The resonant modes of plasmonic nanoparticle structures made of gold or silver endow them with an ability to manipulate light at the nanoscale. However, owing to the high light losses caused by metals at optical wavelengths, only a small fraction of plasmonics applications have been realized. Kuznetsov et al. review how high-index dielectric nanoparticles can offer a substitute for these metals, providing a highly flexible and low-loss route to the manipulation of light at the nanoscale.

Science , this issue p. [10.1126/science.aag2472][1]

 [1]: /lookup/doi/10.1126/science.aag2472

http://science.sciencemag.org/content/sci/354/6314/aag2472.full.pdf

Optically resonant dielectric nanostructures

Metamaterials are composed of periodic subwavelength metal/dielectric structures that resonantly couple to the electric and/or magnetic components of the incident electromagnetic fields, exhibiting properties that are not found in nature. This class of micro- and nano-structured artificial media have attracted great interest during the past 15 years and yielded ground-breaking electromagnetic and photonic phenomena. However, the high losses and strong dispersion associated with the resonant responses and the use of metallic structures, as well as the difficulty in fabricating the micro- and nanoscale 3D structures, have hindered practical applications of metamaterials. Planar metamaterials with subwavelength thickness, or metasurfaces, consisting of single-layer or few-layer stacks of planar structures, can be readily fabricated using lithography and nanoprinting methods, and the ultrathin thickness in the wave propagation direction can greatly suppress the undesirable losses. Metasurfaces enable a spatially varying optical response (e.g. scattering amplitude, phase, and polarization), mold optical wavefronts into shapes that can be designed at will, and facilitate the integration of functional materials to accomplish active control and greatly enhanced nonlinear response. This paper reviews recent progress in the physics of metasurfaces operating at wavelengths ranging from microwave to visible. We provide an overview of key metasurface concepts such as anomalous reflection and refraction, and introduce metasurfaces based on the Pancharatnam-Berry phase and Huygens' metasurfaces, as well as their use in wavefront shaping and beam forming applications, followed by a discussion of polarization conversion in few-layer metasurfaces and their related properties. An overview of dielectric metasurfaces reveals their ability to realize unique functionalities coupled with Mie resonances and their low ohmic losses. We also describe metasurfaces for wave guidance and radiation control, as well as active and nonlinear metasurfaces. Finally, we conclude by providing our opinions of opportunities and challenges in this rapidly developing research field.

A review of metasurfaces: physics and applications.

Metamaterials, artificial electromagnetic media that are structured on the subwavelength scale, were initially suggested for the negative-index 'superlens'. Later metamaterials became a paradigm for engineering electromagnetic space and controlling propagation of waves: the field of transformation optics was born. The research agenda is now shifting towards achieving tunable, switchable, nonlinear and sensing functionalities. It is therefore timely to discuss the emerging field of metadevices where we define the devices as having unique and useful functionalities that are realized by structuring of functional matter on the subwavelength scale. In this Review we summarize research on photonic, terahertz and microwave electromagnetic metamaterials and metadevices with functionalities attained through the exploitation of phase-change media, semiconductors, graphene, carbon nanotubes and liquid crystals. The Review also encompasses microelectromechanical metadevices, metadevices engaging the nonlinear and quantum response of superconductors, electrostatic and optomechanical forces and nonlinear metadevices incorporating lumped nonlinear components.

/pdf/from-metamaterials-to-metadevices-4vgaose515.pdf

From metamaterials to metadevices.

The ideal material for nanophotonic applications will have a large refractive index at optical frequencies, respond to both the electric and magnetic fields of light, support large optical chirality and anisotropy, confine and guide light at the nanoscale, and be able to modify the phase and amplitude of incoming radiation in a fraction of a wavelength. Artificial electromagnetic media, or metamaterials, based on metallic or polar dielectric nanostructures can provide many of these properties by coupling light to free electrons (plasmons) or phonons (phonon polaritons), respectively, but at the inevitable cost of significant energy dissipation and reduced device efficiency. Recently, however, there has been a shift in the approach to nanophotonics. Low-loss electromagnetic responses covering all four quadrants of possible permittivities and permeabilities have been achieved using completely transparent and high-refractive-index dielectric building blocks. Moreover, an emerging class of all-dielectric metamaterials consisting of anisotropic crystals has been shown to support large refractive index contrast between orthogonal polarizations of light. These advances have revived the exciting prospect of integrating exotic electromagnetic effects in practical photonic devices, to achieve, for example, ultrathin and efficient optical elements, and realize the long-standing goal of subdiffraction confinement and guiding of light without metals. In this Review, we present a broad outline of the whole range of electromagnetic effects observed using all-dielectric metamaterials: high-refractive-index nanoresonators, metasurfaces, zero-index metamaterials and anisotropic metamaterials. Finally, we discuss current challenges and future goals for the field at the intersection with quantum, thermal and silicon photonics, as well as biomimetic metasurfaces.

