“…In the context of this model, various non-linear permittivities are studied: cubic (Kerr law) [5-7, 12, 13, 24, 26, 38-41], fifth-(quintic), seventh-(septic) orders [17,31,[42][43][44][45][46], and, more generally, polynomial and power nonlinearity with a positive non-integer degree. In recent years, non-linear properties of modern materials, like metamaterials [34,[47][48][49] and graphene [50,51], have been studied as well.…”