High‐Efficient Switchable Third Harmonic Generation on Thin Film Phase Change Materials

Optical harmonic generation is a cornerstone of nonlinear photonics, with broad applications in medicine, biology, photonics, and telecommunications. A central challenge in this field remains enhancing nonlinear signal efficiency and enabling dynamic intensity control. Here, high‐efficiency third harmonic generation (THG) with an efficiency of 10 −6 at 349 nm from a 20 nm‐thick phase‐change material film is demonstrated. The dependence of the intensity of the third harmonic on the film thickness and excitation wavelength has been shown experimentally and theoretically. On/off switching of the THG signal via reversible phase transitions in the phase‐change material is further demonstrated. Additionally, stable THG intensity modulation is achieved over multiple phase‐cycling iterations, highlighting the robustness of the proposed approach.