Exciton Self‐Trapping Effect in MoSi₂N₄ for Modulating Nonlinear Optical Process

Abstract Coupling between polarized particles (excitons, phonons, and plasmas) in low‐dimension semiconductors can produce rich correlated states by exchange of energy and momentum, thus triggering many attractive electronic and optical applications if these correlated states can be controlled and modulated. Here, the broadband self‐trapped excitons (STEs) states covering visible to optical communication band are first demonstrated in the Mo‐vacancy‐rich MoSi₂N₄ films. A rapid establishment of STEs states (≈1–3 ps) is verified with the assistance of Mo‐vacancy induced defect levels. The modulation effect of STEs states on nonlinear optical processes is first verified, thereby yielding a switched saturable absorption (SA) behavior, which is novel and has not been reported in STEs‐related work and materials. Using Z‐scan technique, a switch of SA and reverse SA is achieved around optical density of 6 GW cm⁻², obtaining a large saturated two‐photon absorption coefficient (β_eff) of −8.66 × 10³ cm GW⁻¹ and a large unsaturated three‐photon absorption coefficient (γ) of 4.91 × 10² cm³ GW⁻², respectively. The findings suggest that MoSi₂N₄ is a promising candidate for constructing novel STEs‐related devices and integrated nonlinear optics platform, and give rise to new horizons for practical implementations of two‐dimensional MoSi₂N₄ family.