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 l/n, where R is the nanoparticle radius, n is its refractive index, and l 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) (C) Unidirectional light scattering by a single dielectric nanoparticle for overlapping electric and magnetic dipole resonances, where k is the wave vector of the incident white light. (D) Light manipulation with highly transparent Huygens metasurfaces. Rapid progress in nanophotonics is driven by the ability of optically resonant nanostructures to enhance near field effects controlling far field scattering through intermodal interference. A majority of such effects are usually associated with plasmonic nanostructures. Recently, a new branch of nanophotonics has emerged that seeks to manipulate the strong, optically induced electric and magnetic Mie resonances in dielectric nanoparticles with high refractive index. In the design of optical nanoantennas and metasurfaces, dielectric nanoparticles offer the opportunity for reducing dissipative losses and achieving large resonant enhancement of both electric and magnetic fields. We review this rapidly developing field and demonstrate that the magnetic response of dielectric nanostructures can lead to novel physical effects and applications.
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