Lead-Free Cs₂AgBiBr₆ Nanocrystals with a Low Mode-Locking Threshold for Femtosecond Fiber Laser Application

Lead-free double perovskites (DPs) have received a lot of attention owing to their good stability and environmental friendliness. They have been widely used in the field of optoelectronics because of their high quantum efficiency and long carrier migration distance. However, the application of lead-free DPs in ultrafast photonics is rarely explored. Cs 2 AgBiBr 6, a typical lead-free double perovskite, has advantages such as toxicity free of Pb, high stability, and strong nonlinear absorption, showing great potential application in ultrafast photonics. However, to date, there is no reported research on the application of Cs 2 AgBiBr 6 in ultrafast optical devices. Herein, we prepared Cs 2 AgBiBr 6 nanocrystals, tested their ultrafast nonlinear absorption properties, and further prepared them as a saturable absorber (SA) to achieve stable mode-locking pulses in Er-doped fiber lasers. The results show that it has a low saturation intensity (0.75 MW/cm 2 ), resulting in a low starting threshold power (50 mW) for mode-locking. The obtained pulsed fiber laser shows a pulse width as narrow as 452 fs. In addition, the Cs 2 AgBiBr 6 SA has unique advantages in long-term stability and it was found that this Cs 2 AgBiBr 6 SA can still work well after 10 months. In general, our results prove that Cs 2 AgBiBr 6 is an excellent nonlinear optical material and has good application potential in ultrafast photonics.