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/SiO quantum well exhibits a second-order susceptibility χ of ~1,800 pm V. 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 χ value. As the χ value is more than 20 times higher than that of the traditional nonlinear LiNbO crystal, our indium tin oxide quantum well design can be an important step towards nonlinear photonic circuit applications.