In Situ Tracking of Saturation Dynamics in Nonlinear Metasurfaces

We present a novel experimental approach to investigate power-dependent nonlinearities in metasurfaces. By monitoring a spectrally distinct, nonpump resonance during SHG excitation, our method enables in situ observation of resonance evolution without interference from the intense pump. This technique allows direct correlation of nonlinear emission with real-time resonance changes, clearly distinguishing between reversible resonance shifts and irreversible structural damage. Combined with numerical simulations incorporating measured refractive index variations, we reveal how resonance detuning governs nonlinear saturation dynamics. Our approach provides a general strategy for probing dynamic nonlinear processes in high-quality metasurfaces and offers actionable insights for designing robust, power-tolerant nonlinear photonic devices, advancing the development of ultracompact SHG sources and integrated nonlinear nanophotonic technologies.