Dynamic Control of Third‐Harmonic Generation in Coupled ENZ‐Dielectric Metasurfaces

We present a time‐varying hybrid metasurface composed of an ultrathin 20 nm indium tin oxide (ITO) film integrated with a silicon (Si) nanodisk array on a glass substrate. Under femtosecond optical pumping, the complex refractive index of the ITO layer is dynamically tuned by non‐equilibrium carrier heating and subsequent lattice relaxation, which are described by a wavelength‐dependent two‐temperature model (TTM). The ultrafast modulation enables the extraction of dispersive third‐order nonlinear coefficients, with the effective third‐order susceptibility reaching the order of 10 −19 –10 −18 m 2 /V 2 in the epsilon‐near‐zero (ENZ) spectral region. The ENZ response of the ITO layer hybridizes with the Mie‐type resonance of the Si nanodisks, leading to a strong, delay‐dependent redistribution of electromagnetic energy within the metasurface. At a fixed pump fluence, the time‐evolving electron temperature induces dynamic modulation of the linear transmission as well as the nonlinear response. As a result, the third‐harmonic generation (THG) is transiently enhanced, accompanied by a notable spectral broadening of the harmonic signal. These results demonstrate ENZ‐dielectric hybridization as an effective strategy for achieving ultrafast and tunable enhancement of nonlinear optical processes, providing a promising platform for dynamic nanophotonic and frequency‐conversion applications in deeply subwavelength structures.