Large Nonlinear Optical Response and Slowed Electron‐Phonon Scattering in a Kagome‐type Flat Band Semiconductor

Abstract Electronic flat bands can result in exotic physical behaviors from ferromagnetism to superconductivity, while the role played by the high density of states associated with the flat band in strong light‐matter interactions remains unexplored. Here, the observation of a large nonlinear optical (NLO) response due to dense photon‐induced carriers in a Kagome‐type flat band semiconductor (Pd 3 P 2 (S 1‐x Se x ) 8 ) at photon energies from E g to 0.67 E g ( E g : bandgap energy) is reported. More importantly, by femtosecond ultrafast spectroscopy It is revealed that the relaxation rate of photo‐generated hot electrons that contribute to the NLO response is slowed down due to the flat electronic band that can be tuned by the S/Se ratio. This observation is interpreted by the enhanced electron heat capacity and the suppressed electron‐phonon coupling as a result of the increase in band flatness and electron (hole) effective mass. The results can have implications for a better understanding of flat band physics from a strong light‐matter interaction perspective.