Negative-Index Metamaterials: Looking into the Unit Cell

Abstract

With their potential for spectacular applications, like superlensing and cloaking, metamaterials are a powerful class of nanostructured materials. All these applications rely on the metamaterials acting as a homogeneous material. We investigate a negative index metamaterial with a phase-sensitive near-field microscope and measure the optical phase as a function of distance. Close to the metamaterial we observe extremely large spatial phase variations within a single unit cell which vanish on a 200 nm length scale from the sample. These deviations of a state-of-the-art metamaterial from a homogeneous medium can be important for nanoscale applications.KEYWORDS Near-field optics, nanophotonics, metamaterials, negative refractive index T he optical properties of "natural" bulk materials arise from a complex interplay between the electronic orbitals of the constituent atoms, which leads to a certain electronic band structure. An externally applied light field interacts with electrons in this band structure. In general, this results in a dispersion of the refractive index and also leads to absorption-edges and even to sharp Lorentz-like absorption lines. Thus a natural material can be considered as an effective medium in which the role of the individual atoms no longer has to be considered. 1 Similarly in the field of metamaterials, 2 we like to think in terms of constituent "meta-atoms" that couple with each other to give rise to the optical properties of the bulk material, which is then considered as an effective medium. 3 Several important experiments have been performed to elucidate the interplay between the meta-atoms that constitute a metamaterial. For instance, the response of a single meta-atom was measured and compared with that of an array of meta-atoms. 4 Also, the response of an array of metaatoms was studied as a function of the periodicity in two directions, to extract both the electric as well as the magnetic coupling between the meta-atoms. 5,6 By now, it is well understood that, as in the case of natural materials, the effective optical properties of a metamaterial are not simply the addition of its constituent meta-atoms, and a monolayer of a metamaterial has not yet developed all the optical properties of the bulk metamaterial. As a result, the properties of a monolayer of meta-atoms that we study in this letter are not necessarily the same as those of a bulk material. 7 Nevertheless, here we use the notion "negative-index metamaterial" for a monolayer to connect to a very large number of corresponding publications.To fully understand how negative index metamaterials can be applied on the nanoscale as in, for instance, a nearfield superlens, 8 it is not sufficient to know the effective medium response of the material that is measured far away from the structure, that is, in the far field. For nanoscale applications, it is crucial to also know the near-field behavior. Hence, we would like to take a look inside the unit cell of a negative index metamaterial. A recent pioneering effort gained ...

Near-field optical experiments on low-symmetry split-ring-resonator arrays

“…For further details, see the Supporting Information of our previous work [12]. The phase lag added here slightly differs from [12]. This is not surprising considering that the SNOM probes used here and in [12] have rather different geometries.…”

“…To further test our interpretation, we compare our experimental results with theoretical modeling. Here, we follow our recent approach [12] that has been successful in the context of negative-index double-fishnet-type photonic metamaterials. We start from numerical calculations of the local electromagnetic fields using the commercial software CST Microwave Studio, which is based on a finite-integration technique.…”

“…The SNOM signal results as the sum of the modulus squared of the two components. For further details, see the Supporting Information of our previous work [12]. The phase lag added here slightly differs from [12].…”

Image formation properties and inverse imaging problem in aperture based scanning near field optical microscopy

“…This can create unprecedented metrological opportunities to characterize electromagnetic fields at the nanoscale. By relying on a polarization and phase resolved measurement, being available with the current state-of-the-art technology [5,6,15,18,19,30,31], two distinct signals are gathered. These two signals correspond to the complex amplitudes of two orthogonally polarized modes of the fiber that transmits the signal to the far field.…”

Realization of negative refractive index with double-layered H-shaped resonator array

“…For the negative refractive index, early designing follows the idea to combine a resonant magnetic structure with another metallic structure that provides a "background" of negative permittivity in a broad spectral range. [8][9][10][11] In other words, the frequency of magnetic resonance is imbedded in a broader spectral range where electric resonance exists. We once proposed an assembly of double-layered metallic Ushaped resonators (USRs), which possesses pure electric and magnetic responses at different frequencies, leading to both negative permeability and permittivity.…”