Observation of polaronic state assisted sub-bandgap saturable absorption

In soft-lattice lead-halide perovskites, the polaronic effects involving stabilization of localized charge character by structural deformations and polarizations have an important effect on the properties of functional materials such as the band gap, which has attracted considerable investigations. However, the concept of polaron assisted nonlinear photonics remains largely unexplored, which has a wide range of applications from optoelectronics to telecommunications and quantum technologies. Here, we report the first observation of the polaronic state assisted saturable absorption through sub-bandgap excitation with a redshift exceeding 60 meV. By combining photoluminescence, transient absorption measurements and density functional theory calculations, we explicate that the anomalous nonlinear saturable absorption under sub-bandgap excitation is caused by the transient picosecond timescale polaronic state formed by strong carrier/exciton-phonon coupling effect. The bandgap fluctuation caused by polaron formation can be further tuned through exciton-phonon coupling of perovskites with different Young’s modulus. This suggests that we can design targeted soft lattice lead-halide perovskite with a specific structure to effectively manipulate exciton-phonon coupling and exciton-polaron formation. These findings profoundly expand our understanding of exciton-polaronic nonlinear optics physics and provide an ideal platform for developing actively tunable nonlinear photonics applications.