Suppression of scattering for small dielectric particles: anapole mode and invisibility

Dynamic modulation of electric and magnetic toroidal dipole resonance and light trapping in Si-GSST hybrid metasurfaces

The weak coupling of a toroidal dipole (TD) to an electromagnetic field offers great potential for the advanced design of photonic devices. However, simultaneous excitation of electric toroidal dipoles (ETDs) and magnetic toroidal dipoles (MTDs) is currently difficult to achieve. In this work, we propose a hybrid metasurface based on Si and phase transition material G e 2 S b 2 S e 4 T e 1 (GSST), which is formed by four Si columns surrounding a GSST column and can simultaneously excite two different TD (ETD and MTD) resonances. We also calculated the electric field distribution, magnetic field distribution, and multipole decomposition of the two resonances, and the results show that the two modes are ETD resonance and MTD resonance, respectively. The polarization characteristics of these two modes are also investigated, and the average field enhancement factor (EF) of the two modes is calculated. The dynamic modulation of the relative transmission and EF is also achieved based on the tunable properties of the phase change material GSST. Our work provides a way to realize actively tunable TD optical nanodevices.

Low-Variance Surface-Enhanced Raman Spectroscopy Using Confined Gold Nanoparticles over Silicon Nanocones

Surface-enhanced Raman spectroscopy (SERS) substrates are of utmost interest in the analyte detection of biological and chemical diagnostics. This is primarily due to the ability of SERS to sensitively measure analytes present in localized hot spots of the SERS nanostructures. In this work, we present the formation of 67 ± 6 nm diameter gold nanoparticles supported by vertically aligned shell-insulated silicon nanocones for ultralow variance SERS. The nanoparticles are obtained through discrete rotation glancing angle deposition of gold in an e-beam evaporating system. The morphology is assessed through focused ion beam tomography, energy-dispersive X-ray spectroscopy, and scanning electron microscopy. The optical properties are discussed and evaluated through reflectance measurements and finite-difference time-domain simulations. Lastly, the SERS activity is measured by benzenethiol functionalization and subsequent Raman spectroscopy in the surface scanning mode. We report a homogeneous analytical enhancement factor of 2.2 ± 0.1 × 10⁷ (99% confidence interval for N = 400 grid spots) and made a comparison to other lithographically derived assemblies used in SERS. The strikingly low variance (4%) of our substrates facilitates its use for many potential SERS applications.

Directional Scattering Switching from an All-Dielectric Phase Change Metasurface

All-dielectric metasurfaces are a blooming field with a wide range of new applications spanning from enhanced imaging to structural color, holography, planar sensors, and directionality scattering. These devices are nanopatterned structures of sub-wavelength dimensions whose optical behavior (absorption, reflection, and transmission) is determined by the dielectric composition, dimensions, and environment. However, the functionality of these metasurfaces is fixed at the fabrication step by the geometry and optical properties of the dielectric materials, limiting their potential as active reconfigurable devices. Herein, a reconfigurable all-dielectric metasurface based on two high refractive index (HRI) materials like silicon (Si) and the phase-change chalcogenide antimony triselenide (Sb₂Se₃) for the control of scattered light is proposed. It consists of a 2D array of Si-Sb₂Se₃-Si sandwich disks embedded in a SiO₂ matrix. The tunability of the device is provided through the amorphous-to-crystalline transition of Sb₂Se₃. We demonstrate that in the Sb₂Se₃ amorphous state, all the light can be transmitted, as it is verified using the zero-backward condition, while in the crystalline phase most of the light is reflected due to a resonance whose origin is the contribution of the electric (ED) and magnetic (MD) dipoles and the anapole (AP) of the nanodisks. By this configuration, a contrast in transmission (ΔT) of 0.81 at a wavelength of 980 nm by governing the phase of Sb₂Se₃ can be achieved.

