Wave propagation control at the deep subwavelength scale in metamaterials

The ability to control wave propagation is of fundamental interest in many areas of physics. Photonic crystals proved very useful for this purpose but, because they are based on Bragg interferences, these artificial media require structures with large dimensions. Metamaterials, on the other hand, can exhibit very deep subwavelength spatial scales. In general they are studied for their bulk effective properties that lead to effects such as negative refraction. Here we go beyond this effective medium paradigm and we use a microscopic approach to study metamaterials based on resonant unit cells. We show that we can tailor unit cells locally to shape the flow of waves at deep subwavelength scales. We validate our approach in experiments with both electromagnetic and acoustic waves in the metre range demonstrating cavities, waveguides, corners and splitters with centimetre-scale dimensions, an order of magnitude smaller than previous proposals. Photonic crystals efficiently control wave propagation on a wavelength scale, but this means they can become very large when long wavelengths are involved. Metamaterials made of resonant unit cells can confine and guide waves even at scales far below their wavelength.