Laser direct writing of nonlinear metasurfaces on phase-change material Ge₂Sb₂Te₅ for enhanced third harmonic generation

Nonlinear metasurfaces enable the precise nanoscale control of light–matter interactions and provide a promising route toward compact and multifunctional nonlinear optical devices by relaxing conventional phase-matching constraints. However, most reported nonlinear metasurfaces rely on materials with large intrinsic nonlinear coefficients, and their fabrication typically relies on complex and costly electron-beam lithography, raising the experimental barrier for nonlinear nanophotonics. A nonlinear metasurface fabricated by laser direct writing on phase-change material Ge₂Sb₂Te₅(GST) is presented in this study. Experimental results show that the structured metasurface exhibits an approximately 13-fold improvement in third harmonic generation (THG) intensity relative to an unpatterned amorphous GST thin film. Notably, the observed THG enhancement is primarily dominated by multipole resonances supported by the metasurface, rather than being affected by laser-induced variations in the crystallization fraction. This work extends the laser direct writing fabrication to nonlinear photonics, providing a flexible approach toward laser-written nonlinear nanophotonic devices and indicating the potential for modulating nonlinear responses within a multipolar framework.