All-dielectric metamaterials

Passive radiative cooling draws heat from surfaces and radiates it into space as infrared radiation to which the atmosphere is transparent. However, the energy density mismatch between solar irradiance and the low infrared radiation flux from a near-ambient-temperature surface requires materials that strongly emit thermal energy and barely absorb sunlight. We embedded resonant polar dielectric microspheres randomly in a polymeric matrix, resulting in a metamaterial that is fully transparent to the solar spectrum while having an infrared emissivity greater than 0.93 across the atmospheric window. When backed with a silver coating, the metamaterial shows a noontime radiative cooling power of 93 watts per square meter under direct sunshine. More critically, we demonstrated high-throughput, economical roll-to-roll manufacturing of the metamaterial, which is vital for promoting radiative cooling as a viable energy technology.

http://science.sciencemag.org/content/sci/early/2017/02/08/science.aai7899.full.pdf

Scalable-manufactured randomized glass-polymer hybrid metamaterial for daytime radiative cooling

Mie resonance-based dielectric metamaterials

We report on a negative-zero-positive metamaterial based on an omega-type microstructure with special attention on the nonvanishing group velocity for a zero refractive index. We first investigate the dispersion characteristics by full wave analysis, by stressing the necessary conditions of equality between the electric and magnetic plasma frequencies which are characteristic of the dispersion of the effective permittivity and permeability. Also, tuning of the gapless transition frequency between the left and right-handed dispersion branches was analyzed when the permittivity of the host substrate is changed. Last, we demonstrate experimentally the balanced composite character of the dispersion by frequency and angle-resolved transmission measurements, carried out at centimeter wavelengths on slabs and wedge-type prototypes, respectively.

Negative-zero-positive metamaterial with omega-type metal inclusions

The condition of both relative permittivity and permeability equal to $\ensuremath{-}1$ in negative index photonic crystals was addressed by the retrieval of the so-called Bloch impedance. The dispersion characteristics of a photonic crystal made of an array of air holes in an InGaAsP semiconductor layer were first compared (i) by solving the eigenvalue problem by plane wave expansion and (ii) by calculating the complex transmission and reflection coefficients for a finite slab. From the latter, the effective refractive index and the Bloch impedance are deduced by using a transfer matrix technique. The criterion of optical index $n=\ensuremath{-}1$, which is the key condition for the same focus for all angle incidence, is shown to be satisfied at a frequency where ${\ensuremath{\epsilon}}_{\mathit{eff}}\ensuremath{\approx}\ensuremath{-}5$ and ${\ensuremath{\mu}}_{\mathit{eff}}\ensuremath{\approx}\ensuremath{-}0.2$ and thus far from the condition for impedance matching. A dielectriclike mode of the negative index branch pointed out by field mapping explains the impossibility of effective permittivity and permeability matching to $\ensuremath{-}1$.

Bloch impedance in negative index photonic crystals

Single (SHBV) and dual (DHBV) heterostructure barrier varactors with AlAs/In0.52Al0.48As bolocking conductionlayers have been fabricated and characterized. The devices, whose electrical properties scale with layer complexity, exhibit, for a DHBV scheme, leakage currents as low as 10 A/cm² at 12V, a zero bias capacitance of 1 fF/micro m², and capacitance ratio of 5:1.

High performance InP-based heterostructure barrier varactors in single and stack configuration

A 2% bandpass filter with an out-of-band rejection in excess of 30 dB was demonstrated experimentally for operation in the C-band (central frequency at 4.5 GHz). Narrow bandwidth and high rejection were achieved by cascading three double split-ring resonators (SRRs) magnetically coupled to a coplanar waveguide (CPW) and three pairs of shunt wires between the central strip and the ground metal layers. This configuration, directly inspired from the so-called metamaterial technology, takes advantage of the high dispersion of the loaded transmission line, induced by a negative magnetic response via the resonators, and of a negative permittivity effect via the shunt wires. The optimization of: 1) the symmetry; 2) the out-of-band rejection; and 3) the fractional bandwidth was carried out in two stages, i.e., by engineering the plasma frequencies of the electrical and magnetic systems and by tapering the tightly coupled cascaded unit cells in order to achieve an overlap of their individual responses.

A 2% Bandwidth C-Band Filter Using Cascaded Split Ring Resonators

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