. Photoluminescence（PL）and time resolved photoluminescence（TRPL）spectroscopy were measured to characterize their ASE properties. And their nonlinear optical properties were studied through the Z-scan technique and the two-photon excited fluorescence method. The stimulated emission properties including one- and two-photon pumped ASE have been realized from FAPbBr₃ NCs. With large two-photon absorption coefficient （0. 27 cm/GW）and high non-linear absorption cross-section（7. 52×10⁵ GM），ASE with threshold as low as 9. 8 μJ/ cm² and 487 μJ/cm² have been obtained from colloidal FAPbBr₃ NCs using one- and two-photon excitations. These results indicate that as a new possible green-emitting frequency-upconversion material with low thresholds，FAPbBr₃ NCs hold great potential in the development of high-performance two-photon pump lasers.