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We present an optical metasurface with symmetrical individual elements sustaining Fano resonances with high Q-factors. This study combines plane-wave illumination and modal analysis to investigate the resonant behavior that results in a suppression of the forward scattering, and we investigate the role of the lattice constant on the excited multipoles and on the spectral position and Q-factor of the Fano resonances, revealing the nonlocal nature of the resonances. The results show that the intrinsic losses play a crucial role in modulating the resonance amplitude in specific conditions and that the optical behavior of the device is extremely sensitive to the pitch of the metasurface. The findings highlight the importance of near-neighbor interactions to achieve high Q resonances and offer an important tool for the design of spectrally tunable metasurfaces using simple geometries. [ABSTRACT FROM AUTHOR]

We present an optical metasurface with symmetrical individual elements sustaining Fano resonances with high Q-factors. This study combines plane-wave illumination and modal analysis to investigate the resonant behavior that results in a suppression of the forward scattering, and we investigate the role of the lattice constant on the excited multipoles and on the spectral position and Q-factor of the Fano resonances, revealing the nonlocal nature of the resonances. The results show that the intrinsic losses play a crucial role in modulating the resonance amplitude in specific conditions and that the optical behavior of the device is extremely sensitive to the pitch of the metasurface. The findings highlight the importance of near-neighbor interactions to achieve high Q resonances and offer an important tool for the design of spectrally tunable metasurfaces using simple geometries.

Abstract Freely switching light transmission and absorption via an achromatic reflectionless screen is highly desired for many photonic applications (e.g., energy-harvesting, cloaking, etc.), but available meta-devices often exhibit reflections out of their narrow working bands. Here, we rigorously demonstrate that an optical metasurface formed by two resonator arrays coupled vertically can be perfectly reflectionless at all frequencies below the first diffraction mode, when the near-field (NF) and far-field (FF) couplings between two constitutional resonators satisfy certain conditions. Tuning intrinsic loss of the system can further modulate the ratio between light transmission and absorption, yet keeping reflection diminished strictly. Designing/fabricating a series of metasurfaces with different inter-resonator configurations, we experimentally illustrate how varying inter-resonator NF and FF couplings can drive the system to transit between different phase regions in a generic phase diagram. In particular, we experimentally demonstrate that a realistic metasurface satisfying the discovered criteria exhibits the desired achromatic reflectionless property within 160–220 THz (0–225 THz in simulation), yet behaving as a perfect absorber at ~ 203 THz. Our findings pave the road to realize meta-devices exhibiting designable transmission/absorption spectra immune from reflections, which may find many applications in practice.

Nanophotonic phenomena, such as zero optical back scattering, nonradiating anapole states, etc. are related to the excitation of single dipolar modes—hence so far, they have only been observed within a few relatively high‐index dielectric materials (refractive index, n > 3.5) in the nanoscale regime at the optical frequencies. Here, dipolar regime is unraveled, close‐to‐zero backscattering is demonstrated, and optical anapoles are excited in mid‐index dielectric spheres (titanium di‐oxide, TiO2; n ≈ 2.6) at the mesoscale regime (particle diameter, d ≈ incident wavelength, λ) under illumination with tightly focused Gaussian beams (TFGBs). Successive scattering minima associated with dipolar excitation are observed satisfying the first Kerker condition in the scattering spectra of single TiO2 spheres with diameters in the micrometer range. Moreover, at specific wavelengths, the electric and magnetic dipolar scattering amplitudes of the dielectric microspheres simultaneously go close‐to‐zero, leading to the excitation of hybrid optical anapoles with a total scattering intensity ≈ 5 times weaker for TFGB illumination with numerical aperture, NA ≈ 0.95 compared to NA ≈ 0.1. The result pushes the boundary of the observation of nanophotonic phenomena to a new regime with regards to type and size of the materials. [ABSTRACT FROM AUTHOR]

