All-dielectric metamaterials

Orientation Control of Helical Nanofilament Phase and Its Chiroptical Applications

Chiral liquid crystal phases show fascinating structural and optical properties due to their inherent helical characteristics. Among the various chiral liquid crystal phases, the helical nanofilament phase, made of achiral bent-shaped molecules, has been of keen research interest due to its unusual polar and chiral properties. This review is intended to introduce the recent progress in orientation control and its application to the helical nanofilament phase, which includes topographic confinement, photoalignment, and chiroptical applications such as photonic crystal and chirality sensor.

A hybrid achromatic metalens

Metalenses, ultra-thin optical elements that focus light using subwavelength structures, have been the subject of a number of recent investigations. Compared to their refractive counterparts, metalenses offer reduced size and weight, and new functionality such as polarization control. However, metalenses that correct chromatic aberration also suffer from markedly reduced focusing efficiency. Here we introduce a Hybrid Achromatic Metalens (HAML) that overcomes this trade-off and offers improved focusing efficiency over a broad wavelength range from 1000–1800 nm. HAMLs can be designed by combining recursive ray-tracing and simulated phase libraries rather than computationally intensive global search algorithms. Moreover, HAMLs can be fabricated in low-refractive index materials using multi-photon lithography for customization or using molding for mass production. HAMLs demonstrated diffraction limited performance for numerical apertures of 0.27, 0.11, and 0.06, with average focusing efficiencies greater than 60% and maximum efficiencies up to 80%. A more complex design, the air-spaced HAML, introduces a gap between elements to enable even larger diameters and numerical apertures.

Metalenses that correct chromatic aberration also suffer from reduced focusing efficiency. Here, the authors introduce a Hybrid Achromatic Metalens which merges a metalens and phase plate to offer improved focusing efficiency over a broad wavelength range and diffraction limited imaging performance.

Liquid crystal integrated metalens with dynamic focusing property

Metalenses are developing fast towards versatile and integrated terahertz (THz) apparatuses, while tunable ones are highly pursued. Here, we propose a strategy that integrates dielectric metasurfaces with liquid crystals (LCs) to realize the dynamic focal spot manipulation. The silicon pillar meta-units of the metasurface are properly selected to generate different phase profiles for two orthogonal linear polarizations, permitting a laterally or axially altered focal spot. After LCs integrated, polarization-multiplexed focusing can be achieved via electrically varying the LC orientations. We demonstrate two metalenses with distinct functions. For the first one, the uniformly aligned LC works as a polarization converter, and further switches the focal length by altering the bias. For the second one, an LC polarization grating is utilized for rear spin-selective beam deflection. Consequently, a THz port selector is presented. This work supplies a promising method towards active THz elements, which may be widely applied in THz sensing, imaging, and communication.

Ceramic resonators for targeted clinical magnetic resonance imaging of the breast

Currently, human magnetic resonance (MR) examinations are becoming highly specialized with a pre-defined and often relatively small target in the body. Conventionally, clinical MR equipment is designed to be universal that compromises its efficiency for small targets. Here, we present a concept for targeted clinical magnetic resonance imaging (MRI), which can be directly integrated into the existing clinical MR systems, and demonstrate its feasibility for breast imaging. The concept comprises spatial redistribution and passive focusing of the radiofrequency magnetic flux with the aid of an artificial resonator to maximize the efficiency of a conventional MR system for the area of interest. The approach offers the prospect of a targeted MRI and brings novel opportunities for high quality specialized MR examinations within any existing MR system.

Here, the authors present a concept for targeted clinical magnetic resonance imaging for relatively small targets in the body. They use an artificial resonator for spatial redistribution and passive focusing of the radiofrequency magnetic flux and demonstrate feasibility for targeted breast imaging.

Superior Resonant Nanocavities Engineering on the Photonic Crystal-Coupled Emission Platform for the Detection of Femtomolar Iodide and Zeptomolar Cortisol

Although luminescence spectroscopy has been a promising sensing technology with widespread applications in point-of-care diagnostics and chem-bio detection, it fundamentally suffers from low signal collection efficiency, considerable background noise, poor photostability, and intrinsic omnidirectional emission properties. In this regard, surface plasmon-coupled emission, a versatile plasmon-enhanced detection platform with >50% signal collection efficiency, high directionality, and polarization has previously been explored to amplify the limit of detection of desired analytes. However, high Ohmic loss in metal-dependent plasmonic platforms has remained an inevitable challenge. Here, we develop a hybrid nanocavity interface on a template-free and loss-less photonic crystal-coupled emission (PCCE) platform by the quintessential integration of high refractive index dielectric Nd₂O₃ "Huygens sources" and sharp-edged silver nanoprisms (NPrs). While efficient forward light scattering characteristics of Nd₂O₃ nanorods (NRs) present 460-fold emission enhancements in PCCE, the tunable localized plasmon resonances of NPrs display high electromagnetic field confinement at sharp nanotips and protrusions, boosting the enhancements 947-fold. The judicious use of silver NPr (AgNPr) metal-Nd₂O₃ dielectric hybrid resonances in conjugation with surface-trapped Bloch surface waves of the one-dimensional photonic crystal (1DPhC) displayed unprecedented >1300-fold enhancements. The experimental results are validated by excellent correlations with numerical calculations. The multifold hotspots generated by zero and nonzero nanogaps between the coassembly of NPrs, NRs, and 1DPhCs are used for (i) determination of hyper and hypothyroidism levels through monitoring the concentration of iodide (I^-) ions and (ii) single-molecule detection (zeptomolar) of the stress hormone, cortisol, through the synthesized cortisol-rhodamine B conjugate obtained using a simple esterification reaction.

Forward and Backward Unidirectional Scattering by the Core-Shell Nanocube Dimer with Balanced Gain and Loss

An optical nanoantenna consisting of a Au-dielectric core-shell nanocube dimer with switchable directionality was designed and described. Our theoretical model and numerical simulation showed that switching between forward and backward directions can be achieved with balanced gain and loss, using a single element by changing the coefficient κ in the core, which can be defined by the relative phase of the polarizability. The optical response indicated a remarkable dependence on the coefficient κ in the core as well as frequency. The location of the electric field enhancement was specified by the different coefficient κ and, furthermore, the chained optical nanoantenna and coupled electric dipole emitted to the optical nanoantenna played significant roles in unidirectional scattering. This simple method to calculate the feasibility of unidirectional and switchable scattering provides an effective strategy to explore the functionalities of nanophotonic devices.

