All-Optical Writing and Erasing of Second-Harmonic Generation in MoS₂ for Reconfigurable Photonics

The development of reconfigurable photonic devices demands dynamic, non-invasive control of nonlinear optical processes at the nanoscale. While second-harmonic generation (SHG) in 2D materials (e.g., MoS₂) is a promising candidate, its active modulation remains challenging. Here, we report an all-optical strategy for on-demand writing and erasing of SHG in few-layer MoS₂. By alternating ultraviolet-ozone treatment and continuous 532 nm laser irradiation, we achieve fully reversible, cyclic modulation of the SHG intensity exceeding 80-fold. Our comprehensive spectroscopic and microscopic characterizations indicate that the tunability arises from the metastable formation and removal of surface S─O bonds, which transiently break lattice inversion symmetry. Leveraging this reversible control, we demonstrate spatially selective patterning of SHG, enabling the writing, erasing, and rewriting of optical information on a single MoS₂ flake. This work establishes a simple, non-destructive platform for reconfigurable nonlinear photonics, with direct implications for optical memory, logic, and encryption technologies.