Freely switching light transmission and absorption via an achromatic reflectionless screen is highly desired for many photonic applications (e.g., energy-harvesting, cloaking, etc.), but available meta-devices often exhibit reflections out of their narrow working bands. Here, we rigorously demonstrate that an optical metasurface formed by two resonator arrays coupled vertically can be perfectly reflectionless at all frequencies below the first diffraction mode, when the near-field (NF) and far-field (FF) couplings between two constitutional resonators satisfy certain conditions. Tuning intrinsic loss of the system can further modulate the ratio between light transmission and absorption, yet keeping reflection diminished strictly. Designing/fabricating a series of metasurfaces with different inter-resonator configurations, we experimentally illustrate how varying inter-resonator NF and FF couplings can drive the system to transit between different phase regions in a generic phase diagram. In particular, we experimentally demonstrate that a realistic metasurface satisfying the discovered criteria exhibits the desired achromatic reflectionless property within 160–220 THz (0–225 THz in simulation), yet behaving as a perfect absorber at ~ 203 THz. Our findings pave the road to realize meta-devices exhibiting designable transmission/absorption spectra immune from reflections, which may find many applications in practice. [ABSTRACT FROM AUTHOR]

Metasurfaces have the ability to control classical and non‐classical states of light to achieve controlled emission even at the level of a single emitter. Here, the Kerker condition induced emission rate enhancement with strong directivity is unveiled from a single emitter integrated within a dielectric metasurface consisting of silicon nano‐disks. The simulation and analytical calculations attest the Kerker condition with unidirectional light scattering evolved by the constructive interference between electric dipole, toroidal dipole, and the magnetic quadrupole. The results evince spatially‐dependent enhanced local density of optical states, which reciprocates localized field intensity. The emission rate enhancement of 400 times is achieved close to the zero phonon line of the nitrogen‐vacancy center with superior emission directivity and collection efficiency. The results have implications in on‐demand single photon generation, spin‐photon interface, many‐body interactions, and strong coupling. [ABSTRACT FROM AUTHOR]

Interference between multipole modes of Mie resonance provides the possibility to tailor a radiation pattern of a subwavelength optical resonator. Unidirectional scattering arising from constructive and destructive interferences between electric and magnetic dipolar modes in a nanoantenna allows to design a variety of metasurfaces. However, experimental determination of radiation patterns of an individual nanoobject and the decomposition into multipoles have not been performed under plane wave illumination due to the inability of angular analysis in a conventional optical microscope. To this end, an angle‐, polarization‐, and wavelength‐resolved microscopy setup is developed in this work to measure radiation patterns of an individual nanoobject. A single spherical silicon nanosphere is employed as an ideal Mie resonator and the angle‐ and polarization‐resolved scattering is measured in the angular range from 30° to 150° against the incident angle. It is shown that Mie scattering from the electric and magnetic dipoles of a single silicon nanosphere is selectively measured in the setup. [ABSTRACT FROM AUTHOR]

Nanostructured materials have provided new freedoms for tailoring thermal emissivity in an ultracompact footprint. However, while many designs have demonstrated large absorption amplitude, this often comes at the expense of angular dispersion or the need for a reflective backplane. Furthermore, metal‐based designs generally have broad linewidths due to large nonradiative damping. Here, a unique approach is outlined for obtaining narrowband near‐unity thermal emissivity through spectrally overlapping two orthogonal lossy Mie‐type modes in dielectric metasurfaces operating in the long‐wave infrared. Operating near the transverse optic phonon frequency of a polar dielectric provides a suitably lossy environment, but also one featuring extremely large permittivity, enabling deeply subwavelength resonator sizes, insensitivity to the angle of incidence, and large quality factors even while operating in the long‐wave infrared. Balancing the oscillator strengths and optical losses of the two overlapped resonances maximizes the absorption, resulting in a 3C‐SiC metasurface with 78% absorptance and a quality factor of 170. This balanced metasurface possesses a quality factor on par with some of the narrowest band thermal emitters in the long‐wave infrared, while possessing a simple architecture and prospects for unity emissivity. [ABSTRACT FROM AUTHOR]

Parity-time (PT) symmetric photonic systems have attracted much attention due to their intriguing properties and asymmetric behaviors. In this paper, we propose a plasmonic nanoantenna with PT-symmetric potential for unidirectional scattering functionality. The studied plasmonic nanoantenna is comprised of three metallic layers separated by two dielectric layers. Such kind of system, with the same coefficient κ of loss and gain in each of the two dielectric layers, holds the characteristic of PT symmetry. We show that the unidirectional scattering is obtained for the passive structure (i.e., κ = 0), and the switching between forward and backward directionality can be achieved with a single structure by changing the excitation wavelength, when the induced electric dipole (ED) and magnetic dipole (MD) modes satisfy the first or second Kerker conditions, respectively. In addition, we find that the forward-to-backward ratio spectra can be strongly affected by the non-Hermiticity parameter κ. In particular, it is possible to reverse the radiation direction at the same wavelength in a wide spectra band by adjusting κ. Moreover, putting the nanoantennas in an array of transverse configuration can efficiently narrow the main lobe angular beam width to be

