Negative refraction by a virtual photonic lattice
  Research on photonics and metamaterials constantly challenges our intuitive understanding of the behaviour of light. In recent years we have seen negative refraction, focusing of light by a flat slab, a ``perfect'' prism, and an ``invisibility cloak'' [1-6]. It is generally understood that the cause of this unusual behaviour is the strong (anomalous) dispersion, i.e., dependence of the material properties on the frequency of light. Dispersion can be either due to a natural microscopic resonance of the material as with surface plasmons-polaritons, or due to an effective resonance (band-gap) of the periodic lattice as in photonics [7-9]. Metamaterials take the better of the two approaches representing a periodic array of designer subwavelength particles tuned to resonate at a specific frequency-band. At present, however, we have only a very basic understanding of the effect which a finite size of a sample of a periodic photonic crystal or metamaterial has on the macroscopic properties such as refraction. Yet every finite dielectric object is a moderate-quality resonator whose eigenmodes form a virtual photonic lattice with its own angular band-gaps and preferred directions of propagation. Here we show that this virtual lattice produces nontrivial real effects and that even a homogeneous dielectric resonator may refract negatively without either negative or periodically modulated permittivity/permeability. We also propose a simple way to control the period of this virtual photonic lattice by varying the transverse dimension of the resonator. Our research shows the importance of three-dimensional resonant phenomena in optics and may result in new optical devices with unusual properties. 
Neil V. Budko and Shreyas B. Raghunathan