Vectorial polarisation control with epsilon-near-zero metamaterials

Abstract

We report a theoretical and experimental study of tightly focused vectorial vortex beams propagating through a strongly anisotropic epsilon-near-zero metamaterial. The longitudinal field generated upon focusing can couple to the longitudinal plasmonic resonance featured by the metamaterial with an efficiency which depends on the numerical aperture of the objective, the initial state of polarisation and the quality of the metamaterial. Theoretical predictions and experimental observations prove this interaction to be able to transform any vectorial vortex into an azimuthal beam, aside from the special case of an ideal radial beam. The latter resilience is broken with the introduction of defects in the initial state of polarisation, so that an azimuthal beam is obtained also in the case of non-ideal radial polarisation. We investigate how a change in the metamaterial quality influences the efficiency of the process, as well as its spectral dependence.