Polarization Insensitive, Broadband, Near Diffraction-Limited Metalens in Ultraviolet Region

Metasurfaces in the ultraviolet spectrum have stirred up prevalent research interest due to the increasing demand for ultra-compact and wearable UV optical systems. The limitations of conventional plasmonic metasurfaces operating in transmission mode can be overcome by using a suitable dielectric material. A metalens holds promising wavefront engineering for various applications. Metalenses have developed a breakthrough technology in the advancement of integrated and miniaturized optical devices. However, metalenses utilizing the Pancharatnam–Berry (PB) phase or resonance tuning methodology are restricted to polarization dependence and for various applications, polarization-insensitive metalenses are highly desirable. We propose the design of a high-efficiency dielectric polarization-insensitive UV metalens utilizing cylindrical nanopillars with strong focusing ability, providing full phase delay in a broadband range of Ultraviolet light (270–380 nm). The designed metalens comprises Silicon nitride cylindrical nanopillars with spatially varying radii and offers outstanding polarization-insensitive operation in the broadband UV spectrum. It will significantly promote and boost the integration and miniaturization of the UV photonic devices by overcoming the use of Plasmonics structures that are vulnerable to the absorption and ohmic losses of the metals. The focusing efficiency of the designed metalens is as high as 40%.

Tunable narrowband shortwave-infrared absorber made of a nanodisk-based metasurface and a phase-change material Ge2Sb2Te5 layer

A tunable absorber made of a nanodisk-based metasurface is proposed to realize a narrowband shortwave-infrared (SWIR) perfect absorption. By introducing a phase-change material Ge₂Sb₂Te₅ (GST) layer, we produce a selective and active control of the optical response. It is found that the narrowband absorption of 99.9% can be achieved for amorphous GST (aGST) with a modulation depth of 54.6% at 1931 nm, which is attributed to the strong electric dipole resonance in the germanium nanodisks. Moreover, under the aGST state, the full width at half-maximum of 22 nm can be acquired for a normal TM-polarized wave, and such a nanodisk-based absorber enables a tunable operating wavelength by adjusting the geometrical parameters to realize the spectral selectivity. In addition, the nanodisk-based metasurface nanostructure, combined with a dielectric Bragg reflector with alternately stacked SiO₂ and TiO₂ layers, can realize the SWIR dual-band absorption for aGST and single-band absorption for crystalline GST through the adjustment of electric and magnetic resonances. The designed absorbers have the potential applications in tunable absorption filter, thermal sensing, and optical signal processing.

Hybrid exciton-plasmon-polaritons in van der Waals semiconductor gratings

Van der Waals materials and heterostructures that manifest strongly bound exciton states at room temperature also exhibit emergent physical phenomena and are of great promise for optoelectronic applications. Here, we demonstrate that nanostructured, multilayer transition metal dichalcogenides (TMDCs) by themselves provide an ideal platform for excitation and control of excitonic modes, paving the way to exciton-photonics. Hence, we show that by patterning the TMDCs into nanoresonators, strong dispersion and avoided crossing of exciton, cavity photons and plasmon polaritons with effective separation energy exceeding 410 meV can be controlled with great precision. We further observe that inherently strong TMDC exciton absorption resonances may be completely suppressed due to excitation of hybrid light-matter states and their interference. Our work paves the way to the next generation of integrated exciton optoelectronic nano-devices and applications in light generation, computing, and sensing.

The authors investigate the optical properties of a heterostructure formed by a metallic substrate and a nanostructured transition metal dichalcogenide multilayer by measuring the reflectance spectrum at different multilayer thicknesses, filling factors and grating periods. The spectra show strong dispersion and avoided crossing of excitons, plasmons and cavity photons along with excitonic mode suppression at the anti-crossing point.

Spatially Broadband Coupled-Surface Plasmon Wave Assisted Transmission Effect in Azo-Dye-Doped Liquid Crystal Cell

Active tuning on a plasmonic structure is discussed in this report. We examined the transient transmission effects of an azo-dye-doped liquid crystal cell on a metallic surface grating. The transition between isotropic and nematic phases in liquid crystal generated micro-domains was shown to induce the dynamic scattering of light from a He-Ne laser, thereby allowing transmission through a non-transparent aluminum film overlaying a dielectric grating. Various grating pitches were tested in terms of transmission effects. The patterned gratings include stripe ones and circular forms. Our results indicate that surface plasmon polariton waves are involved in the transmission process. We also demonstrated how momentum diagrams of gratings and Surface Plasmon Polariton (SPP) modes combined with Mie scattering effects could explain the broadband coupling phenomenon. This noteworthy transition process could be applied to the development of spatially broadband surface plasmon polariton coupling devices.

Manipulation Technique for Precise Transfer of Single Perovskite Nanoparticles

In this article, we present the pick-and-place technique for the manipulation of single nanoparticles on non-conductive substrates using a tungsten tip irradiated by a focused electron beam from a scanning electron microscope. The developed technique allowed us to perform the precise transfer of single BaTiO₃ nanoparticles from one substrate to another in order to carry out measurements of elastic light scattering as well as second harmonic generation. Also, we demonstrate a fabricated structure made by finely tuning the position of a BaTiO₃ nanoparticle on top of a dielectric nanowaveguide deposited on a glass substrate. The presented technique is based on the electrostatic interaction between the sharp tungsten tip charged by the electron beam and the nanoscale object. A mechanism for nanoparticle transfer to a non-conductive substrate is proposed and the forces involved in the manipulation process are evaluated. The presented technique can be widely utilized for the fabrication of nanoscale structures on optically transparent non-conductive substrates, which presents a wide range of applications for nanophotonics.

Chromatic Dispersion Manipulation Based on Metalenses

Metasurfaces are 2D metamaterials composed of subwavelength nanoantennas according to specific design. They have been utilized to precisely manipulate various parameters of light fields, such as phase, polarization, amplitude, etc., showing promising functionalities. Among all meta-devices, the metalens can be considered as the most basic and important application, given its significant advantage in integration and miniaturization compared with traditional lenses. However, the resonant dispersion of each nanoantenna in a metalens and the intrinsic chromatic dispersion of planar devices and optical materials result in a large chromatic aberration in metalenses that severely reduces the quality of their focusing and imaging. Consequently, how to effectively suppress or manipulate the chromatic aberration of metalenses has attracted worldwide attention in the last few years, leading to variety of excellent achievements promoting the development of this field. Herein, recent progress in chromatic dispersion control based on metalenses is reviewed.

