Trapped-modes, slow light and collective resonances in metamaterials

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

Plasmonic absorption properties of bimetallic metamaterials

We demonstrate polarization controlled absorption in plasmonic bimetallic metamaterials. We fabricate and experimentally characterize Au/Ni ring resonator arrays, where by varying the wavelength and polarization of the incident wave, local electromagnetic fields and dissipation can be suppressed or enhanced in the Au and Ni areas of the rings

Light localization in disordered metamaterials

Localization due to multiple scattering of photons in disordered electromagnetic media has triggered a recent paradigm shift in photonics, where disorder is no longer considered an unwanted disturbance on perfect periodicity, but is being used to achieve enhancement of luminescence, nonlinear optical interactions, Raman scattering and occupies a central position in random lasers. Here for the first time we extend the study of light localization to planar metamaterials. We study a metamaterial system consisting of asymmetrically-split ring (ASR) resonators that support both collective and individual modes. In such a metamaterial, a high-quality subradiant (trapped) modes can be excited that corresponds to a collective excitation of magnetic dipoles normal to the array plane and hence uncoupled to the magnetic field of the incident wave. This leads to a nearly-closed system, very weakly coupled to free-space, ideal for the study of localization effects

Evolutionary trade-offs between reproduction and dispersal in populations at expanding range boundaries

During recent climate warming, some species have expanded their ranges northwards to keep track of climate changes. Evolutionary changes in dispersal have been demonstrated in these expanding populations and here we show that increased dispersal is associated with reduced investment in reproduction in populations of the speckled wood butterfly, Pararge aegeria. Evolutionary changes in flight versus reproduction will affect the pattern and rate of expansion at range boundaries in the future, and understanding these responses will be crucial for predicting the distribution of species in the future as climates continue to warm

Magneto-optical response in bimetallic metamaterials

We demonstrate resonant Faraday polarization rotation in plasmonic arrays of bimetallic nano-ring resonators consisting of Au and Ni sections. This metamaterial design allows to optimize the trade-off between the enhancement of magneto-optical effects and plasmonic dissipation. Although Ni sections correspond to as little as ~6% of the total surface of the metamaterial, the resulting magneto-optically induced polarization rotation is equal to that of a continuous film. Such bimetallic metamaterials can be used in compact magnetic sensors, active plasmonic components and integrated photonic circuits

"Slow" light in metamaterials

We demonstrate that propagation of microwave pulses can be significantly affected by the presence of a planar fish-scale metamaterial, which is at least 30 times thinner than the wavelength. In the resonant band of the fish-scale structure, a spectrally narrow pulse (18 ns) can be significantly delayed (by 5.6 ns) as if propagating through an 84 cm thick dielectric (epsilon=3.77), while a short pulse (220 ps) will split in two roughly equal pulses propagating with subluminal and superluminal velocity respectively. We also interpret the response of the metamaterial in terms of effective material parameters

Wavevector Selective Metasurfaces and Tunnel Vision Filters

Metasurfaces offer unprecedented flexibility in the design and control of light propagation, replacing bulk optical components and exhibiting exotic optical effects. One of the basic properties of the metasurfaces, which renders them as frequency selective surfaces, is the ability to transmit or reflect radiation within a narrow spectral band that can be engineered on demand. Here we introduce and demonstrate experimentally in the THz domain the concept of wavevector selective surfaces -- metasurfaces transparent only within a narrow range of light propagation directions operating effectively as tunnel vision filters. Practical implementations of the new concept include applications in wavefront manipulation, observational instruments, vision and free-space communication in light-scattering environments, as well as passive camouflage