Controlling magnetic and electric dipole modes in hollow silicon nanocylinders

Multiple Scattering-Enhanced Fluorescence Within Randomly Oriented Low-Index Polymer Nanofiber Sensors

Fluorescence enhancement technologies play a crucial role in biological and chemical sensors. Currently, effective fluorescence sensors primarily rely on noble metals and high-index dielectric nanostructures. While effective, they are plagued by optical losses and complex fabrication processes. In contrast, low-index material nanostructures offer significant advantages, including the absence of optical losses, ease of fabrication, and cost-effectiveness, but they face the challenge of weaker electric field enhancement. Here, we designed a low-index, randomly oriented polyvinyl acetate (PVAc) nanofiber sensor via scalable electrospinning, enabling multiple scattering within the disordered nanofibers and resulting in an impressive surface-enhanced fluorescence factor of 1170. This sensor achieves a detection limit for rhodamine 6G as low as 7.24 fM, outperforming the reported fluorescence biosensors. Further results of photoluminescence decay dynamics and random lasing validate the effectiveness of multiple scattering in enhancing fluorescence within the polymer nanofiber sensor. With its excellent performance and scalable production process, this randomly oriented, low-index polymer nanofiber sensor offers a promising new pathway for efficient surface-enhanced fluorescence based on multiple scattering. Furthermore, PVAc nanofibers can be extended to other low-index materials capable of forming randomly oriented nanostructures, offering significant potential for cost-effective, high-performance fluorescence sensor applications.

Dark-field microscopy studies of single silicon nanoparticles fabricated by e-beam evaporation technique: effect of thermal annealing, polarization of light and deposition parameters

Herein, we report the dark-field microscopy studies on single silicon nanoparticles (SiNPs) fabricated using different deposition parameters in the electron beam evaporation technique. The morphology of the fabricated SiNPs is studied using theAtomic Force Microscope. Later, for the first time, the effect of thermal annealing and deposition parameters (i.e. beam current and deposition time) on the far-field scattering images and spectra of single SiNPs is studied using a transmission-mode dark-field optical microscope to estimate the wavelength locations and full-width at half maxima of the optical resonances of single SiNPs. Finally, the role of polarization of incident light on the optical resonances of single SiNPs is also studied by recording their scattering images and spectra.

Enhancing surface sensing performance of cascaded high contrast gratings using bound states in the continuum

We proposed the cascaded high contrast grating (CHCG) structure to enhance surface sensing capabilities through bound states in the continuum (BICs). Utilizing the finite element method (FEM) and rigorous coupled-wave analysis (RCWA), we studied the dispersion relations, far-field contribution CHCGs, and near-field distributions of BICs corresponding to resonance peaks at different wavelengths. Results demonstrate the ability to precisely control symmetry-protected BIC (SP-BIC) and Friedrich-Wintgen BIC (FW-BIC) resonance peaks by altering incident angles and structural parameters, enhancing structure robustness and tunability. Significantly, modes 1 and 2 have demonstrated substantial enhancement in surface refractive index sensing, achieving highest sensitivities at 51 nm/RIU and the figure of merit reaching 490.8 RIU-1, indicating notable advancement in detecting subtle surface changes. In contrast, mode 3 has shown robust performance in bulk refractive index sensing, achieving a sensitivity of 602 nm/RIU and a figure of merit of 5189.65 RIU-1. These findings underscore the significant potential of the structure as a high-performance integrated sensor, particularly for precise environmental and biological monitoring in surface refractive index sensing.

Efficient and accurate numerical-projection of electromagnetic multipoles for scattering objects

In this paper, we develop an efficient and accurate procedure of electromagnetic multipole decomposition by using the Lebedev and Gaussian quadrature methods to perform the numerical integration. Firstly, we briefly review the principles of multipole decomposition, highlighting two numerical projection methods including surface and volume integration. Secondly, we discuss the Lebedev and Gaussian quadrature methods, provide a detailed recipe to select the quadrature points and the corresponding weighting factor, and illustrate the integration accuracy and numerical efficiency (that is, with very few sampling points) using a unit sphere surface and regular tetrahedron. In the demonstrations of an isotropic dielectric nanosphere, a symmetric scatterer, and an anisotropic nanosphere, we perform multipole decomposition and validate our numerical projection procedure. The obtained results from our procedure are all consistent with those from Mie theory, symmetry constraints, and finite element simulations.