Rapid large-scale fabrication of multipart unit cell metasurfaces

Periodic diffractive elements known as metasurfaces constitute platform technology whereby exceptional optical properties, not attainable by conventional means, are attained. Generally, with increasing unit-cell complexity, there emerges a wider design space and bolstered functional capability. Advanced devices deploying elaborate unit cells are typically generated by electron-beam patterning which is a tedious, slow process not suitable for large surfaces and quick turnaround. Ameliorating this condition, we present a novel route towards facile fabrication of complex periodic metasurfaces based on sequential exposures by laser interference lithography. Our method is fast, cost-effective, and can be applied to large surface areas. It is enabled by precise control over periodicity and exposure energy. With it we have successfully patterned and fabricated one-dimensional (1D) and two-dimensional (2D) multipart unit cell devices as demonstrated here. Thus, zero-order transmission spectra of an etched four-part 1D grating device are simulated and measured for both transverse-electric (TE) and transverse-magnetic (TM) polarization states of normally incident light. We confirm non-resonant wideband antireflection (∼800 nm) for TM-polarized light and resonance response for TE-polarized light in the near-IR band spanning 1400-2200 nm in a ∼100 mm² device. Furthermore, it is shown that this method of fabrication can be implemented not only to pattern periodic symmetric/asymmetric designs but also to realize non-periodic metasurfaces. The method will be useful in production of large-area photonic devices in the realm of nanophotonics and microphotonics.

Global Mie Scattering: Polarization Morphologies and the Underlying Topological Invariant

In various subdisciplines of optics and photonics, Mie theory has been serving as a fundamental language and playing indispensable roles widely. Conventional studies related to Mie scattering largely focus on local properties such as differential cross sections and angular polarization distributions. Though spatially integrated features of total cross sections in terms of both scattering and absorption are routine for investigations, they are intrinsically dependent on the specific morphologies of both the scattering bodies and the incident waves, consequently manifesting no sign of global invariance. Here, we propose a global Mie scattering theory to explore topological invariants for the characterization of scatterings by any obstacles of arbitrarily structured or polarized coherent light. It is revealed that, independent of distributions and interactions among the scattering bodies of arbitrary geometric and optical parameters, in the far field, inevitably, there are directions where the scatterings are either zero or circularly polarized. Furthermore, for each such singular direction, we can assign a half-integer index and the index sum of all those directions are bounded to be a global topological invariant of 2. The global Mie theory we propose, which is mathematically simple but conceptually penetrating, can render new perspectives for light scattering and topological photonics in both linear and nonlinear regimes and would potentially shed new light on the scattering of acoustic and matter waves of various forms.

Laser-Induced Periodic Ag Surface Structure with Au Nanorods Plasmonic Nanocavity Metasurface for Strong Enhancement of Adenosine Nucleotide Label-Free Photoluminescence Imaging

The label-free detection of biomolecules by means of fluorescence spectroscopy and imaging is topical. The developed surface-enhanced fluorescence technique has been applied to achieve progress in the label-free detection of biomolecules including deoxyribonucleic acid (DNA) bases. In this study, the effect of a strong enhancement of photoluminescence of 5'-deoxyadenosine-monophosphate (dAMP) by the plasmonic nanocavity metasurface composed of the silver femtosecond laser-induced periodic surface structure (LIPSS) and gold nanorods or nanospheres has been realized at room temperature. The highest value of 1220 for dAMP on the Ag-LIPSS/Au nanorod metasurface has been explained to be a result of the synergetic effect of the generation of hot spots near the sharp edges of LIPSS and Au nanorod tips together with the excitation of collective gap mode of the cavity due to strong near-field plasmonic coupling. A stronger plasmonic enhancement of the phosphorescence compared to the fluorescence is achieved due to a greater overlap of the phosphorescence spectrum with the surface plasmon spectral region. The photoluminescence imaging of dAMP on the metasurfaces shows a high intensity in the blue range. The comparison of Ag-LIPSS/Au nanorod and Ag-LIPSS/Au-nanosphere metasurfaces shows a considerably higher enhancement for the metasurface containing Au nanorods. Thus, the hybrid cavity metasurfaces containing metal LIPSS and nonspherical metal nanoparticles with sharp edges are promising for high-sensitive label-free detection and imaging of biomolecules at room temperature.

Anapole mediated giant photothermal nonlinearity in nanostructured silicon

Featured with a plethora of electric and magnetic Mie resonances, high index dielectric nanostructures offer a versatile platform to concentrate light-matter interactions at the nanoscale. By integrating unique features of far-field scattering control and near-field concentration from radiationless anapole states, here, we demonstrate a giant photothermal nonlinearity in single subwavelength-sized silicon nanodisks. The nanoscale energy concentration and consequent near-field enhancements mediated by the anapole mode yield a reversible nonlinear scattering with a large modulation depth and a broad dynamic range, unveiling a record-high nonlinear index change up to 0.5 at mild incident light intensities on the order of MW/cm². The observed photothermal nonlinearity showcases three orders of magnitude enhancement compared with that of unstructured bulk silicon, as well as nearly one order of magnitude higher than that through the radiative electric dipolar mode. Such nonlinear scattering can empower distinctive point spread functions in confocal reflectance imaging, offering the potential for far-field localization of nanostructured Si with an accuracy approaching 40 nm. Our findings shed new light on active silicon photonics based on optical anapoles.

Effective dipolar polarizability of amorphous arrays of size-dispersed nanoparticles

Inhomogeneity of nanoparticle size, shape, and distribution is ubiquitous and inherent in fabricated arrays or may be a deliberate attempt to engineer the optical response. It leads to a spread of polarizabilities of interacting elements and phases of scattered light, and quantitative understanding of these effects is important. Focusing on random/amorphous arrays of optical antennas, we combine T-matrix calculations and an analytical approach based on an effective dipolar polarizability within a film of dipoles framework to quantify the spectral response as a function of the particle inhomogeneity and stochastic clustering. The interplay of position-dependent stochastic coupling and size distribution of antennas determines the optical properties of such arrays as a function of mean/standard deviation of diameter and minimum separation. The resonance wavelength, amplitude, and scattering-to-absorption ratio exhibit oscillations around their size-averaged values with periods and amplitudes given by average structural factors.

Generalized Maxwell projections for multi-mode network Photonics

The design of optical resonant systems for controlling light at the nanoscale is an exciting field of research in nanophotonics. While describing the dynamics of few resonances is a relatively well understood problem, controlling the behavior of systems with many overlapping states is considerably more difficult. In this work, we use the theory of generalized operators to formulate an exact form of spatio-temporal coupled mode theory, which retains the simplicity of traditional coupled mode theory developed for optical waveguides. We developed a fast computational method that extracts all the characteristics of optical resonators, including the full density of states, the modes quality factors, and the mode resonances and linewidths, by employing a single first principle simulation. This approach can facilitate the analytical and numerical study of complex dynamics arising from the interactions of many overlapping resonances, defined in ensembles of resonators of any geometrical shape and in materials with arbitrary responses.

