Despite the extraordinary degree of interest in optical metamaterials in recent years the hoped--for devices and applications have, in large part, yet to emerge, and it is becoming clear that the first generation of metamaterial--based devices will more likely arise from their two--dimensional equivalents, metasurfaces. In this review we describe the recent progress made in the area of metasurfaces formed from plasmonic meta--atoms. In particular we approach the subject from the perspective of the fundamental excitations supported by the meta--atoms and the interactions between them. We also identify some areas ripe for future research, and indicate likely avenues for future device development.

We investigate the chiral properties of near fields around plasmonic nanostructures and their relation to the electromagnetic chirality C. By combining chiral metal nanoparticles with achiral dye molecules and measuring the circular polarization dependence of the enhanced photoluminescence, we find a correlation between the dissymmetry of the luminescence enhancement and the calculated values of C. These effects are strong (∼10 −1 ), despite the weak circular dichroism of the particles (∼10 −5 ). We further show that C represents the chiral selectivity of the near-field coupling between an emitter and a nanoantenna. Structural chirality, i.e., the handedness of an object that makes it incongruent with its mirror image, is a common property of many biologically relevant molecules, ranging from simple amino acids to complex helical DNA strands. This quality is often characterized by optical techniques that exploit the enantioselective interaction with circularly polarized light, e.g., by measuring circular dichroism (CD), 1,2 or by detecting molecular transitions in chiral molecules. 3,4 Such studies of chiroptical effects require an understanding of the chirality-or helicity-of light, which is not as simple a concept as structural chirality: It is, more fundamentally, associated with angular momentum. However, determining the connection between helicity and angular momentum is not trivial, 5 not even for a plane electromagnetic wave, and it breaks down entirely for evanescent fields.6 Nevertheless, it has been shown that the evanescent near fields around certain plasmonic nanostructures can undergo chiroptical interactions.7-12 However, the chiral properties of these electromagnetic near fields are very different from circularly polarized light; indeed, the concept of polarization is not applicable to evanescent fields: They exhibit phase-shifted electric and magnetic field vector components without temporal vector rotation. Such solutions to the Maxwell equations are only allowed in local regions of space, such as in the vicinity of scattering nanostructures or at the center of Laguerre-Gaussian beams. 13A more general description of the chiral symmetry of arbitrary electromagnetic fields is therefore needed and several theoretical approaches have been proposed. a pseudoscalar that serves as a measure of the chiral nature of an electromagnetic field with E and B denoting complex electric and magnetic field vectors, respectively. The formalism developed in Ref.15 is based on the interaction of a time-varying electromagnetic field with the electric and magnetic dipole moments that describe a chiral probe molecule. The electromagnetic chirality is then linked to the dissymmetry in the excitation rate of this probe for left-and right-handed systems. It contains only the chirality intrinsic to the electromagnetic field and is independent of the probe.Chirality is usually quantified through far-field measurements, more specifically, of the CD. Such a measurement does not probe the near-field chirality C. Here, we dire...

We study arrays of silver split-ring resonators operating at around 1.5-µm wavelength coupled to an MBE-grown single 12.7-nm thin InGaAs quantum well separated only 4.8 nm from the wafer surface. The samples are held at liquid-helium temperature and are pumped by intense femtosecond optical pulses at 0.81-µm center wavelength in a pump-probe geometry. We observe much larger relative transmittance changes (up to about 8%) on the split-ring-resonator arrays as compared to the bare quantum well (not more than 1-2%). We also observe a much more rapid temporal decay component of the differential transmittance signal of 15 ps for the case of split-ring resonators coupled to the quantum well compared to the case of the bare quantum well, where we find about 0.7 ns. These observations are ascribed to the evanescent coupling of the split-ring resonators to the quantum-well gain. All experimental results are compared with a recently introduced analytical toy model that accounts for this evanescent coupling, leading to excellent overall qualitative agreement.

Abstract:We propose, solve, and discuss a simple model for a metamaterial incorporating optical gain: A single bosonic resonance is coupled to a fermionic (inverted) two-level-system resonance via local-field interactions. For given steady-state inversion, this model can be solved analytically, revealing a rich variety of (Fano) absorption/gain lineshapes. We also give an analytic expression for the fixed inversion resulting from gain pinning under steady-state conditions. Furthermore, the dynamic response of the "lasing SPASER", i.e., its relaxation oscillations, can be obtained by simple numerical calculations within the same model. As a result, this toy model can be viewed as the near-field-optical counterpart of the usual LASER rate equations.

We study regular wavelength scale arrays of metallic dimers. By employing dimers made up of two different sized discs, we are able to couple to array-based collective surface lattice resonances of both bright and dark, that is symmetric and antisymmetric, dimer modes and to show that the degree of asymmetry can be used to control the relative strength of the two surface-lattice modes. The collective nature of these excitations can even lead to an antisymmetric surface-lattice resonance that is stronger than the symmetric one; this is in stark contrast to the dark and bright nature of the underlying modes of the individual dimers. We verify these experimental findings, derived from extinction measurements, by comparison with both analytical and numerical modeling.