Multipolar silicon-based resonant meta-surface for electro-optical modulation and sensing

A multipolar silicon-based resonant meta-surface scheme is proposed and numerically presented via intercalating oblique slits into the silicon patches, leading to an ultra-sharp resonant spectrum via the excitation of electric and magnetic quadrupoles and their hybridization coupling. High-performance electro-optical modulator is demonstrated, showing a spectrally shifted modulation sensitivity up to 1.546 nm/V. Moreover, novel, to the best of our knowledge, optical sensing for ion solution concentration with the detection limitation down to 5.15 × 10^-3 is demonstrated as another application. These findings provide an impressive strategy for resonant silicon-based nano-photonics and opto-electronic devices.

Giant localized electromagnetic field of highly doped silicon plasmonic nanoantennas

In this work, we present the analysis and design of an efficient nanoantenna sensor based on localized surface plasmon resonance (LSPR). A high refractive index dielectric nanostructure can exhibit strong radiation resonances with high electric field enhancement inside the gap. The use of silicon instead of metals as the material of choice in the design of such nanoantennas is advantageous since it allows the integration of nanoantenna-based structures into integrated-optoelectronics circuits manufactured using common fabrication methods in the electronic industry. It also allows the suggested devices to be mass-produced at a low cost. The proposed nanoantenna consists of a highly doped silicon nanorod and is placed on a dielectric substrate. Different shapes and different concentrations of doping for the nanoantenna structures that are resonant in the mid-infrared region are investigated and numerically analyzed. The wavelength of the enhancement peak as well as the enhancement level itself vary as the surrounding material changes. As a result, sensors may be designed to detect molecules via their characteristic vibrational transitions. The 3D FDTD approach via Lumerical software is used to obtain the numerical results. The suggested nanoantennas exhibit ultra-high local field enhancement inside the gap of the dipole structure.

Probing the Optical Response and Local Dielectric Function of an Unconventional Si@MoS2 Core-Shell Architecture

Heterostructures of optical cavities and quantum emitters have been highlighted for enhanced light-matter interactions. A silicon nanosphere, core, and MoS₂, shell, structure is one such heterostructure referred to as the core@shell architecture. However, the complexity of the synthesis and inherent difficulties to locally probe this architecture have resulted in a lack of information about its localized features limiting its advances. Here, we utilize valence electron energy loss spectroscopy (VEELS) to extract spatially resolved dielectric functions of Si@MoS₂ with nanoscale spatial resolution corroborated with simulations. A hybrid electronic critical point is identified ∼3.8 eV for Si@MoS₂. The dielectric functions at the Si/MoS₂ interface is further probed with a cross-sectioned core-shell to assess the contribution of each component. Various optical parameters can be defined via the dielectric function. Hence, the methodology and evolution of the dielectric function herein reported provide a platform for exploring other complex photonic nanostructures.

Substrate-Modulated Electric and Magnetic Resonances of Lithium Niobite Nanoparticles Illuminated by White Light

The manipulation of light at the nanoscale is important for nanophotonic research. Lithium niobite (LiNbO₃), as an ideal building block for metamaterials, has attracted great interest for its unique properties in the field of nonlinear optics. In this paper, we numerically studied the effect of different substrates on the optical resonances of a LiNbO₃ nanoparticle. The results show that the electric and magnetic resonances of such a system can be effectively adjusted by changing the substrate. Compared to the impact of dielectric substrate, the interaction between the LiNbO₃ nanoparticle and the Au film shows a fascinating phenomenon that a sharp resonance peak appears. The multipole decomposition of the scattering spectrum shows that the size, shape of the LiNbO₃ nanoparticle, and the thickness of the SiO₂ film between the particle and the Au film have a significant impact on the electromagnetic resonance of the LiNbO₃ nanoparticle. This work provides a new insight into LiNbO₃ nanoparticles, which may have potential use in the design of dielectric nanomaterials and devices.