On the Core-Shell Nanoparticle in Fractional Dimensional Space

The investigation of core-shell nanoparticles has been greatly exciting in biomedical applications, as this remains of prime importance in targeted drug delivery, sensing, etc. In the present work, the polarizability and scattering features of nanoparticles comprised of nano-sized dielectric/metallic core-shell structures were investigated in the fractional dimensional (FD) space, which essentially relates to the confinement of charged particles. For this purpose, three different kinds of metals-namely aluminum, gold and silver-were considered to form the shell, having a common silicon dioxide (SiO2) nanoparticle as the core. It is noteworthy that the use of noble metal-SiO2 mediums interface remains ideal to realize surface plasmon resonance. The core-shell nanoparticles were considered to have dimensions smaller than the operating wavelength. Under such conditions, the analyses of polarizability and the scattering and absorption cross-sections, and also, the extinction coefficients were taken up under Rayleigh scattering mechanism, emphasizing the effects of a varying FD parameter. Apart from these, the tuning of resonance peaks and the magnitude of surface plasmons due to FD space parameter were also analyzed. It was found that the increase of FD space parameter generally results in blue-shifts in the resonance peaks. Apart from this, the usage of gold and silver shells brings in fairly large shifts in the peak positions of wavelengths, which allows them to be more suitable for a biosensing purpose.

Magnetic toroidal dipole response in individual all-dielectric nanodisk clusters

Multipole electromagnetic resonances and their couplings are of crucial importance for both the fundamental understanding of light scattering by high-index all-dielectric nanostructures and lots of nanophotonic applications based on those nanostructures. Here, we show that magnetic dipole modes in a dielectric nanodisk cluster can easily form a magnetic toroidal dipole (MTD) mode. The cluster consists of five silicon nanodisks, where each nanodisk holds a magnetic dipole mode. These magnetic dipole modes can collectively couple with each other and form a MTD mode under suitable excitation. The MTD mode is confirmed by multipole expansion calculations and near field distributions, where two closed loops of magnetic field with opposite directions are seen. The response of the MTD is strong and comparable to that of a common electric dipole or magnetic dipole mode. It is also found that the MTD resonance is accompanied by an electric toroidal quadrupole mode in the cluster. The MTD mode is tunable by varying the geometries. We also fabricated silicon nanoparticle clusters and verified the MTD mode in the experiment. Our results illustrate the controllable excitation of strong high-order electromagnetic modes and these modes may open new opportunities for light manipulation at the nanoscale.

Increasing efficiency of high numerical aperture metasurfaces using the grating averaging technique

One of the important advantages of optical metasurfaces over conventional diffractive optical elements is their capability to efficiently deflect light by large angles. However, metasurfaces are conventionally designed using approaches that are optimal for small deflection angles and their performance for designing high numerical aperture devices is not well quantified. Here we introduce and apply a technique for the estimation of the efficiency of high numerical aperture metasurfaces. The technique is based on a particular coherent averaging of diffraction coefficients of periodic blazed gratings and can be used to compare the performance of different metasurface designs in implementing high numerical aperture devices. Unlike optimization-based methods that rely on full-wave simulations and are only practicable in designing small metasurfaces, the gradient averaging technique allows for the design of arbitrarily large metasurfaces. Using this technique, we identify an unconventional metasurface design and experimentally demonstrate a metalens with a numerical aperture of 0.78 and a measured focusing efficiency of 77%. The grating averaging is a versatile technique applicable to many types of gradient metasurfaces, thus enabling highly efficient metasurface components and systems.

Direct etching at the nanoscale through nanoparticle-directed capillary condensation

We report a method to locally deliver a chemical etchant at the nanoscale in the vapor phase by capillary condensation forming a meniscus at the nanoparticle/substrate interface. The process is simple, scalable and does not require functionalization of the nanoparticles. Furthermore, it does not rely on any specific chemical properties of the materials other than the solution being aqueous and the wettability properties of the surfaces involved, which should enable its application to other material and chemical combinations. In particular, in this work we demonstrate the proposed process by periodically pattering a SiO₂ layer using a self-assembled monolayer of polystyrene particles exposed to HF vapors. The patterned SiO₂ layer is then used as a mask to etch a pattern of inverted nanopyramids on Si. The silicon nanopatterning has been demonstrated for particles sizes ranging from 800 nm down to 100 nm, providing pyramids with a size down to 50 nm for 100 nm nanoparticles.

All-dielectric metasurface for high-performance structural color

The achievement of structural color has shown advantages in large-gamut, high-saturation, high-brightness, and high-resolution. While a large number of plasmonic/dielectric nanostructures have been developed for structural color, the previous approaches fail to match all the above criterion simultaneously. Herein we utilize the Si metasurface to demonstrate an all-in-one solution for structural color. Due to the intrinsic material loss, the conventional Si metasurfaces only have a broadband reflection and a small gamut of 78% of sRGB. Once they are combined with a refractive index matching layer, the reflection bandwidth and the background reflection are both reduced, improving the brightness and the color purity significantly. Consequently, the experimentally demonstrated gamut has been increased to around 181.8% of sRGB, 135.6% of Adobe RGB, and 97.2% of Rec.2020. Meanwhile, high refractive index of silicon preserves the distinct color in a pixel with 2 × 2 array of nanodisks, giving a diffraction-limit resolution.

Position-guided Fano resonance and amended GaussAmp model for the control of slow light in hybrid graphene-silicon metamaterials

Position-guided Fano resonance is observed in hybrid graphene-silicon metamaterials. An outstanding application of such resonance is slow-light metadevices. The maximum group delay is 9.73 ps, which corresponds to a group delay in free-space propagation of 2.92 mm. We employ a coupled oscillator model to illustrate anomalous transmission, where the intensity of the Fano peak increases with the Fermi level. Furthermore, we amend the GaussAmp model to serve as a suitable control equation for the group delay. The coefficient of correlation (R²) is as high as 0.99998, while the lowest values of the root-mean-square error and sum of squared errors are respectively 0.00421 and 0.00156. These results indicate that the amended GaussAmp model accurately controls the trend of the group delay. This work not only clarifies the mechanism of Fano resonance generation but also provides a promising platform for dynamically adjustable optical switches and multidimensional information sensors.

Wafer-level fabrication of 3D nanoparticles assembled nanopillars and click chemistry modification for sensitive SERS detection of trace carbonyl compounds

In this work, we develop a new method for fabricating wafer-level gold nanoparticles covered silicon nanopillars (SNPs) combined with surface chemical modification to detect trace level carbonyl compounds based on surface-enhanced Raman scattering (SERS) technique. The SNPs are fabricated with an etching process using nano masks synthesized in oxygen-plasma bombardment of photoresist, and further deposited with gold nanoparticles on the surface, thus forming a 3D 'particles on pillars' nanostructure for sensitive SERS detection. The enhancement factor (EF) of the devices for R6G detection can achieve 1.56 × 10⁶ times compared with a flat Si substrate. We also developed an oximation click chemistry reaction procedure by chemically modifying the nanostructures with aminooxy dodecane thiol (ADT) self-assemble modification. The chip is further integrated with a polydimethylsiloxane (PDMS) microfluidic chamber, which allows fast and convenient detection of trace carbonyl compounds in liquid samples. The SERS detection capability was demonstrated by the dropwise addition of fluorescent carbonyl compounds before and after elution. Furthermore, the device was proved with high surface consistency(<70%) for repeated measurement, which has the potential for ppb(parts per billion) level concentration of carbonyl compounds detection.

