Abstract

Materials with a linear refractive index approaching zero, such as epsilon-near-zero (ENZ) materials, can exhibit very large nonlinear effects, the description of which leads to the violation of standard approximations in nonlinear optics. We generalize the theoretical description of the intensity-dependent index of refraction in this context. We apply our treatment to measurements of refraction in indium tin oxide (ITO) and find that the nonlinear contributions to refraction exceed the linear term; we thus conclude that the nonlinear response can no longer be interpreted as a perturbation in this class of materials.

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