Near-Field Mapping of Photonic Eigenmodes in Patterned Silicon Nanocavities by Electron Energy-Loss Spectroscopy

Recently, there has been significant interest in using dielectric nanocavities for the controlled scattering of light, owing to the diverse electromagnetic modes that they support. For plasmonic systems, electron energy-loss spectroscopy (EELS) is now an established method enabling structure-optical property analysis at the scale of the nanostructure. Here, we instead test its potential for the near-field mapping of photonic eigenmodes supported in planar dielectric nanocavities, which are lithographically patterned from amorphous silicon according to standard photonic principles. By correlating results with finite element simulations, we demonstrate how many of the EELS excitations can be directly corresponded to various optical eigenmodes of interest for photonic engineering. The EELS maps present a high spatial definition, displaying intensity features that correlate precisely to the impact parameters giving the highest probability of modal excitation. Further, eigenmode characteristics translate into their EELS signatures, such as the spatially and energetically extended signal of the low Q-factor electric dipole and nodal intensity patterns emerging from excitation of toroidal and second-order magnetic modes within the nanocavity volumes. Overall, the spatial-spectral nature of the data, combined with our experimental-simulation toolbox, enables interpretation of subtle changes in the EELS response across a range of nanocavity dimensions and forms, with certain simulated resonances matching the excitation energies within ±0.01 eV. By connecting results to far-field simulations, perspectives are offered for tailoring the nanophotonic resonances via manipulating nanocavity size and shape.

Multipolar scattering analysis of hybrid metal-dielectric nanostructures

We perform a systematic study showing the evolution of the multipoles along with the spectra for a hybrid metal-dielectric nanoantenna, a Si cylinder and an Ag disk stacked one on top of another, as its dimensions are varied one by one. We broaden our analysis to demonstrate the "magnetic light" at energies above 1 eV by varying the height of the Ag on the Si cylinder and below 1 eV by introducing insulating spacing between them. We also explore the appearance of the anapole state along with some exceptionally narrow spectral features by varying the radius of the Ag disk.

Novel silicon bipodal cylinders with controlled resonances and their use as beam steering metasurfaces

Metasurfaces have paved the way for high performance wavefront shaping and beam steering applications. Phase-gradient metasurfaces (PGM) are of high importance owing to the powerful and relatively systematic tool they offer for manipulating electromagnetic wave fronts and achieving various functionalities. Herein, we numerically present a novel unit cell known as bipodal cylinders (BPC), made of Silicon (Si) and placed on a Silicon dioxide (SiO2) substrate to be compatible with CMOS fabrication techniques and to avoid field leakage into a high index substrate. Owing to its geometrical structure, the BPC structure provides a promising unit cell for electromagnetic wave manipulation. We show that BPC offers a way to shift the electric dipole mode to a frequency higher than that of the magnetic dipole mode. We investigate the effect of varying different geometrical parameters on the performance of such unit cell. Building on that, a metasurface is then presented that can achieve efficient electromagnetic beam steering with high transmission of 0.84 and steering angle of 15.2°; with very good agreement with the theoretically predicted angle covering the whole phase range from 0 to 2[Formula: see text].

Flat Bands in Magic-Angle Bilayer Photonic Crystals at Small Twists

The new physics of magic-angle twisted bilayer graphene (TBG) motivated extensive studies of flat bands hosted by moiré superlattices in van der Waals structures, inspiring the investigations into their photonic counterparts with potential applications including Bose-Einstein condensation. However, correlation between photonic flat bands and bilayer photonic moiré systems remains unexplored, impeding further development of moiré photonics. In this work, we formulate a coupled-mode theory for low-angle twisted bilayer honeycomb photonic crystals as a close analogy of TBG, discovering magic-angle photonic flat bands with a non-Anderson-type localization. Moreover, the interlayer separation constitutes a convenient degree of freedom in tuning photonic moiré bands without high pressure. A phase diagram is constructed to correlate the twist angle and separation dependencies to the photonic magic angles. Our findings reveal a salient correspondence between fermionic and bosonic moiré systems and pave the avenue toward novel applications through advanced photonic band or state engineering.