Omnidirectional Photodetectors Based on Spatial Resonance Asymmetric Facade via a 3D Self-Standing Strategy

Integration of photovoltaic materials directly into 3D light-matter resonance architectures can extend their functionality beyond traditional optoelectronics. Semiconductor structures at subwavelength scale naturally possess optical resonances, which provides the possibility to manipulate light-matter interactions. In this work, a structure and function integrated printing method to remodel 2D film to 3D self-standing facade between predesigned gold electrodes, realizing the advancement of structure and function from 2D to 3D, is demonstrated. Due to the enlarged cross section in the 3D asymmetric rectangular structure, the facade photodetectors possess sensitive light-matter interaction. The single 3D facade photodetectors can measure the incident angle of light in 3D space with a 10° angular resolution. The resonance interaction of the incident light at different illumination angles and the 3D subwavelength photosensitive facade is analyzed by the simulated light flow in the facade. The 3D facade structure enhances the manipulation of the light-matter interaction and extends metasurface nanophotonics to a wider range of materials. The monitoring of dynamic variation is achieved in a single facade photodetector. Together with the flexibility of structure and function integrated printing strategy, three and four branched photodetectors extend the angle detection to omnidirectional ranges, which will be significant for the development of 3D angle-sensing devices.

Narrowband Perfect Absorber Based on Dielectric-Metal Metasurface for Surface-Enhanced Infrared Sensing

We proposed a narrowband perfect absorber that was based on dielectric-metal metasurface for wide-band surface-enhanced infrared sensing. It is found that the narrowband perfect absorber can generate the hybrid guided modes with high quality-factor at infrared frequencies, which make the absorber highly sensitive to the surrounded analyte. Moreover, tuning the incident angle can actively modulate the resonant wavelength of absorber. Such an absorber with excellent features is employed to realize both refractive index sensing and infrared vibrational fingerprint sensing on a single substrate. It is demonstrated that a refractive index sensitivity of 1800 nm/RIU and figure of merit of 62 RIU−1 can be obtained as the refractive index sensor. While, as a surface enhanced infrared absorption spectroscopy substrate, two closed vibrational modes of analyte with nanometer thick layers can be effectively identified and selectively detected with 50-folds enhancement by actively tuning the incident angle without any change in the structural parameters (periodicity, width, height, and refractive index of the grating) of the device after fabricating. Our study offers a promising approach for designing high-performance surface-enhanced infrared optical sensors in the infrared region.

Slow light using magnetic and electric Mie resonances

The ability to slow down light leads to strong light-matter interaction, which is important for a number of optical applications such as sensing, nonlinear optics, and optical pulse manipulation. Here, we show that a dramatic reduction in the speed of light can be realized through the interference of electric and magnetic dipole resonances in Mie-type resonators made of a dielectric material with a high refractive index. We present a general theory that links the maximal speed reduction of light to resonator radiation losses and then consider a specific realization based on silicon nanodisk arrays.

Multiple Resonance Metamaterial Emitter for Deception of Infrared Emission with Enhanced Energy Dissipation

Artificial camouflage surfaces for assimilating with the environment have been utilized for controlling optical properties. Especially, the optical properties of infrared (IR) camouflage materials should be satisfied with two requirements: deception of IR signature in a detected band through reduced emissive energy and dissipation of reduced emissive energy for preventing thermal instability through an undetected band. Most reported articles suggest the reduction of emissive energy in the detected band; however, broadband emission for enough energy dissipation through the undetected band simultaneously is still a challenging issue. Here, we demonstrate the multiresonance emitter for broadband emission with IR camouflage utilizing the electromagnetic properties of dielectric material. We reveal that the interaction between the magnetic resonance and dielectric layer's property in a metal-dielectric-metal structure induces the multiple resonance at the specific band. We present an IR camouflage behavior of multiresonance emitter on a curved surface through the IR camera (8-14 μm). We evaluate the energy dissipation in the undetected band, which is 1613% higher than metal and 26% higher than conventional selective emitters. This study paves the way to develop broadband emitters for radiative cooling and thermophotovoltaic applications.

Metasurfaces-based imaging and applications: from miniaturized optical components to functional imaging platforms

This review focuses on the imaging applications of metasurfaces. These optical elements provide a unique platform to control light; not only do they have a reduced size and complexity compared to conventional imaging systems but they also enable novel imaging modalities, such as functional-imaging techniques. This review highlights the development of metalenses, from their basic principles, to the achievement of achromatic and tunable lenses, and metasurfaces implemented in functional optical imaging applications.

Quasi-triply-degenerate states and zero refractive index in two-dimensional all-dielectric photonic crystals

We introduce the concept of a quasi-triply-degenerate state (QTDS) and demonstrate its relation to an effective zero refractive index (ZRI) in a two-dimensional (2D) square lattice photonic crystal (PC) of all dielectric pillars. A QTDS is characterized by a triple band structure (TBS), wherein two of the bands manifest a linear dispersion around the Γ-point, i.e. a Dirac-like cone, while the third is a flat zero refractive index (ZRI) band with a frequency that is degenerate with one of the other bands. Significantly, we find that while triple degeneracy of the bands is not observed, the three bands approach one another so close that the observable properties of PCs adapted to the QTDS frequency perform as expected of a ZRI material. We closely examine the ZRI band at the Γ-point and show that by varying the PC material and structure parameters, the ZRI band behavior extends over a wide range of dielectric refractive indices enabling materials made with polymeric constituents. Moreover, the ZRI characteristics are robust and tolerant over a range of frequencies. Furthermore, the computational screening we employ to identify QTDS parameters enables the rational design of low-loss 2D ZRI materials for a broad range of photonic applications, including distributing a common reference phase, cloaking and focusing light.

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.

On the constraints of electromagnetic multipoles for symmetric scatterers: eigenmode analysis

The scattering and resonant properties of optical scatterers/resonators are determined by the relative ratios among the associated multipole components, the calculation of which usually is analytically tedious and numerically complicated for complex structures. Here we identify the constraints as well as the relative relations among electromagnetic multipoles for the eigenmodes of symmetric scatterers/resonators. By reducing the symmetry properties of the vector spherical harmonic waves to those of the modified generating functions, we systematically study the required conditions for electromagnetic multipoles under several fundamental symmetry operations, i.e., 2D rotation and reflection operations and 3D proper and improper rotations. Taking a 2D scatterer with C_4v as an example, we show that each irreducible representation of C_4v can be assigned to corresponding electromagnetic multipoles, and consequently the constraints of the electromagnetic multipoles can be easily extracted. Such group approach can easily be extended to more complex 3D scatterers with higher symmetry group. Subsequently, we use the same procedure to map out the complete relation and constraint on the electromagnetic multipoles of a 3D scatterer imposed by D_3h symmetry. Our theoretical analyses are in perfect agreements with the fullwave finite element calculations of the eigenmodes of the symmetric scatters.

