Broadband Slow Light Metamaterial Based on a Double-Continuum Fano Resonance

We propose a concept of a low-symmetry three-dimensional metamaterial exhibiting a double-continuum Fano (DCF) optical resonance. Such metamaterial is described as a birefringent medium supporting a discrete dark electromagnetic state weakly coupled to the continua of two nondegenerate bright bands of orthogonal polarizations. It is demonstrated that light propagation through such DCF metamaterial can be slowed down over a broad frequency range when the medium parameters (e.g., frequency of the dark mode) are adiabatically changed along the optical path. Using a specific metamaterial implementation, we demonstrate that the DCF approach to slow light is superior to that of the electromagnetically induced transparency because it enables spectrally uniform group velocity and transmission coefficient.