<i>In Situ</i> 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 <i>in situ</i> 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 <i>in situ</i> 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 <i>in situ</i> electrochemical Z-scan technique to control surface oxidation and real-time monitor oxidation-mediated nonlinear optical properties.