A composite material (102) comprising a dielectric material and a plurality of non-overlapping local resonant cell groups (106) disposed across the dielectric material is described. Each local resonant cell group (106) comprises a plurality of resonant cells (108) that are small relative to a first wavelength (Lambada c) of electromagnetic radiation that is incident upon the composite material (102). Each local resonant cell group (106) has a spatial extent that is not larger than an order of the first wavelength. For each of the local resonant cell groups, the resonant cells (108) therein are chirped with respect to at least one geometric feature thereof such that a plurality of different subsets of the resonant cells resonate for a respective plurality of wavelengths in a spectral neighborhood (203) of the first wavelength (Lambada c). The composite material (102) exhibits at least one of a negative effective permeability and a negative effective permittivity for each of the plurality of wavelengths in that spectral neighborhood (203).