Large second-order nonlinearity in asymmetric metallic quantum wells

Investigation of new plasmonic material platforms with large optical nonlinearity is crucial for the continued development of nonlinear optics and its applications. Here we report an enhanced second order nonlinear effect in metallic quantum wells (QWs) where the intersubband transition plays a dominant role. Centrosymmetry in these metallic QWs is broken by forming multilayers with chemically and structurally distinct barrier oxides above and below a metal nanofilm. For Au-based QWs, we show that a large χ(2), measured by second harmonic generation (SHG), around 229.6 pm/V at the near infrared (NIR) wavelength of 940 nm was achieved in an asymmetric metallic QW of SiO2|Au|HfO2 on a fused silica substrate.