Invertible Optical Nonlinearity in Epsilon-near-zero Materials*

Epsilon-near-zero (ENZ) materials have attracted great interest recently for their astonishing optical properties, especially the potential in realizing large nonlinearities. Indium tin oxide(ITO), as one of the most popular ENZ materials, is known for its mature fabrication technology and easy tunability of plasma frequency. In the talk, we present our exploration on the origin of a large, spectrally-invertible nonlinearity in ITO thin films. We investigate the two underlying competing factors, Fermi energy shift and band nonparabolicity. We propose a theoretical model including both interband and intraband transitions in ITO during nonlinear generation. It is confirmed by femtosecond pump-probe experiments on a series of ITO films with various ENZ wavelengths under UV/NIR excitations. Typically, the spectral inversion of nonlinearity based on this competing mechanism is observed for the first time. Our work provides a useful insight of tailoring the intrinsic material properties of ENZ materials to create a large optical nonlinearity.