Efficient nonlinear optical interactions are essential for many applications in modern photonics. However, they typically require intense laser sources and long interaction lengths, requirements that often render nonlinear optics incompatible with new nanophotonic architectures in integrated optics and metasurface devices. Obtaining materials with stronger nonlinear properties is a crucial step towards applications that require lower powers and smaller footprints. Recently, a new class of materials with a vanishing permittivity, known as epsilon-near-zero (ENZ) materials, has been reported to exhibit unprecedented ultrafast nonlinear efficiencies within sub-wavelength propagation lengths. In this Review, we survey the work that has been performed on ENZ materials and the related near-zero-index materials, focusing on the observation of various nonlinear phenomena (such as intensity-dependent refraction, four-wave mixing and harmonic generation), the identification of unique field-enhancement mechanisms and the study of non-equilibrium dynamics. Degenerately doped semiconductors (such as tin-doped indium oxide and aluminium-doped zinc oxide) are particularly promising candidates for ENZ-enhanced nonlinear optical applications. We conclude by pointing towards possible future research directions, such as the search for ENZ materials with low optical losses and the elucidation of the mechanisms underlying nonlinear enhancements.
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The authors acknowledge support through the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canada Research Chairs programme and the Canada First Research Excellence Fund. O.R. acknowledges the support of the Banting Postdoctoral Fellowship of the NSERC. I.D.L. acknowledges support from CONACyT (Ciencia Básica) grant no. 286150. R.W.B. also acknowledges support from the Defense Advanced Research Projects Agency (DARPA) Defense Sciences Office (DSO) Nascent programme and from the US Army Research Office.
The authors declare that they have no competing interests.
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Reshef, O., De Leon, I., Alam, M.Z. et al. Nonlinear optical effects in epsilon-near-zero media. Nat Rev Mater 4, 535–551 (2019). https://doi.org/10.1038/s41578-019-0120-5
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