Trapped-modes, slow light and collective resonances in metamaterials

Abstract

A new class of metamaterials exhibiting coherent, collective response has been introduced. It is shown that the sharp resonant behaviour of coherent metamaterials can only be observed in arrays of metamaterial elements and is absent from the response of a single isolated unit cell. As a result, such arrays are extremely sensitive to positional disorder and resonances degrade rapidly with increasing randomization. These observed strong inter-element interactions render coherent metamaterials ideal candidates for gain-assisted functionalities as demonstrated by the suggestion and numerical study of a novel amplifying/lasing device, termed the 'lasing spaser'. An antipode class of incoherent metamaterials is also presented, where the resonant response of a single unit and of an infinite array are very similar resulting in weak dependence on disorder. The first metamaterial analogue of electromagnetically induced transparency is demonstrated experimentally and theoretically in essentially planar structures. The phenomenon arises from destructive interference of fields radiated by strongly coupled metamaterial elements that support anti-symmetric weakly-radiating current configurations, termed trapped-modes. This behaviour is accompanied by sharp resonances and steep normal dispersion which leads to long pulse delays. It is shown that cascading of metamaterial slabs increases the bandwidth of the pulse delay effect, while extension to all-angles and all-polarizations is demonstrated by appealing to incoherent metamaterials. The first experimental study of metamaterials with toroidal symmetry is reported. Resonant circular dichroism is observed in a metamaterial consisting of toroidal wire windings. Further numerical investigation attributes the gyrotropic behaviour to current standing waves corresponding to the eigenmodes of the unit cell winding. Multipole expansion of the resonant current configurations indicates a dominant electric dipole-magnetic dipole contribution to gyrotropy followed by electric dipole-electric quadrupole order effects, while a non-negligible toroidal response comparable to electric quadrupole in scattering efficiency also emerges. Finally, collective effects are studied in quasicrystal hole arrays and it is demonstrated that non-resonant scatterers can lead to strong lattice resonances and extraordinary transmission even in the case of quasi-periodicity. Microwave and optical quasicrystal patterns exhibit similar response exceeding predictions based on absence of inter-element interactions and even reaching a nearly invisible state in the microwave part of the spectrum