Andrey E. Miroshnichenko

Fano resonances in nanoscale structures

Modern nanotechnology allows one to scale down various important devices (sensors, chips, fibers, etc.) and thus opens up new horizons for their applications. The efficiency of most of them is based on fundamental physical phenomena, such as transport of wave excitations and resonances. Short propagation distances make phase-coherent processes of waves important. Often the scattering of waves involves propagation along different paths and, as a consequence, results in interference phenomena, where constructive interference corresponds to resonant enhancement and destructive interference to resonant suppression of the transmission. Recently, a variety of experimental and theoretical work has revealed such patterns in different physical settings. The purpose of this review is to relate resonant scattering to Fano resonances, known from atomic physics. One of the main features of the Fano resonance is its asymmetric line profile. The asymmetry originates from a close coexistence of resonant transmission and resonant reflection and can be reduced to the interaction of a discrete (localized) state with a continuum of propagation modes. The basic concepts of Fano resonances are introduced, their geometrical and/or dynamical origin are explained, and theoretical and experimental studies of light propagation in photonic devices, charge transport through quantum dots, plasmon scattering in Josephson-junction networks, and matter-wave scattering in ultracold atom systems, among others are reviewed.

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

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 nanodisks 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 particles resonant and off-resonant scattering patterns. Most importantly, measured and numerically calculated transmittance spectra reveal that backward scattering can be suppressed and forward scattering can be enhanced at resonance for the particular case of overlapping electric and magnetic resonances. Our experimental results are in good agreement with calculations based on the discrete dipole approach as well as finite-integral frequency-domain simulations. Furthermore, we show useful applications of silicon nanodisks with tailored resonances as optical nanoantennas with strong unidirectional emission from a dipole source.

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)

Enhanced Third-Harmonic Generation in Silicon Nanoparticles Driven by Magnetic Response

We observe enhanced third-harmonic generation from silicon nanodisks exhibiting both electric and magnetic dipolar resonances. Experimental characterization of the nonlinear optical response through third-harmonic microscopy and spectroscopy reveals that the third-harmonic generation is significantly enhanced in the vicinity of the magnetic dipole resonances. The field localization at the magnetic resonance results in two orders of magnitude enhancement of the harmonic intensity with respect to unstructured bulk silicon with the conversion efficiency limited only by the two-photon absorption in the substrate.

Fano resonances in all-dielectric oligomers

We demonstrate that light scattering by all-dielectric oligomers exhibits well-pronounced Fano resonances with strong suppression of the scattering cross section. Our analysis reveals that this type of the Fano resonance originates from the optically induced magnetic dipole modes of individual high-dielectric nanoparticles. By comparing to the plasmonic analogues, we observe that Fano resonances in all-dielectric oligomers are less sensitive to structural variations, which makes them promising for future applications in nanophotonics.

Broadband unidirectional scattering by magneto-electric core-shell nanoparticles.

Core-shell nanoparticles have attracted surging interests due to their flexibly tunable resonances and various applications in medical diagnostics, biosensing, nanolasers, and many other fields. The core-shell nanoparticles can support simultaneously both electric and magnetic resonances, and when the resonances are properly engineered, entirely new properties can be achieved. Here we study core-shell nanoparticles that support both electric and artificial magnetic dipolar modes, which are engineered to coincide spectrally with the same strength. We reveal that the interferences of these two resonances result in azimuthally symmetric unidirectional scattering, which can be further improved by arranging the nanoparticles in a chain, with both azimuthal symmetry and vanishing backward scattering preserved over a wide spectral range. We also demonstrate that the vanishing backward scattering is preserved, even for random particle distributions, which can find applications in the fields of nanoantennas, photovoltaic devices, and nanoscale lasers that require backward scattering suppressions.

Ultrafast All-Optical Switching with Magnetic Resonances in Nonlinear Dielectric Nanostructures

We demonstrate experimentally ultrafast all-optical switching in subwavelength nonlinear dielectric nanostructures exhibiting localized magnetic Mie resonances. We employ amorphous silicon nanodisks to achieve strong self-modulation of femtosecond pulses with a depth of 60% at picojoule-per-disk pump energies. In the pump-probe measurements, we reveal that switching in the nanodisks can be governed by pulse-limited 65 fs-long two-photon absorption being enhanced by a factor of 80 with respect to the unstructured silicon film. We also show that undesirable free-carrier effects can be suppressed by a proper spectral positioning of the magnetic resonance, making such a structure the fastest all-optical switch operating at the nanoscale.

Observation of Fano Resonances in All‐Dielectric Nanoparticle Oligomers

It is well-known that oligomers made of metallic nanoparticles are able to support sharp Fano resonances originating from the interference of two plasmonic resonant modes with different spectral width. While such plasmonic oligomers suffer from high dissipative losses, a new route for achieving Fano resonances in nanoparticle oligomers has opened up after the recent experimental observations of electric and magnetic resonances in low-loss dielectric nanoparticles. Here, light scattering by all-dielectric oligomers composed of silicon nanoparticles is studied experimentally for the first time. Pronounced Fano resonances are observed for a variety of lithographically-fabricated heptamer nanostructures consisting of a central particle of varying size, encircled by six nanoparticles of constant size. Based on a full collective mode analysis, the origin of the observed Fano resonances is revealed as a result of interference of the optically-induced magnetic dipole mode of the central particle with the collective mode of the nanoparticle structure. This allows for effective tuning of the Fano resonance to a desired spectral position by a controlled size variation of the central particle. Such optically-induced magnetic Fano resonances in all-dielectric oligomers offer new opportunities for sensing and nonlinear applications.

Nonlinearly PT-symmetric systems: Spontaneous symmetry breaking and transmission resonances

We consider a class of PT-symmetric systems which include mutually matched nonlinear loss and gain (in other words, a class of PT-invariant Hamiltonians in which both the harmonic and anharmonic parts are non-Hermitian). For a basic system in the form of a dimer, symmetric and asymmetric eigenstates, including multistable ones, are found analytically. We demonstrate that, if coupled to a linear chain, such a nonlinear PT-symmetric dimer generates previously unexplored types of nonlinear Fano resonances, with completely suppressed or greatly amplified transmission, as well as a regime similar to the electromagnetically induced transparency. The implementation of the systems is possible in various media admitting controllable linear and nonlinear amplification of waves.

Invited Article: Broadband highly efficient dielectric metadevices for polarization control

Metadevices based on dielectric nanostructured surfaces with both electric and magnetic Mie-type resonances have resulted in the best efficiency to date for functional flat optics with only one disadvantage: a narrow operational bandwidth. Here we experimentally demonstrate broadband transparent all-dielectric metasurfaces for highly efficient polarization manipulation. We utilize the generalized Huygens principle, with a superposition of the scattering contributions from several electric and magnetic multipolar modes of the constituent meta-atoms, to achieve destructive interference in reflection over a large spectral bandwidth. By employing this novel concept, we demonstrate reflectionless (~90% transmission) half-wave plates, quarter-wave plates, and vector beam q-plates that can operate across multiple telecom bands with ~99% polarization conversion efficiency.