Large second-order susceptibility from a quantized indium tin oxide monolayer

Due to their high optical transparency and electrical conductivity, indium tin oxide thin films are a promising material for photonic circuit design and applications. However, their weak optical nonlinearity has been a substantial barrier to nonlinear signal processing applications. In this study, we show that an atomically thin (~1.5 nm) indium tin oxide film in the form of an air/indium tin oxide/SiO₂ quantum well exhibits a second-order susceptibility χ² of ~1,800 pm V^-1. First-principles calculations and quantum electrostatic modelling point to an electronic interband transition resonance in the asymmetric potential energy of the quantum well as the reason for this large χ² value. As the χ² value is more than 20 times higher than that of the traditional nonlinear LiNbO₃ crystal, our indium tin oxide quantum well design can be an important step towards nonlinear photonic circuit applications.