Collective photonic response of high refractive index dielectric metasurfaces

Sub-wavelength periodic nanostructures give rise to interesting optical phenomena like effective refractive index, perfect absorption, cloaking, etc. However, such structures are usually metallic which results in high dissipative losses and limitations for use; therefore, dielectric nanostructures are increasingly considered as a strong alternative to plasmonic (metallic) materials. In this work, we show light-matter interaction in a high refractive index dielectric metasurface consisting of an array of cubic dielectric nano-structures made of very high refractive index material, Te in air, using computer modelling. We observe a distinct band-like structure in both transmission and reflection spectra resulting from the near-field coupling of the field modes from neighboring dielectric structures followed by a sharp peak in the transmission at higher frequencies. From the spatial distribution of the electric and magnetic fields and a detailed multipole analysis in both spherical harmonics and Cartesian components, the dominant resonant modes are identified to be electric and magnetic dipoles. Specifically at lower frequency (60 THz) a novel anapole-like state characterized by strong-suppression in reflection and absorption is observed, reported very recently as 'lattice-invisibility' state. Differently, at higher frequency (62 THz), strong absorption and near-zero far field scattering are observed, which combined with the field profiles and the multipole analysis of the near-fields indicate the excitation of an anapole. Notably the observed novel modes occur in the simple geometry of dielectric cubes and are a result of collective response of the metasurfaces. Periodicity of the cubic metasurface is shown as the significant material tuning parameter, allowing for the near-field and far-field coupling effects of anapole metasurface. More generally, our work is a contribution towards developing far-fetching applications based on metamaterials such as integrated devices and waveguides consisting of non-radiating modes.

Mie Resonant Structural Colors

Structural colors refer to colors produced by the interference of light scattered by judiciously arranged nano- or microscopic structures. In this Forum Article, we discuss the use of Mie resonant scattering in structural colors with dielectric and metal-dielectric hybrid structures to achieve notable figures of merit in pixel size and gamut range. Compared with plasmonic structures, resonant dielectric and hybrid structures are subjected to less loss while providing strong field confinement and large scattering cross sections, making them appealing for realizing vibrant colors at ultrahigh resolutions. We outline the basic principles behind Mie resonances in analytically solvable structures and highlight the relation between these resonances and color with demonstrations in dielectric metasurfaces. Mie resonant colors occurring in nonplanar designs including disordered systems are also explored. We review recent advances in dynamic and reversibly tunable Mie resonant colors and conclude by providing an outlook for future research directions.

Ultra-narrowband polarization insensitive transmission filter using a coupled dielectric-metal metasurface

A coupled dielectric-metal metasurface (CDMM) filter consisting of amorphous silicon (a-Si) rings and subwavelength holes in Au layer separated by a SiO₂ layer is presented. The design parameters of the CDMM filter is numerically optimized to have a polarization independent peak transmittance of 0.55 at 1540 nm with a Full Width at Half Maximum (FWHM) of 10 nm. The filter also has a 100 nm quiet zone with ∼10^-2 transmittance. A radiating two-oscillator model reveals the fundamental resonances in the filter which interfere to produce the electromagnetically induced transparency (EIT) like effect. Multipole expansion of the currents in the structure validates the fundamental resonances predicted by the two-oscillator model. The presented CDMM filter is robust to artifacts in device fabrication and has performances comparable to a conventional Fabry-Pérot filter. However, it is easier to be integrated in image sensors as the transmittance peak can be tuned by only changing the periodicity resulting in a planar structure with a fixed height.

Identical emission enhancement for arbitrary-orientation magnetic dipole emitters in silicon hollow nanocavity

Benefiting from low dissipative absorption, dielectric nanoparticles with high-refractive-index have become efficient platforms in producing magnetic dipole resonances and thus harvest functionality in enhancing spontaneous decay rate of optical magnetic dipole emitters, which is also named as magnetic Purcell effect. However, this effect is highly sensitive to emitter orientations, which brings difficulty in practical experiments. Here, we propose an effective method to suppress this orientation sensitivity based on an engineered silicon nanocavity. We will demonstrate that by balancing the resonant wavelengths and emission enhancement for two perpendicularly orientated MD emitters, the whole emission spectrum could be almost orientation-independent, thus accomplishing a completely isotropic magnetic Purcell effect. Further simulations show such effects could survive against a 10 nm spatial deviation of the emitter and will be slightly influenced by the presence of substrate. We anticipate the results of this paper could bring new possibilities in enhancing emission intensity from magnetic dipole transitions in experimental investigations.

