Spectral phase singularity in a transmission-type double-layer metamaterial

Abstract

Phase singularities are seen in optical vortex beams, which are located in a two-dimensional spatial plane. Phase singularities in optical spectra are not common, but exploiting the extreme phase behavior around the singularity point could improve conventional optical devices for molecular-/bio-sensing, large phase modulation, etc. Recently, spectral phase singularities have been reported in reflection-type metamaterials but have not yet been reported in transmission. In this work, we demonstrate a spectral phase singularity in a transmission-type double-layer metamaterial. We attribute the mechanism to two different types of resonances in the double-layer metamaterial, namely a hybrid resonance and an anti-parallel dipole resonance. By controlling the lattice size of the double-layer metamaterial, we can manipulate the coupling of the two types of resonances. At a critical-coupling point, the phase singularity occurs in a spectrum-lattice diagram. We developed film metamaterial devices to experimentally demonstrate the phase singularity. Our results could enable investigation of coupled resonator systems, and may have applications in tunable metamaterial device design and improved sensing techniques.