Speaker
Description
We present an experimental and theoretical study of second harmonic generation (SHG) in an ITO/gold bilayer with emphasis on the role of interfacial effective mass and charge discontinuities. First, we measure the SHG response of individual components- ITO and gold nanolayers- separately, and as expected, the SH signal is triggered by Coulomb, magnetic Lorentz and convective sources. In ITO the response is strongest near the epsilon near zero (ENZ) regime. The linear transmission through the ITO layer displays a minimum near the ENZ crossing point, (fig. 1a) while gold (fig. 1b, red curve) reflects like a mirror. However, the bilayer behaves in an unusual manner, by showing a resonance in linear reflection at a shifted ENZ wavelength (fig. 1b, blue curve). The nonlinear response of the bilayer is also unsual. The measured reflected SHG spectrum (fig. 1c, markers only curve) shows two distinct features: a small wavelength bump and plateau region, and a pronounced peak at longer wavelengths for a fixed incidence angle. To explain this behavior,- we use a hydrodynamic-Maxwell model for the coupled bilayer system. The model suggests that at shorter wavelengths the response is dominated by gold-related surface (Coulomb and convective) and Lorentz contributions, while at longer wavelengths the identification of SH sources is more subtle: near the ENZ crossing point a discontinuity in both effective mass and free electron density at ITO-gold interface triggers nonlinear convective sources that enhance the overall efficiency. Mass and density discontinuities are pivotal and establish angle-resolved SHG as a sensitive probe of interface-driven nonlinearities in conductive-oxide and metal-based nanophotonic systems.