Ideal Kerker scattering by homogeneous spheres: the role of gain or loss

We investigate how the optical gain or loss (characterized by isotropic complex refractive indexes) influence the ideal Kerker scattering of exactly zero backward scattering. It was previously shown that, for non-magnetic homogeneous spheres with incident plane waves, either gain or loss prohibit ideal Kerker scattering, provided that only electric and magnetic multipoles of a specific order are present and contributions from other multipoles can all be made precisely zero. Here we reveal that, when two multipoles of a fixed order are perfectly matched in terms of both phase and magnitude, multipoles of at least the next two orders cannot possibly be tuned to be all precisely zero or even perfectly matched, and consequently cannot directly produce ideal Kerker scattering. Moreover, we further demonstrate that, when multipoles of different orders are simultaneously taken into consideration, loss or gain can serve as helpful rather than harmful contributing factors, for the elimination of backward scattering.

Enhanced Thermo-optical Response by Means of Anapole Excitation

High refractive index (HRI) dielectric nanostructures offer a versatile platform to control the light-matter interaction at the nanoscale as they can easily support electric and magnetic modes with low losses. An additional property that makes them extraordinary is that they can support low radiative modes, so-called anapole modes. In this work, we propose a spectrally tunable anapole nanoheater based on the use of a dielectric anapole resonator. We show that a gold ring nanostructure, a priori nonresonant, can be turned into a resonant unit by just filling its hole with an HRI material supporting anapole modes, resulting in a more efficient nanoheater able to amplify the photothermal response of the bare nanoring. As proof of concept, we perform a detailed study of the thermoplasmonic response of a gold nanoring used as heating source and a silicon disk, designed to support anapole modes, located in its center acting as an anapolar resonator. Furthermore, we utilize the anapole excitation to easily shift the thermal response of these structures from the shortwave infrared range to the near-infrared range.

Transverse Kerker effect in all-dielectric spheroidal particles

Kerker effect is one of the unique phenomena in modern electrodynamics. Due to overlapping of electric and magnetic dipole moments, all-dielectric particles can be invisible in forward or backward directions. In our paper we propose new conditions between resonantly excited electric dipole and magnetic quadrupole in ceramic high index spheroidal particles for demonstrating transverse Kerker effect. Moreover, we perform proof-of-concept microwave experiment and demonstrate dumbbell radiation pattern with suppressed scattering in both forward and backward directions and enhanced scattering in lateral directions. Our concept is promising for future planar lasers, nonreflected metasurface and laterally excited waveguides and nanoantennas.

Colossal magnetic fields in high refractive index materials at microwave frequencies

Resonant scattering of electromagnetic waves is a widely studied phenomenon with a vast range of applications that span completely different fields, from astronomy or meteorology to spectroscopy and optical circuitry. Despite being subject of intensive research for many decades, new fundamental aspects are still being uncovered, in connection with emerging areas, such as metamaterials and metasurfaces or quantum and topological optics, to mention some. In this work, we demonstrate yet one more novel phenomenon arising in the scattered near field of medium sized objects comprising high refractive index materials, which allows the generation of colossal local magnetic fields. In particular, we show that GHz radiation illuminating a high refractive index ceramic sphere creates instant magnetic near-fields comparable to those in neutron stars, opening up a new paradigm for creation of giant magnetic fields on the millimeter's scale.

Radiationless anapole states in on-chip photonics

High-index nanoparticles are known to support radiationless states called anapoles, where dipolar and toroidal moments interfere to inhibit scattering to the far field. In order to exploit the striking properties arising from these interference conditions in photonic integrated circuits, the particles must be driven in-plane via integrated waveguides. Here, we address the excitation of electric anapole states in silicon disks when excited on-chip at telecom wavelengths. In contrast to normal illumination, we find that the anapole condition-identified by a strong reduction of the scattering-does not overlap with the near-field energy maximum, an observation attributed to retardation effects. We experimentally verify the two distinct spectral regions in individual disks illuminated in-plane from closely placed waveguide terminations via far-field and near-field measurements. Our finding has important consequences concerning the use of anapole states and interference effects of other Mie-type resonances in high-index nanoparticles for building complex photonic integrated circuitry.