Ultra-narrowband dielectric metamaterial absorber with ultra-sparse nanowire grids for sensing applications

Due to their low losses, dielectric metamaterials provide an ideal resolution to construct ultra-narrowband absorbers. To improve the sensing performance, we present numerically a near-infrared ultra-narrowband absorber by putting ultra-sparse dielectric nanowire grids on metal substrate in this paper. The simulation results show that the absorber has an absorption rate larger than 0.99 with full width at half-maximum (FWHM) of 0.38 nm. The simulation field distribution also indicates that the ultra-narrowband absorption is originated from the low loss in the guided-mode resonance. Thanks to the ultra-narrow absorption bandwidths and the electric field mainly distributed out of the ultra-sparse dielectric nanowire grids, our absorber has a high sensitivity S of 1052 nm/RIU and a large figure of merit (FOM) of 2768 which mean that this ultra-narrowband absorber can be applied as a high-performance refractive index sensor.

Complete Complex Amplitude Modulation with Electronically Tunable Graphene Plasmonic Metamolecules

Dynamic high-resolution wavefront modulation of light is a long-standing quest in photonics. Metasurfaces have shown potential for realizing light manipulation with subwavelength resolution through nanoscale optical elements, or metaatoms, to overcome the limitations of conventional spatial light modulators. State-of-the-art active metasurfaces operate via phase modulation of the metaatoms, and their inability to also independently control the scattered amplitude leads to an inferior reconstruction of the desired wavefronts. This fundamental problem posed severe performance limitations particularly for applications relying on subwavelength spatiotemporal complex field modulation, which includes dynamic holography, high-resolution imaging, optical tweezing, and optical information processing. Here, we present the "metamolecule" strategy, which incorporates two independent subwavelength scatterers composed of noble metal antennas coupled to gate-tunable graphene plasmonic nanoresonators. The two-parametric control of the metamolecule secures the complete control of both amplitude and phase of light, enabling 2π phase shift as well as large amplitude modulation including perfect absorption. We further develop a generalized graphical model to examine the underlying requirements for complete complex amplitude modulation, offering intuitive design guidelines to maximize the tunability in metasurfaces. To illustrate the reconfigurable capability of our designs, we demonstrate dynamic beam steering and holographic wavefront reconstruction in periodically arranged metamolecules.

Multi-bit dielectric coding metasurface for EM wave manipulation and anomalous reflection

In this paper, a multi-bit dielectric reflective metasurface is presented for control of electromagnetic (EM) wave scattering and anomalous reflection. The unit cell is designed to act as a 1-, 2-, and 3-bit coding metasurface to attain better control of EM waves. For the 3-bit coding metasurface, the eight digital states have phase responses of 0°, 45°, 90°, 135°, 180°, 225°, 270°, and 315°. The top layer of the proposed metasurface consists of high permittivity material to realize a high Q factor. The proposed multi-bit coding metasurface can reflect the incident EM wave to the desired angle with more than 93% power efficiency. For radar cross section reduction applications, the discrete water cycle algorithm is utilized to obtain an optimal coding matrix for the unit cell arrangement, leading to better diffusion-like scattering, dispersion of the EM wave in all directions, and hence minimal specular reflection. The simulation and experimental results verify that the proposed metasurface is a suitable candidate for control of EM wave scattering and anomalous reflection.

Engineering novel tunable optical high-Q nanoparticle array filters for a wide range of wavelengths

The interaction of non-monochromatic radiation with arrays comprising plasmonic and dielectric nanoparticles has been studied using the finite-difference time-domain electrodynamics method. It is shown that LiNbO₃, TiO₂, GaAs, Si, and Ge all-dielectric nanoparticle arrays can provide a complete selective reflection of an incident plane wave within a narrow spectral line of collective lattice resonance with a Q-factor of 10³ or larger at various spectral ranges, while plasmonic refractory TiN and chemically stable Au nanoparticle arrays provide high-Q resonances with moderate reflectivity. Arrays with fixed dimensional parameters make it possible to fine-tune the position of a selected resonant spectral line by tilting the array relative to the direction of the incident radiation. These effects provide grounds for engineering novel selective tunable optical high-Q filters in a wide range of wavelengths, from visible to middle-IR.

Laser-induced spatially-selective tailoring of high-index dielectric metasurfaces

Optically resonant high-index dielectric metasurfaces featuring Mie-type electric and magnetic resonances are usually fabricated by means of planar technologies, which limit the degrees of freedom in tunability and scalability of the fabricated systems. Therefore, we propose a complimentary post-processing technique based on ultrashort (≤ 10 ps) laser pulses. The process involves thermal effects: crystallization and reshaping, while the heat is localized by a high-precision positioning of the focused laser beam. Moreover, for the first time, the resonant behavior of dielectric metasurface elements is exploited to engineer a specific absorption profile, which leads to a spatially-selective heating and a customized modification. Such technique has the potential to reduce the complexity in the fabrication of non-uniform metasurface-based optical elements. Two distinct cases, a spatial pixelation of a large-scale metasurface and a height modification of metasurface elements, are explicitly demonstrated.

Mie-type GaAs nanopillar array resonators for negative electron affinity photocathodes

This paper presents modeling results of Mie-type GaAs nanopillar array resonant structures and the design of negative electron affinity photocathodes based on Spicer's three-step model. For direct-bandgap GaAs with high intrinsic absorption coefficient in the 500 ∼ 850 nm spectral range, photoelectrons were found to be highly localized inside the nanopillars near the top and side surfaces where electrons can be efficiently transported and emitted into vacuum, and the light reflectance can be reduced to ∼1% level at resonance wavelengths. Predictions of spectrally resolved photoemission indicate that these nanophotonics resonators, when properly optimized, can increase the photo-electron emission quantum efficiency at resonance wavelengths to levels limited only by the surface-electron escape probability, significantly outperforming traditional flat wafer photocathodes. Ultrafast photoelectric response is also expected from these nanostructured photocathodes due to the much shorter photoelectron transport distance in nanopillars compared to flat wafers. Given these unique optoelectronic properties, GaAs nanophotonic resonance structured photocathodes represent a very promising alternative to photocathodes with flat surfaces that are widely used in many applications today.