Optical quantum emitters near nanostructures have access to additional relaxation channels and thus exhibit structure-dependent emission properties, including quantum yield and emission directionality. A well-engineered quantum emitter-plasmonic nanostructure hybrid can be considered as an optical meta-emitter consisting of a transmitting nanoantenna driven by an optical-frequency generator. In this work, the DNA origami fabrication method is used to construct ultracompact unidirectional meta-emitters composed of a plasmonic trimer nanoantenna driven by a single dye molecule. The origami is designed to bring the dye to the gap to simultaneously excite the electric and magnetic dipole modes of the trimer nanoantenna. The interference of these modes fulfills the Kerker condition at the fluorophore's emission band, enabling unidirectional emission. We report unidirectional emission from a single molecule with a front-to-back ratio of up to 10.7 dB accompanied by a maximum emission enhancement of 23-fold.

To achieve efficient light control at subwavelength dimensions, plasmonic and all-dielectric nanoparticles have been utilized both as a single element as well as in the arrays. Here we study 2D periodic nanoparticle arrays (metasurfaces) that support lattice resonances near the Rayleigh anomaly due to the electric dipole (ED) and magnetic dipole (MD) resonant coupling between the nanoparticles. Silicon and core-shell particles are considered. Our investigations are carried out using two independent numerical techniques, namely, the finite-element method and the method of coupled-dipole equations based on the Green function approach. We numerically demonstrate that choosing of lattice periods independently in each mutual-perpendicular direction, it is possible to achieve a full overlap between the ED-lattice resonance and MD resonances of nanoparticles in certain spectral range and to realize the resonant lattice Kerker effect (resonant suppression of the backward scattering or reflection). At the effect conditions, the strong suppression of light reflectance in the structure is appeared due to destructive interference between electromagnetic waves scattered by ED and MD moments of every nanoparticle in the backward direction with respect to the incident light wave. Influence of the array size on the revealed reflectance and transmittance behavior is discussed. The resonant lattice Kerker effect based on the overlap of both ED and MD lattice resonances is also demonstrated. [ABSTRACT FROM AUTHOR]

Dielectric metasurfaces that exploit the different Mie resonances of nanoscale dielectric resonators are a powerful platform for manipulating electromagnetic fields and can provide novel optical behavior. In this work, we experimentally demonstrate independent tuning of the magnetic dipole resonances relative to the electric dipole resonances of split dielectric resonators (SDRs). By increasing the split dimension, we observe a blue shift of the magnetic dipole resonance toward the electric dipole resonance. Therefore, SDRs provide the ability to directly control the interaction between the two dipole resonances within the same resonator. For example, we achieve the first Kerker condition by spectrally overlapping the electric and magnetic dipole resonances and observe significantly suppressed backward scattering. Moreover, we show that a single SDR can be used as an optical nanoantenna that provides strong unidirectional emission from an electric dipole source. [ABSTRACT FROM AUTHOR]

Phase change materials, such as GeTe that exhibits reversible phase transitions between amorphous and crystalline states, are promising for reconfigurable photonic devices. Here we propose a nanoparticle composed of Au core and GeTe shell that can realize active tuning of directional scattering by adjusting the crystallinities of the GeTe shell to manipulate the electric and magnetic Mie responses of the nanoparticle. The scattering, the far-field radiation patterns and the ratio of the forward (backward) to backward (forward) scattering cross section of the hybridized nanoparticle are calculated analytically with Mie theory. The theoretical results show that when the GeTe shell is switched from amorphous phase to crystalline phase, the phase differences between the overlapped electric and magnetic modes can be changed from 0 to π. As a result, the Kerker's first and second conditions are satisfied simultaneously, leading to tunable forward and backward scattering in near-infrared wavelength. Our results are useful for designing reconfigurable optical nanoantennas that may find applications in compact photonic circuits and all-optical wireless communications. • Active tuning of the directional scattering of hybridized nanoparticles. • The directional scattering of core–shell nanoparticles loaded with the PCMs. • The directional scattering is manipulated flexibly in near-infrared wavelength. • The directional forward and backward scattering are realized simultaneously. [ABSTRACT FROM AUTHOR]