Plasmons in strongly coupled metallic nanostructures.

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

J-aggregates are of significant interest for organic materials conceived by supramolecular approaches. Their discovery in the 1930s represents one of the most important milestones in dye chemistry as well as the germination of supramolecular chemistry. The intriguing optical properties of J-aggregates (in particular, very narrow red-shifted absorption bands with respect to those of the monomer and their ability to delocalize and migrate excitons) as well as their prospect for applications have motivated scientists to become involved in this field, and numerous contributions have been published. This Review provides an overview on the J-aggregates of a broad variety of dyes (including cyanines, porphyrins, phthalocyanines, and perylene bisimides) created by using supramolecular construction principles, and discusses their optical and photophysical properties as well as their potential applications. Thus, this Review is intended to be of interest to the supramolecular, photochemistry, and materials science communities.

J-aggregates: from serendipitous discovery to supramolecular engineering of functional dye materials.

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

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.

Interference of optically induced electric and magnetic modes in high-index all-dielectric nanoparticles offers unique opportunities for tailoring directional scattering and engineering the flow of light. In this article we demonstrate theoretically and experimentally that the interference of electric and magnetic optically induced modes in individual subwavelength silicon nanodisks can lead to the suppression of resonant backscattering and to enhanced resonant forward scattering of light. To this end we spectrally tune the nanodisk’s fundamental electric and magnetic resonances with respect to each other by a variation of the nanodisk aspect ratio. This ability to tune two modes of different character within the same nanoparticle provides direct control over their interference, and, in consequence, allows for engineering the particle’s resonant and off-resonant scattering patterns. Most importantly, measured and numerically calculated transmittance spectra reveal that backward scattering can be suppresse...

Tailoring Directional Scattering through Magnetic and Electric Resonances in Subwavelength Silicon Nanodisks

Stable Au nanoshell-J-aggregate complexes are formed that exhibit coherent coupling between the localized plasmons of a nanoshell and the excitons of molecular J-aggregates adsorbed on its surface. By tuning the nanoshell plasmon energies across the exciton line of the J-aggregate, plasmon-exciton coupling energies for these complexes are obtained. The strength of this interaction is dependent on the specific plasmon mode of the nanoparticle coupled to the J-aggregate exciton. From a model based on Gans theory, we obtain an expression for the plasmon-exciton hybridized states of the complex.

Plexcitonic nanoparticles: plasmon-exciton coupling in nanoshell-J-aggregate complexes.

Coherently coupled plasmons and excitons give rise to new optical excitations- plexcitons − due to the strong coupling of these two oscillator systems Time-resolved studies of J-aggregate-Au nanoshell complexes when the nanoshell plasmon and J-aggregate exciton energies are degenerate probe the dynamical behavior of this coupled system Transient absorption of the interacting plasmon-exciton system is observed, in dramatic contrast to the photoinduced transmission of the pristine J-aggregate An additional, transient Fano-shaped modulation within the Fano dip is also observable The behavior of the J-aggregate-Au nanoshell complex is described by a combined one-exciton and two-exciton state model coupled to the nanoshell plasmon

Plexciton Dynamics: Exciton−Plasmon Coupling in a J-Aggregate−Au Nanoshell Complex Provides a Mechanism for Nonlinearity

We measure the near-infrared photoluminescence spectra of colloidal quantum dots coupled to the localized electric and magnetic resonances of subwavelength silicon nanodisks. The spectral position of the resonances with respect to each other is controlled via the nanodisk geometry. We observe a strong influence of the nanodisk resonance positions on the quantum dot photoluminescence spectra. For separate resonances, the spectral density observed in transmittance measurements correlates with the spectral range covered by a broad emission spectrum. For the case of spectral overlap of the electric and magnetic dipolar resonances we enter a new regime for coupling, where the characteristic transparency effect evident in the transmittance spectra is accompanied by a pronounced single emission maximum. Our experimental observations are in good qualitative agreement with numerical calculations.

Shaping photoluminescence spectra with magnetoelectric resonances in all-dielectric nanoparticles

Enhanced light trapping is an attractive technique for improving the efficiency of thin film silicon solar cells. In this paper, we use FDTD simulations to study the scattering properties of silicon nanostructures on a silicon substrate and their application as enhanced light trappers. We find that the scattered spectrum and angular scattering distribution strongly depend on the excitation direction, that is, from air to substrate or from substrate to air. At the dipole resonance wavelength the scattering angles tend to be very narrow compared to those of silicon nanostructures in the absence of a substrate. Based on these properties, we propose a new thin film silicon solar cell design incorporating silicon nanostructures on both the front and back surfaces for enhanced light trapping.

Nche T. Fofang

Papers

Tailoring Directional Scattering through Magnetic and Electric Resonances in Subwavelength Silicon Nanodisks

Plexcitonic nanoparticles: plasmon-exciton coupling in nanoshell-J-aggregate complexes.

Plexciton Dynamics: Exciton−Plasmon Coupling in a J-Aggregate−Au Nanoshell Complex Provides a Mechanism for Nonlinearity

Shaping photoluminescence spectra with magnetoelectric resonances in all-dielectric nanoparticles

Substrate-modified scattering properties of silicon nanostructures for solar energy applications.