Strong field enhancement in individual Φ-shaped dielectric nanostructures based on anapole mode resonances

Due to their ability to produce high electric field enhancements in relatively large nanoscale volumes with minimum absorption and nonradiating properties, anapole modes excited in high index dielectric nanostructures have attracted considerable attentions in these years. We propose a design strategy to simultaneously excite the anapole mode efficiently and maintain its resonant wavelength, which has been remained as a challenge in the conventional dielectric nanostructures. Based on analyzing the relationship between the field enhancement factor and scattering intensity of the electric and toroidal dipoles, we introduce two and four nanocuboids into the nil field intensity areas in the silicon disk system, respectively. The geometric volume of the system can be increased effectively and the electric field enhancement is boosted to be 190% and 250% while the resonant wavelength of the anapole mode is almost maintained constant. The systems combined with a slot in the strongest field intensity area also follow the same law, revealing that the design strategy can be easily extended to other geometric, material and frequency systems. Different from the design strategy to add new components into the areas with strong field intensity, the incorporations occurring at the minimum intensity area is another design scheme to engineer the properties of the resonant systems and can find broad applications in nano-device designs.

Plasmonic nanosponges filled with silicon for enhanced white light emission

Plasmonic nanosponges are a powerful platform for various nanophotonic applications owing to extremely high local field enhancement in metallic nanopores. The filling of the nanopores with high-refractive index semiconductors (e.g. Si, Ge, GaP, etc.) opens up opportunities for the enhancement of nonlinear effects in these materials. However, this task remains challenging due to the lack of knowledge on the integration process of metal and high-index semiconductor components in such nanoobjects. Here, we investigate metal-dielectric nanoparticles fabricated from bilayer Si/Au films by the laser printing technique via a combination of theoretical and experimental methods. We reveal that these hybrid nanoparticles represent the Au sponge-like nanostructure filled with Si nanocrystallites. We also demonstrate that the Au net provides strong near-field enhancement in the Si grains increasing the white light photoluminescence in the hybrid nanostructures compared to uniform Si nanoparticles. These results pave the way for engineering the internal structure of the sponge-like hybrid nanoparticles possessing white light luminescence and control of their optical properties on demand.

Metasurface-Empowered Optical Multiplexing and Multifunction

Metasurfaces are planar photonic elements composed of subwavelength nanostructures, which can deeply interact with light and exploit new degrees of freedom (DOF) to manipulate optical fields. In the past decade, metasurfaces have drawn great interest from the scientific community due to their profound potential to arbitrarily control light. Here, recent developments of multiplexing and multifunctional metasurfaces, which enable concurrent tasks through a dramatic compact design, are reviewed. The fundamental properties, design strategies, and applications of multiplexing and multifunctional metasurfaces are then discussed. First, recent progress on angular momentum multiplexing, including its behavior under different incident conditions, is considered. Second, a detailed overview of polarization-controlled, wavelength-selective, angle-selective, and reconfigurable multiplexing/multifunctional metasurfaces is provided. Then, the integrated and on-chip design of multifunctional metasurfaces is addressed. Finally, future directions and potential applications are presented.

MEMS-actuated metasurface Alvarez lens

Miniature lenses with a tunable focus are essential components for many modern applications involving compact optical systems. While several tunable lenses have been reported with various tuning mechanisms, they often face challenges with respect to power consumption, tuning speed, fabrication cost, or production scalability. In this work, we have adapted the mechanism of an Alvarez lens - a varifocal composite lens in which lateral shifts of two optical elements with cubic phase surfaces give rise to a change in the optical power - to construct a miniature, microelectromechanical system (MEMS)-actuated metasurface Alvarez lens. Implementation based on an electrostatic MEMS generates fast and controllable actuation with low power consumption. The utilization of metasurfaces - ultrathin and subwavelength-patterned diffractive optics - as optical elements greatly reduces the device volume compared to systems using conventional freeform lenses. The entire MEMS Alvarez metalens is fully compatible with modern semiconductor fabrication technologies, granting it the potential to be mass-produced at a low unit cost. In the reported prototype operating at 1550 nm wavelength, a total uniaxial displacement of 6.3 µm was achieved in the Alvarez metalens with a direct-current (DC) voltage application up to 20 V, which modulated the focal position within a total tuning range of 68 µm, producing more than an order of magnitude change in the focal length and a 1460-diopter change in the optical power. The MEMS Alvarez metalens has a robust design that can potentially generate a much larger tuning range without substantially increasing the device volume or energy consumption, making it desirable for a wide range of imaging and display applications.

Tailorable Dynamics in Nonlinear Optical Metasurfaces

Controlling light with light is essential for all-optical switching, data processing in optical communications and computing. Until now, all-optical control of light has relied almost exclusively on nonlinear optical interactions in materials. Achieving giant nonlinearities under low light intensity is essential for weak-light nonlinear optics. In the past decades, such weak-light nonlinear phenomena have been demonstrated in photorefractive and photochromic materials. However, their bulky size and slow speed have hindered practical applications. Metasurfaces, which enhance light-matter interactions at the nanoscale, provide a new framework with tailorable nonlinearities for weak-light nonlinear dynamics. Current advances in nonlinear metasurfaces are introduced, with a special emphasis on all-optical light controls. The tuning of the nonlinearity values using metasurfaces, including enhancement and sign reversal is presented. The tailoring of the transient behaviors of nonlinearities in metasurfaces to achieve femtosecond switching speed is also discussed. Furthermore, the impact of quantum effects from the metasurface on the nonlinearities is introduced. Finally, an outlook on the future development of this energetic field is offered.

Probing optical resonances of silicon nanostructures using tunable-excitation Raman spectroscopy

Optical materials with a high refractive index enable effective manipulation of light at the nanoscale through strong light confinement. However, the optical near field, which is mainly confined inside such high-index nanostructures, is difficult to probe with existing measurement techniques. Here, we exploit the connection between Raman scattering and the stored electric energy to detect resonance-induced near-field enhancements in silicon nanostructures. We introduce a Raman setup with a wavelength-tunable laser, which allows us to tune the Raman excitation wavelength and thereby identify Fabry-Pérot and Mie type resonances in silicon thin films and nanodisk arrays, respectively. We measure the optical near-field enhancement by comparing the Raman response on and off resonance. Our results show that tunable-excitation Raman spectroscopy can be used as a complimentary far-field technique to reflection measurements for nanoscale characterization and quality control. As proof-of-principle for the latter, we demonstrate that Raman spectroscopy captures fabrication imperfections in the silicon nanodisk arrays, enabling an all-optical quality control of metasurfaces.

