Ultrafast Charge Transfer‐Induced Unusual Nonlinear Optical Response in Monoclinic BiVO ₄ ‐SAM for Tunable Mid‐Infrared Ultrafast Fiber Laser

Surface terminations can fundamentally influence the nonlinear optical (NLO) properties of semiconductor materials by regulating exciton dissociation and surface‐state‐mediated charge transfer, yet their direct relationship between surface termination and NLO response has not been systematically investigated. In this work, monoclinic bismuth vanadate (BiVO 4 ) is synthesized using a coordination‐assisted solution route, and its carrier dynamics and tunable broadband NLO responses were systematically investigated through transient absorption (TA) and Z‐scan measurements. Open‐aperture Z‐scan measurements reveal that monoclinic BiVO 4 exhibits saturable absorption (SA) and reversible saturable absorption (RSA) at 532 nm with an effective two‐photon absorption (TPA) coefficient (β eff ) of 1.72 × 10 −10 m∙W −1 , while exhibiting multiphoton absorption with a β eff of 4.32 × 10 −6 cm∙W − 1 at 2.78 µm, attributed to higher‐order nonlinear carrier excitation processes. Our study first reports a new inhibition for watt‐level output power scaling enhancement, the transition from Q‐switched mode‐locked (QML) to continuous‐wave mode‐locked (CWML) due to RSA of the SA, and reveals that the optimal SA for power scaling enhancement should be characterized by a large effective modulation depth without RSA. Density functional theory (DFT)‐based density of states (DOS) calculations confirm the oxidation‐state‐dependent electronic structure underlying the enhanced NLO behavior. By employing a BiVO 4 ‐SAM with ≈ 43% modulation depth, a mid‐infrared fiber laser achieves output power up to 1.14 W with switchable pulse durations of 745 ns and 27 ps, respectively. The enhanced ultrafast carrier dynamics and large modulation depth enable high‐performance pulse switching with shorter pulse durations and highly scalable output power.