Arseniy I. Kuznetsov

Optically resonant dielectric nanostructures

A clear approach to nanophotonics 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 BACKGROUND Nanoscale optics is usually associated with plasmonic structures made of metals such as gold or silver. However, plasmonics suffers from high losses of metals, heating, and incompatibility with complementary metal oxide semiconductor fabrication processes. Recent developments in nanoscale optical physics have led to a new branch of nanophotonics aiming at the manipulation of optically induced Mie resonances in dielectric and semiconductor nanoparticles with high refractive indices. Such particles offer unique opportunities for reduced dissipative losses and large resonant enhancement of both electric and magnetic near-fields. Semiconductor nanostructures also offer longer excited-carrier lifetimes and can be electrically doped and gated to realize subwavelength active devices. These recent developments revolve closely around the nature of the optical resonances of the structures and how they can be manipulated in individual entities and in complex particle arrangements such as metasurfaces. Resonant high-index dielectric nanostructures form new building blocks to realize unique functionalities and novel photonic devices. ADVANCES We discuss the key advantages of resonant high-index nanostructures, associated new physical effects, and applications for nanoantennas, optical sensors, nonlinear devices, and flat optics. For a subwavelength high-index dielectric particle illuminated by a plane wave, electric and magnetic dipole resonances have comparable strengths. The resonant magnetic response results from a coupling of incoming light to the circular displacement currents of the electric field, when the wavelength inside the particle becomes comparable to its diameter d = 2R ≈ λ/n, where R is the nanoparticle radius, n is its refractive index, and λ is the wavelength of light. At the wavelength of a magnetic resonance, the excited magnetic dipole mode of a high-index dielectric sphere may provide a dominant contribution to the scattering efficiency exceeding the contribution of other multipoles by orders of magnitude. Nanophotonic structures composed of dielectric resonators can exhibit many of the same features as plasmonic nanostructures, including enhanced scattering, high-frequency magnetism, and negative refractive index. The specific design and parameter engineering of all-dielectric nanoantennas and metasurfaces give rise to superior performance in comparison to their lossy plasmonic counterparts. Spectral signatures of the Mie-type resonances of these structures are revealed by using far-field spectroscopy while tuning geometrically their resonance properties. A special case is realized when the electric and magnetic resonances spectrally overlap; the impedance matching eliminates the backward scattering, leading to unidirectional scattering and Huygens metasurfaces. A variety of nanoparticle structures have been studied, including dielectric oligomers as well as metasurfaces and metadevices. The magnetic resonances lead to enhanced nonlinear response, Raman scattering, a novel Brewster effect, sharp Fano resonances, and highly efficient sensing and photodetection. OUTLOOK The study of resonant dielectric nanostructures has been established as a new research direction in modern nanophotonics. Because of their unique optically induced electric and magnetic resonances, high-index nanophotonic structures are expected to complement or even replace different plasmonic components in a range of potential applications. The unique low-loss resonant behavior allows reproduction of many subwavelength resonant effects demonstrated in nanophotonics without much energy dissipation into heat. In addition, the coexistence of strong electric and magnetic resonances, their interference, and resonant enhancement of the magnetic field in dielectric nanoparticles bring entirely novel functionalities to simple geometries largely unexplored in plasmonic structures, especially in the nonlinear regime or in optoelectronic device applications. Manifestations of all-dielectric resonant nanophotonics. (A) Structure of the fields near the magnetic dipole resonance. (B) Experimental demonstration of optical magnetic response shown through optical dark-field and scanning electron microscope images (top and bottom, respectively)

Laser Fabrication of Large-Scale Nanoparticle Arrays for Sensing Applications

A novel method for high-speed fabrication of large scale periodic arrays of nanoparticles (diameters 40-200 nm) is developed. This method is based on a combination of nanosphere lithography and laser-induced transfer. Fabricated spherical nanoparticles are partially embedded into a polymer substrate. They are arranged into a hexagonal array and can be used for sensing applications. An optical sensor with the sensitivity of 365 nm/RIU and the figure of merit of 21.5 in the visible spectral range is demonstrated.

Magnetic and electric hotspots with silicon nanodimers

The study of the resonant behavior of silicon nanostructures provides a new route for achieving efficient control of both electric and magnetic components of light. We demonstrate experimentally and numerically that enhancement of localized electric and magnetic fields can be achieved in a silicon nanodimer. For the first time, we experimentally observe hotspots of the magnetic field at visible wavelengths for light polarized across the nanodimers primary axis, using near-field scanning optical microscopy.

Generalized Brewster effect in dielectric metasurfaces.

