Sub-picosecond Magneto-optical Response Probed by Kerr Rotation Technique in PbI₂ Film

The ultrafast optical dynamics in solution-processed ${\rm PbI}_2$ film on quartz substrate around the band edge have been systematically studied by transient reflectance spectroscopy and femtosecond resolved Kerr rotation spectroscopy at room temperature. The Kerr rotation angle shows a maximum at the excitonic photobleaching peak and increases with the pump fluence until reaching a saturation value of ${\sim}{0.8}\;{\rm deg}$ (${\sim}{4}\;{\rm deg}/\unicode{x00B5}{\rm m}$), which is on the same order as that for ${\rm CH}_3{\rm NH}_3{\rm PbI}_3$ perovskite film. The spin lifetime gradually decreases from 2.24 to 0.48 ps when increasing the pump fluence from 13 to ${234}\;\unicode{x00B5} {{{\rm J}/{\rm cm}}^2}$, which is attributed to the strong spin-orbital coupling and the dominance of the Elliot–Yafet process in spin relaxation. Our findings suggest that ${\rm PbI}_2$ film is a promising candidate for ultrafast spin switching with a subpicosecond time scale and optical isolator in optical communications owing to the giant rotation angle.