Grayscale-to-Color: Scalable Fabrication of Custom Multispectral Filter Arrays

Snapshot multispectral image (MSI) sensors have been proposed as a key enabler for a plethora of multispectral imaging applications, from diagnostic medical imaging to remote sensing. With each application requiring a different set, and number, of spectral bands, the absence of a scalable, cost-effective manufacturing solution for custom multispectral filter arrays (MSFAs) has prevented widespread MSI adoption. Despite recent nanophotonic-based efforts, such as plasmonic or high-index metasurface arrays, large-area MSFA manufacturing still consists of many-layer dielectric (Fabry-Perot) stacks, requiring separate complex lithography steps for each spectral band and multiple material compositions for each. It is an expensive, cumbersome, and inflexible undertaking, but yields optimal optical performance. Here, we demonstrate a manufacturing process that enables cost-effective wafer-level fabrication of custom MSFAs in a single lithographic step, maintaining high efficiencies (∼75%) and narrow line widths (∼25 nm) across the visible to near-infrared. By merging grayscale (analog) lithography with metal-insulator-metal (MIM) Fabry-Perot cavities, whereby exposure dose controls cavity thickness, we demonstrate simplified fabrication of MSFAs up to N-wavelength bands. The concept is first proven using low-volume electron beam lithography, followed by the demonstration of large-volume UV mask-based photolithography with MSFAs produced at the wafer level. Our framework provides an attractive alternative to conventional MSFA manufacture and metasurface-based spectral filters by reducing both fabrication complexity and cost of these intricate optical devices, while increasing customizability.

Geometric Nanophotonics: Light Management in Single Nanowires through Morphology

Comprehensive control of light-matter interactions at the nanoscale is increasingly important for the development of miniaturized light-based technologies that have applications ranging from information processing to sensing. Control of light in nanoscale structures-the realm of nanophotonics-requires precise control of geometry on a few-nanometer length scale. From a chemist's perspective, bottom-up growth of nanoscale materials from chemical precursors offers a unique opportunity to design structures atom-by-atom that exhibit desired properties. In this Account, we describe our efforts to create chemically and morphologically precise Si nanowires (NWs) with designed nanophotonic properties using a vapor-liquid-solid (VLS) growth process. A synthetic technique termed "Encoded Nanowire Growth and Appearance through VLS and Etching" (ENGRAVE) combines optimized VLS growth, dopant modulation, and dopant-dependent wet-chemical etching to produce NWs with precisely designed diameter modulations, yielding lithographic-like morphological control that can vary from sinusoids to fractals. The ENGRAVE NWs thus provide a versatile playground for coupling, trapping, and directing light in a nanoscale geometry. Previously, the nanophotonic functionality of NWs primarily relied on uniform-diameter structures that exhibit Mie scattering resonances and longitudinally oriented guided modes, two key photonic properties that typically cannot be utilized simultaneously due to their orthogonality. However, when the NW diameter is controllably modulated along the longitudinal axis on a scale comparable to the wavelength of light-a geometry we term a geometric superlattice (GSL)-we found that NWs can exhibit a much richer and tunable set of nanophotonic properties, as described herein. To understand these unique properties, we first summarize the basic optical properties of uniform-diameter NWs using Mie scattering theory and dispersion relations, and we describe both conventional and relatively new microscopy methods that experimentally probe the optical properties of single NWs. Next, delving into the properties of NW GSLs, we summarize their ability to couple a Mie resonance with a guided mode at a select wavevector (or wavelength) dictated by their geometric pitch. The coupling forms a bound guided state (BGS) with a standing wave profile, which allows a NW GSL to serve as a spectrally selective light coupler and to act as optical switch or sensor. We also summarize the capacity of a GSL to trap light by serving as an ultrahigh (theoretically infinite) quality factor optical cavity with an optical bound state in the continuum (BIC), in which destructive interference prevents coupling to and from the far field. Finally, we discuss a future research outlook for using ENGRAVE NWs for nanoscale light control. For instance, we highlight research avenues that could yield light-emitting devices by interfacing a NW-based BIC with emissive materials such as quantum dots, 2D materials, and hybrid perovskite. We also discuss the design of photonic band gaps, generation of high-harmonics with quasi-BIC structures, and the possibility for undiscovered nanophotonic properties and phenomena through more complex ENGRAVE geometric designs.

Generating Third Harmonic Vacuum Ultraviolet Light with a TiO2 Metasurface

Dielectric metasurfaces have recently been shown to provide an excellent platform for the harmonic generation of light due to their low optical absorption and to the strong electromagnetic field enhancement that can be designed into their constituent meta-atoms. Here, we demonstrate vacuum ultraviolet (VUV) third harmonic generation from a specially designed dielectric metasurface consisting of a titanium dioxide (TiO₂) nanostructure array. The metasurface was designed to enhance the generation of VUV light at a wavelength of 185 nm by tailoring its geometric design parameters to achieve an optical resonance at the fundamental laser wavelength of 555 nm. The metasurface exhibits an enhancement factor of nominally 180 compared to an unpatterned TiO₂ thin film of the same thickness, evidence of strong field enhancement at the fundamental wavelength. Mode analysis reveals that the origin of the enhancement is an anapole resonance near the pump wavelength. This work demonstrates an effective strategy for the compact generation of VUV light that could enable expanded access to this useful region of the electromagnetic spectrum.

Symmetry-broken square silicon patches for ultra-narrowband light absorption

The effect of ultra-narrowband light absorption enhancement is presented by using metamaterials with symmetry-broken square silicon patches (SSPs). The symmetry of the SSP can be broken by introducing a narrow slit deviating from its center. By breaking the symmetry of the SSPs, slit resonance mode with standing wave patterns can be excited, and the locations of the absorption peaks can be well estimated by using the Fabry-Pérot (F-P) cavity model. Although there is no excitation of surface plasmon resonance, ultra-narrowband light absorption can be achieved by minimizing the reflectance through perfect impedance matching and simultaneously eliminating the transmittance by the metallic substrate. Good ultra-narrowband absorption features can be maintained as the parameters of the buffer layer and the SSPs are altered. When this type of symmetry-broken SSPs-based metamaterial is used in refractive-index sensors, it shows excellent sensing properties due to its stable ultra-narrowband absorption enhancement.

Homogenization of plasmonic crystals: seeking the epsilon-near-zero effect

By using an asymptotic analysis and numerical simulations, we derive and investigate a system of homogenized Maxwell's equations for conducting material sheets that are periodically arranged and embedded in a heterogeneous and anisotropic dielectric host. This structure is motivated by the need to design plasmonic crystals that enable the propagation of electromagnetic waves with no phase delay (epsilon-near-zero effect). Our microscopic model incorporates the surface conductivity of the two-dimensional (2D) material of each sheet and a corresponding line charge density through a line conductivity along possible edges of the sheets. Our analysis generalizes averaging principles inherent in previous Bloch-wave approaches. We investigate physical implications of our findings. In particular, we emphasize the role of the vector-valued corrector field, which expresses microscopic modes of surface waves on the 2D material. We demonstrate how our homogenization procedure may set the foundation for computational investigations of: effective optical responses of reasonably general geometries, and complicated design problems in the plasmonics of 2D materials.