Non-isolated sources of electromagnetic radiation by multipole decomposition for photonic quantum technologies on a chip with nanoscale apertures

The creation of single photon sources on a chip is a mid-term milestone on the road to chip-scale quantum computing. An in-depth understanding of the extended multipole decomposition of non-isolated sources of electromagnetic radiation is not only relevant for a microscopic description of fundamental phenomena, such as light propagation in a medium, but also for emerging applications such as single-photon sources. To design single photon emitters on a chip, we consider a ridge dielectric waveguide perturbed with a cylindrical inclusion. For this, we expanded classical multipole decomposition that allows simplifying and interpreting complex optical interactions in an intuitive manner to make it suitable for analyzing light-matter interactions with non-isolated objects that are parts of a larger network, e.g. individual components such as a single photon source of an optical chip. It is shown that our formalism can be used to design single photon sources on a chip.

Manifestation of extremely high-Q pseudo-modes in scattering of a Bessel light beam by a sphere

The exact analytical solution of Maxwell equations for a Bessel light beam scattered by a sphere is found. Scattered power, stored energy, and a generalized Q factor as a function of frequency, the sphere radius, permittivity, and the Bessel beam angle are found. On the basis of this solution, modes and pseudo-modes of a dielectric sphere are extracted by calculation of the generalized Q factor. It is shown that an appropriate choice of Bessel beam parameters can provide excitation of a single given mode and an unlimited value of the radiative Q factor of pseudo-modes.

Selective magnetic responses of silicon nanoparticles modulated by waveguide structures

High-refractive-index nanoparticles (NPs), such as silicon NPs, were considered as effective carriers in their response to a magnetic field at optical frequencies. Such NPs play an important role in many state-of-the-art technologies in nano-optics. Although the resonance properties of these NPs when varying their structural parameters have been studied intensely in the past few years, their interaction with the underlying substrate has seldom been discussed, in particular, when the substrate is a waveguide structure that significantly modulates the optical responses of the NPs. We proposed and studied a selective magnetic coupling system comprising a Si-NP on a metal-dielectric waveguide (MDW). The MDW structure supports either a transverse electric (TE) or a transverse magnetic (TM) mode that induces a large polarization dependence in the magnetic resonance. A new manifestation of the optical spin Hall effect was demonstrated in which a vertical rotating magnetic dipole excites a TE-type waveguide mode with a specific unidirectional emission. Making use of this polarization response, we developed a scanning imaging system that can selectively map the transverse or longitudinal magnetic field component of a focused beam depending on the type of MDW used in the system. This selective magnetic resonance coupling system is expected to be valuable for studying the fundamental interactions between the magnetic field and matter and for developing related nano-applications.

Light emission driven by magnetic and electric toroidal dipole resonances in a silicon metasurface

Dielectric nanoparticles supporting pronounced toroidal and anapole resonances have enabled a new class of optical antennas with unprecedented functionalities. In this work, we propose a light-emitting silicon metasurface which simultaneously supports both magnetic toroidal dipole and electric toroidal dipole resonances in the near-infrared region. The metasurface consists of a square array of split nanodisks with embedded germanium quantum dots. By varying the width of the split air-gap, the spectral positions and quality factors of the two toroidal dipoles are flexibly tuned. Large photoluminescence enhancement is experimentally demonstrated at the toroidal resonances, which is attributed to the unique near- and far-field characteristics of the resonant modes. Moreover, the light emissions driven by the two toroidal dipoles are of different polarization, which further suggests versatile polarization-engineered radiation properties. Our work shows enormous potential in light emission manipulation and provides a route for high-efficiency, ultra-compact LEDs and potentially functional dielectric metasurface lasers.

