Invertible Optical Nonlinearity in Epsilon-near-zero Materials

TL;DR

This paper investigates the origins of optical nonlinearities in epsilon-near-zero ITO materials, revealing how intraband and interband transitions, along with material properties, contribute to their unique nonlinear behavior.

Contribution

It provides a combined theoretical and experimental analysis of nonlinear mechanisms in ITO, highlighting the roles of band structure and Fermi energy shifts near the ENZ region.

Findings

Nonlinearities are dominated by intraband and interband transitions.

Spectrally-invertible nonlinearity arises from band non-parabolicity and Fermi energy shifts.

Large nonlinear response is linked to intrinsic material properties of ITO.

Abstract

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 thin films that are dominated by two mechanisms based on 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 non-parabolicity of the band structure that results in a larger effective mass in the intraband transition and the Fermi energy shift, which determines the free carrier density. Our work reveals the relationship between the large nonlinearity and the intrinsic material properties of the ITO films.