Anisotropic light scattering by prismatic semiconductor nanowires

Anisotropic transverse light scattering by prismatic nanowires is a natural outcome of their geometry. In this work, we perform numerical calculations of the light scattering characteristics for nanowires in the optical and near-infrared range and explore the possibility of tuning the directivity by changing the angle of light incidence. The scattering cross section and the directivity of the scattered light when it is incident perpendicular to a facet or to an edge of the prism are investigated both with transverse electric and with transverse magnetic polarizations. The phenomenology includes Mie resonances and guided modes yielding together rich and complex spectra. We consider nanowires with hexagonal, square and triangular cross sections. The modes that are most sensitive to the incidence angle are the hexapole for the hexagonal case and the quadrupole for the square case. Higher order modes are also sensitive, but mostly for the square geometry. Our results indicate the possibility of a flexible in-situ tunability of the directivity simply by rotating the nanowire profile relatively to the direction of the incident light which could offer potential advantages in applications such as switching or sensing.

Direct measurement and analytical description of the mode alignment in inversely tapered silicon nano-resonators

Inversely tapered silicon photonic resonators on silicon substrates were shown to host multiple high–Q whispering gallery modes and constitute versatile building blocks for CMOS compatible solid state lighting, optical sensing and modulator devices. So far, numerical analyses by the finite difference time domain method have been used to predict the height distribution of whispering gallery modes in such resonators. In this study, we provide an experimental evidence of this mode distribution along the resonator height by selectively exciting whispering gallery modes using cathodoluminescence spectroscopy. Further we derive analytical functions that permit to relate the height distribution of modes with a defined polarization, symmetry and effective refractive index to the geometrical shape of the inversely tapered resonators.

Laser printing of optically resonant hollow crystalline carbon nanostructures from 1D and 2D metal-organic frameworks

Using a hybrid approach involving a slow diffusion method to synthesize 1D and 2D MOFs followed by their treatment with femtosecond infrared laser radiation, we generated 100-600 nm well-defined hollow spheres and hemispheres of graphite. This ultra-fast technique extends the library of shapes of crystalline MOF derivatives appropriate for all-dielectric nanophotonics.

Multiband enhancement of magnetic dipole emission with tapered hollow hyperbolic metamaterials

Based on a single resonance, nanostructures often provide narrowband enhancement for magnetic dipole emissions. Here, tapered hollow hyperbolic metamaterial is designed in order to produce a multiband emission enhancement. Specifically, a series of coaxial magnetic hot spots is excited inside the structure in five discrete bands. Meanwhile, we demonstrate that the emission enhancement can be achieved at both multiple wavelengths and multiple spatial positions in one single device. An enhancement factor of radiative decay rate up to 694 is obtained. Results of this paper might open new possibilities for nanostructures to achieve multiband light emission enhancement in the magneto-optical domain.

Engineering scattering patterns with asymmetric dielectric nanorods

By controlling interference of Mie resonance modes of various nanostructures, we can achieve a large number of nontrivial effects in nanophotonics. In this work, we propose a cylindrical structure in which the spectral overlap of the Mie-type modes can be controlled by drilling a hole parallel to the axis, thus changing unidirectional scattering. We further demonstrate that the scattering patterns can be tailored by rotating the structure to achieve almost arbitrary scattered wave direction.

