In Situ Nonlinear Optical Absorption Response during Electrochemically Controlled ReS₂ Surface Oxidation

Oxidation-mediated surface functionalization of two-dimensional semiconductors provides an effective method to precisely tune electronic and optical properties, garnering significant interest across electronics and photonics. However, the lack of an in situ nonlinear optical characterization technique significantly limits the in-depth exploration of oxidation-dependent nonlinear optical properties. Herein, we developed an electrochemical method to selectively control the surface oxidation process of ReS₂. Furthermore, we integrate this electrochemical oxidation method with a Z-scan setup, establishing an in situ electrochemical Z-scan system, to detect the nonlinear absorption conversion from two-photon absorption to saturable absorption with increasing ReS₂ surface oxidation. This transition is primarily due to the two-photon absorption reduction of ReS₂ and the saturable absorption enhancement of ReO₃, which is demonstrated by theoretical calculations of band alignment and density of states. Our work develops a simple and nondestructive in situ electrochemical Z-scan technique to control surface oxidation and real-time monitor oxidation-mediated nonlinear optical properties.