Nonradiating anapole states in nanophotonics: from fundamentals to applications

Nonradiating sources are nontrivial charge-current distributions that do not generate fields outside the source domain. The pursuit of their possible existence has fascinated several generations of physicists and triggered developments in various branches of science ranging from medical imaging to dark matter. Recently, one of the most fundamental types of nonradiating sources, named anapole states, has been realized in nanophotonics regime and soon spurred considerable research efforts and widespread interest. A series of astounding advances have been achieved within a very short period of time, uncovering the great potential of anapole states in many aspects such as lasing, sensing, metamaterials, and nonlinear optics. In this review, we provide a detailed account of anapole states in nanophotonics research, encompassing their basic concepts, historical origins, and new physical effects. We discuss the recent research frontiers in understanding and employing optical anapoles and provide an outlook for this vibrant field of research.

Multipolar passive cloaking by nonradiating anapole excitation

In this paper, we demonstrate the relation between cloaking effect and its nonradiating state by considering the destructive multipolar interaction between near-field scattering by bare object and surrounding coating located in its proximity. This cloaking effect is underpinned by anapole mode excitation and it occurs as destructive interference between the electric dipole moment, generated by a bare object (here a central metallic scatterer) and the toroidal moment, formed inside the cloak (a surrounding cluster of dielectric cylinders). Numerical results show how a cloaking effect based on the formation of the anapole mode can lead to an overall nonradiating metamolecule with all-dielectric materials in the coating region.

Anapoles in Free-Standing III-V Nanodisks Enhancing Second-Harmonic Generation

Nonradiating electromagnetic configurations in nanostructures open new horizons for applications due to two essential features: a lack of energy losses and invisibility to the propagating electromagnetic field. Such radiationless configurations form a basis for new types of nanophotonic devices, in which a strong electromagnetic field confinement can be achieved together with lossless interactions between nearby components. In our work, we present a new design of free-standing disk nanoantennas with nonradiating current distributions for the optical near-infrared range. We show a novel approach to creating nanoantennas by slicing III-V nanowires into standing disks using focused ion-beam milling. We experimentally demonstrate the suppression of the far-field radiation and the associated strong enhancement of the second-harmonic generation from the disk nanoantennas. With a theoretical analysis of the electromagnetic field distribution using multipole expansions in both spherical and Cartesian coordinates, we confirm that the demonstrated nonradiating configurations are anapoles. We expect that the presented procedure of designing and producing disk nanoantennas from nanowires becomes one of the standard approaches to fabricating controlled chains of standing nanodisks with different designs and configurations. These chains can be essential building blocks for new types of lasers and sensors with low power consumption.

Excitation of nonradiating magnetic anapole states with azimuthally polarized vector beams

Nonradiating current configurations have been drawing the attention of the physics community for many years. It has been demonstrated recently that dielectric nanoparticles provide a unique platform to host such nonradiating modes, called "anapoles". Here we study theoretically the excitation of such exotic anapole modes in silicon nanoparticles using structured light. Alternative illumination configurations, properly designed, are able to unlock hidden behavior of scatterers. Particularly, azimuthally polarized focused beams enable us to excite ideal anapole modes of magnetic type in dielectric nanoparticles. Firstly, we perform the decomposition of this type of excitation into its multipolar content and then we employ the T-matrix method to calculate the far-field scattering properties of nanoparticles illuminated by such beams. We propose several configuration schemes where magnetic anapole modes of simple or hybrid nature can be detected in silicon nanospheres, nanodisks and nanopillars.

Metamaterial engineered transparency due to the nullifying of multipole moments

Here we propose, to the best of our knowledge, a novel transparency effect in cylindrical all-dielectric metamaterials. We show that the cancellation of multipole moments of the same kind leads to almost zero radiation losses in all-dielectric metamaterials due to the counter-directed multipolar moments in metamolecule. The nullifying of multipoles, mainly dipoles, and suppression of higher multipoles, results in the ideal transmission of an incident wave through the designed metamaterial. The observed effect could pave the road to the new generation of light-manipulating transparent metadevices such as filters, waveguides, and cloaks.