Modelling of free-form conformal metasurfaces

Artificial electromagnetic surfaces, metasurfaces, control light in the desired manner through the introduction of abrupt changes of electromagnetic fields at interfaces. Current modelling of metasurfaces successfully exploits generalised sheet transition conditions (GSTCs), a set of boundary conditions that account for electric and magnetic metasurface-induced optical responses. GSTCs are powerful theoretical tools but they are not readily applicable for arbitrarily shaped metasurfaces. Accurate and computationally efficient algorithms capable of implementing artificial boundary conditions are highly desired for designing free-form photonic devices. To address this challenge, we propose a numerical method based on conformal boundary optics with a modified finite difference time-domain (FDTD) approach which accurately calculates the electromagnetic fields across conformal metasurfaces. Illustrative examples of curved meta-optics are presented, showing results in good agreement with theoretical predictions. This method can become a powerful tool for designing and predicting optical functionalities of conformal metasurfaces for new lightweight, flexible and wearable photonic devices.

Predicting and modelling the responses of free-from photonics devices remain challenging with conventional computational tools. Here, the authors propose an efficient algorithm based on conformal boundary optics and modified finite difference time-domain to calculate the electromagnetic fields across conformal metasurfaces.

Exploring the Magnetic and Electric Side of Light through Plasmonic Nanocavities

Light-matter interactions are often considered to be mediated by the electric component of light only, neglecting the magnetic contribution. However, the electromagnetic energy density is equally distributed between both parts of the optical fields. Within this scope, we experimentally demonstrate here, in excellent agreement with numerical simulations, that plasmonic nanostructures can selectively manipulate and tune the magnetic versus electric emission of luminescent nanocrystals. In particular, we show selective enhancement or decay of magnetic and electric emission from trivalent europium-doped nanoparticles in the vicinity of plasmonic nanocavities, designed to efficiently couple to either the electric or magnetic emission of the quantum emitter. Specifically, by precisely controlling the spatial position of the emitter with respect to our plasmonic nanostructures, by means of a near-field optical microscope, we record local distributions of both magnetic and electric radiative local densities of states (LDOS) with nanoscale precision. The distribution of the radiative LDOS reveals the modification of both the magnetic and electric optical quantum environments induced by the presence of the metallic nanocavities. This manipulation and enhancement of magnetic light-matter interaction by means of plasmonic nanostructures opens up new possibilities for the research fields of optoelectronics, chiral optics, nonlinear and nano-optics, spintronics, and metamaterials, among others.

Enhancing Magnetic Light Emission with All-Dielectric Optical Nanoantennas

Electric and magnetic optical fields carry the same amount of energy. Nevertheless, the efficiency with which matter interacts with electric optical fields is commonly accepted to be at least 4 orders of magnitude higher than with magnetic optical fields. Here, we experimentally demonstrate that properly designed photonic nanoantennas can selectively manipulate the magnetic versus electric emission of luminescent nanocrystals. In particular, we show selective enhancement of magnetic emission from trivalent europium-doped nanoparticles in the vicinity of a nanoantenna tailored to exhibit a magnetic resonance. Specifically, by controlling the spatial coupling between emitters and an individual nanoresonator located at the edge of a near-field optical scanning tip, we record with nanoscale precision local distributions of both magnetic and electric radiative local densities of states (LDOS). The map of the radiative LDOS reveals the modification of both the magnetic and electric quantum environments induced by the presence of the nanoantenna. This manipulation and enhancement of magnetic light-matter interaction by means of nanoantennas opens up new possibilities for the research fields of optoelectronics, chiral optics, nonlinear and nano-optics, spintronics, and metamaterials, among others.

Unidirectional emission in an all-dielectric nanoantenna

All-dielectric nanoantennas are a promising alternative to plasmonic optical antennas for engineering light emission because of their low-loss nature in the optical spectrum. Nevertheless, it is still challenging to manipulate directional light emission with subwavelength all-dielectric nanoantennas. Here, we propose and numerically demonstrate that a hollow silicon nanodisk can serve as a versatile antenna for directing and enhancing the emission from either an electric or magnetic dipole emitter. When primarily coupled to both electric and magnetic dipole modes of a nanoantenna, broadband nearly-unidirectional emission can be realized by the interference of two modes, which can be spectrally tuned via the geometric parameters in an easy way. More importantly, the emission directions for the magnetic and electric dipole emitters are shown as opposite to each other through control of the phase difference between the induced magnetic and electric dipole modes of the antenna. Meanwhile, the Purcell factors can be enhanced by more than one order of magnitude and high quantum efficiencies can be maintained at the visible spectrum for both kinds of dipole emitters. We further show that these unidirectional emission phenomena can withstand small disorder effects of in-plane dipole orientation and location. Our study provides a simple yet versatile platform that can shape the emission of both magnetic and electric dipole emitters.

