Giant Second Harmonic Generation by Photoinduced Phase Engineering

Addressing the growing demand for ever-shrinking nonlinear optical devices requires a paradigm shift to two-dimensional (2D) materials to bypass phase-matching limitations encountered in bulk crystals. While strategies like strain, electrical gating, and surface decoration have been explored to enhance the nonlinear susceptibility of 2D materials, the potential of laser-driven effects, with its exceptional controllability and accessibility, remains largely underexplored. Here, we demonstrate an optical approach to achieve a giant second-order parametric response in a metastable noncentrosymmetric 1M-WS₂ by selectively controlling laser pulse duration. We report a record effective second harmonic generation coefficient of ∼2000 pm/V, surpassing typical values of conventional nonlinear crystals by several orders of magnitude. Detailed analyses reveal that the structural distortions unique to the 1M phase are crucial for the enhanced nonlinear response. These findings establish an alternative way to amplify nonlinear optical properties and set 1M-WS₂ as a promising candidate for compact optical modulators and nanolasers.