Ultrafast Exciton and Spin Dynamics of Monolayer MoSi ₂ N ₄ Studied by Non‐Degenerate Pump‐Probe Transient Transmission Spectroscopy

2D materials have attracted numerous attention for their potential applications in nano-photoelectronic and valleytronic devices. Recently, a new 2D material, MoSi₂N₄ monolayer, is synthesized and reported, and predicted to have many unique properties. Here, its ultrafast photoelectron and spin dynamics using femtosecond-resolved transient differential transmission and Faraday rotation spectroscopies are investigated. Complex and diverse ultrafast dynamics of excitons are observed with increasing probe wavelength from 510 to 640 nm, including fully positive and negative monotonic decaying dynamics as well as an initial positive (negative) peak followed by slow negative (positive) recovery dynamics, and are explained well based on the band structure of MoSi₂N₄ monolayer containing a deep defect level locating above the midpoint of direct bandgap at K valley. Spin polarization of A and B excitons is found controlled by the circular helicity of exciting light. The spin relaxation lifetime of B excitons is determined as ≈0.73 ps.