Plasmonic- and dielectric-based structural coloring: from fundamentals to practical applications

Structural coloring is production of color by surfaces that have microstructure fine enough to interfere with visible light; this phenomenon provides a novel paradigm for color printing. Plasmonic color is an emergent property of the interaction between light and metallic surfaces. This phenomenon can surpass the diffraction limit and achieve near unlimited lifetime. We categorize plasmonic color filters according to their designs (hole, rod, metal-insulator-metal, grating), and also describe structures supported by Mie resonance. We discuss the principles, and the merits and demerits of each color filter. We also discuss a new concept of color filters with tunability and reconfigurability, which enable printing of structural color to yield dynamic coloring at will. Approaches for dynamic coloring are classified as liquid crystal, chemical transition and mechanical deformation. At the end of review, we highlight a scale-up of fabrication methods, including nanoimprinting, self-assembly and laser-induced process that may enable real-world application of structural coloring.

Boosting magnetic field enhancement with radiative couplings of magnetic modes in dielectric nanostructures

Dielectric nanostructures can readily support considerable magnetic field enhancements that offer great potential applications in field enhanced spectroscopies. However, the magnetic fields of dielectric structures are usually distributed within the entire volume, which brings challenge to the further increment of the magnetic field enhancement. Here, we theoretically demonstrate that the magnetic field enhancement in dielectric nanostructures can be boosted through the radiative couplings of magnetic modes. Our concentric structure consists of a hollow disk and a ring. The disk has a magnetic dipole mode. The ring has two magnetic dipole modes that are out of phase. Strong radiative interactions between the modes on the disk and the ring can occur, which result in a net constructive coupling effect. For a lossless material with n = 3.3, a sharp peak can be obtained on the scattering spectrum of the coupled system due to the radiative interactions. The corresponding resonant magnetic field enhancement at the disk center reaches 96 times. This enhancement is about 7 times higher than that of an individual disk. The structure with a lossy material Si is also considered, where radiative couplings and boosted magnetic field can also be obtained. Our research reveals the strong radiative mode couplings in dielectric structures and is important for furthering our understanding on the light-matter interactions at the nanoscale.

Optically resonant dielectric nanostructures

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.

All-dielectric hollow nanodisk for tailoring magnetic dipole emission

We propose a silicon hollow nanodisk for enhancing magnetic dipole (MD) emission. The Purcell factor can be more than 300, which is one order of magnitude larger than the silicon nanosphere case. It is demonstrated that the silicon hollow nanodisk resembles the function of an azimuthally polarized beam for tailoring the magnetic and electric dipole (ED) emission. It is shown that MD emission can be significantly enhanced, while ED emission will be suppressed when emitters are located in the hollow of the nanodisk. The dependence of the Purcell factor on the geometry parameters is also studied. Our results might facilitate the on-chip engineering of magnetic light emission.

Partially hollowed ultra-thin dielectric meta-surface for transmission manipulation

Impressive optical properties are numerically demonstrated in the partially hollowed dielectric meta-surface (p-HDMS), which consists of an air cavity array intercalated in an ultra-thin (~λ/6) high-index dielectric film. Multispectral transmission band-stop response with near-perfect spectral modulation depth is achieved. The spectral slop is up to 80%/nm, indicating the sharp and narrowband transmission behavior. Classical Malus law is confirmed by this sub-wavelength platform. Moreover, the multispectral light propagation manipulation can be perfectly reproduced by using the actual dielectric with absorption loss. In this all-dielectric meta-surface, conduction loss is avoided compared to its metallic plasmonic counterpart. Such configurations can therefore serve as excellent alternatives for plasmonic meta-surfaces and constitute an important step in nanophotonics.