Polarization is a key property defining the state of light. It was discovered by Brewster, while studying light reflected from materials at different angles. This led to the first polarizers, based on Brewsters effect. Now, one of the trends in photonics is the study of miniaturized devices exhibiting similar, or improved, functionalities compared with bulk optical elements. In this work, it is theoretically predicted that a properly designed all-dielectric metasurface exhibits a generalized Brewsters effect potentially for any angle, wavelength and polarization of choice. The effect is experimentally demonstrated for an array of silicon nanodisks at visible wavelengths. The underlying physics is related to the suppressed scattering at certain angles due to the interference between the electric and magnetic dipole resonances excited in the nanoparticles. These findings open doors for Brewster phenomenon to new applications in photonics, which are not bonded to a specific polarization or angle of incidence.

Optimum Forward Light Scattering by Spherical and Spheroidal Dielectric Nanoparticles with High Refractive Index

High-refractive index dielectric nanoparticles may exhibit strong directional forward light scattering at visible and near-infrared wavelengths due to interference of simultaneously excited electric and magnetic dipole resonances. For a spherical particle shape, the so-called first Kerker’s condition can be realized, at which the backward scattering practically vanishes for some combination of refractive index and particle size. However, realization of Kerker’s condition for spherical particles is only possible at the tail of the scattering resonances, when the particle scatters light weakly. Here we demonstrate that significantly higher forward scattering can be realized if spheroidal particles are considered instead. For each value of refractive index n exists an optimum shape of the particle, which produces minimum backscattering efficiency together with maximum forward scattering. This effect is achieved due to the overlapping of magnetic and electric dipole resonances of the spheroidal particle at th...

Plasmon-Enhanced Sub-Wavelength Laser Ablation: Plasmonic Nanojets

In response to the incident lights electric field, the electron density oscillates in the plasmonic hotspots producing an electric current. Associated Ohmic losses raise the temperature of the material within the plasmonic hotspot above the melting point. A nanojet and nanosphere ejection can then be observed precisely from the plasmonic hotspots.

Laser-induced transfer of metallic nanodroplets for plasmonics and metamaterial applications

A novel approach, to our knowledge, for the fabrication of metallic micro- and nanostructures based on femtosecond laser-induced transfer of metallic nanodroplets is developed. The controllable fabrication of high-quality spherical gold micro- and nanoparticles with radius of 100-800 nm is realized. In combination with the two-photon polymerization technique, this approach provides unique possibilities for the realization of plasmonic components and metamaterials. Polymer woodpile structures filled with gold nanoparticles are demonstrated. Scattering of surface plasmon polaritons on an individual spherical gold nanoparticle fabricated by the proposed method is investigated. The obtained results are supported by a numerical modeling using the Greens tensor approach.

Split-ball resonator as a three-dimensional analogue of planar split-rings

Split-ring resonators are basic elements of metamaterials, which can induce a magnetic response in metallic nanosctructures. Tunability of such response up to the visible frequency range is still a challenge. Here we introduce the concept of the split-ball resonator and demonstrate the strong magnetic response in the visible for both gold and silver spherical plasmonic nanoparticles with nanometre scale cuts. We realize this concept experimentally by employing the laser-induced transfer method to produce near-perfect metallic spheres and helium ion beam milling to make cuts with the clean straight sidewalls and nanometre resolution. The magnetic resonance is observed at 600 nm in gold and at 565 nm in silver nanoparticles. This method can be applied to the structuring of arbitrary three-dimensional features on the surface of nanoscale resonators. It provides new ways for engineering hybrid resonant modes and ultra-high near-field enhancement.

Traditional and emerging materials for optical metasurfaces

Abstract One of the most promising and vibrant research areas in nanotechnology has been the field of metasurfaces. These are two dimensional representations of metaatoms, or artificial interfaces designed to possess specialized electromagnetic properties which do not occur in nature, for specific applications. In this article, we present a brief review of metasurfaces from a materials perspective, and examine how the choice of different materials impact functionalities ranging from operating bandwidth to efficiencies. We place particular emphasis on emerging and non-traditional materials for metasurfaces such as high index dielectrics, topological insulators and digital metamaterials, and the potentially transformative role they could play in shaping further advances in the field.

Probing magnetic and electric optical responses of silicon nanoparticles

We study experimentally both magnetic and electric optically induced resonances of silicon nanoparticles by combining polarization-resolved dark-field spectroscopy and near-field scanning optical microscopy measurements. We reveal that the scattering spectra exhibit strong sensitivity of electric dipole response to the probing beam polarization and attribute the characteristic asymmetry of measured near-field patterns to the excitation of a magnetic dipole mode. The proposed experimental approach can serve as a powerful tool for the study of photonic nanostructures possessing both electric and magnetic optical responses.