Plano-concave microlenses with epsilon-near-zero surface-relief coatings for efficient shaping of nonparaxial optical beams

Epsilon-near-zero (ENZ) materials, including artificial metamaterials, have been advanced to mold laser beams and antenna-mediated radiated waves. Here we propose an efficient method to control Ohmic losses inherent to natural ENZ materials by the assembly of subwavelength structures in a nonperiodic matrix constituting an ENZ metacoating. Implemented over plano-concave transparent substrates whose radius can be of only a few wavelengths, ENZ surface-relief elements demonstrate to adequately shape a plane wave into highly localized fields. Furthermore, our proposal provides an energy efficiency even higher than an ideally-lossless all-ENZ plano-concave lens. Our procedure is satisfactory to generate aberration-free nonparaxial focused waves and accelerating beams in miniaturized spaces.