Invertible optical nonlinearity in epsilon-near-zero materials

Epsilon-near-zero (ENZ) materials such as indium tin oxide (ITO) have recently emerged as a new platform to enhance optical nonlinearities. Here we report a theoretical and experimental study on the origin of nonlinearities in ITO films that are dominated by intraband and interband transitions. We show that there are two competing factors that jointly contribute to a spectrally invertible nonlinearity of ITO near its ENZ region, i.e., the nonparabolicity of the bands that results in a larger effective mass, and the Fermi energy shift that determines the free carrier density. Our work reveals the relationship between the large nonlinearity and the intrinsic material properties of the ITO films, which will enable design and development of photonic materials and nonlinear devices made of transparent conductive oxides.