Ultranarrow plasmon resonances from annealed nanoparticle lattices

Liquid lasing from solutions of ligand-engineered semiconductor nanocrystals

Semiconductor nanocrystals (NCs) can function as efficient gain materials with chemical versatility because of their surface ligands. Because the properties of NCs in solution are sensitive to ligand-environment interactions, local chemical changes can result in changes in the optical response. However, amplification of the optical response is technically challenging because of colloidal instability at NC concentrations needed for sufficient gain to overcome losses. This paper demonstrates liquid lasing from plasmonic lattice cavities integrated with ligand-engineered CdZnS/ZnS NCs dispersed in toluene and water. By taking advantage of calcium ion-induced aggregation of NCs in aqueous solutions, we show how lasing threshold can be used as a transduction signal for ion detection. Our work highlights how NC solutions and plasmonic lattices with open cavity architectures can serve as a biosensing platform for lab-on-chip devices.

Strong Linearly Polarized Light Emission by Coupling Out-of-Plane Exciton to Anisotropic Gap Plasmon Nanocavity

With exceptional quantum confinement, 2D monolayer semiconductors support a strong excitonic effect, making them an ideal platform for exploring light-matter interactions and as building blocks for novel optoelectronic devices. Different from the well-known in-plane excitons in transition metal dichalcogenides (TMD), the out-of-plane excitons in indium selenide (InSe) usually show weak emission, which limits their applications as light sources. Here, by embedding InSe in an anisotropic gap plasmon nanocavity, we have realized plasmon-enhanced linearly polarized photoluminescence with an anisotropic ratio up to ∼140, corresponding to degree of polarization (DoP) of ∼98.6%. Such polarization selectivity, originating from the polarization-dependent plasmonic enhancement supported by the "nanowire-on-mirror" nanocavity, can be well tuned by the InSe thickness. Moreover, we have also realized an InSe-based light-emitting diode with polarized electroluminescence. Our research highlights the role of excitonic dipole orientation in designing nanophotonic devices and paves the way for developing InSe-based optoelectronic devices with polarization control.

Self-Confined Dewetting Mechanism in Wafer-Scale Patterning of Gold Nanoparticle Arrays with Strong Surface Lattice Resonance for Plasmonic Sensing

A self-confined solid-state dewetting mechanism is reported that can fundamentally reduce the use of sophisticated nanofabrication techniques, enabling efficient wafer-scale patterning of non-closely packed (ncp) gold nanoparticle arrays. When combined with a soft lithography process, this approach can address the reproducibility challenges associated with colloidal crystal self-assembly, allowing for the batch fabrication of ncp gold arrays with consistent ordering and even optical properties. The resulting dewetted ncp gold nanoparticle arrays exhibit strong surface lattice resonance properties when excited in inhomogeneous environments under normal white-light incidence. With these SLR properties, the sensitive plasmonic sensing of molecular interactions is achieved using a simple transmission setup. This study will advance the development of miniaturized and portable devices.

Tip-based Lithography with a Sacrificial Layer

The fabrication of a highly controlled gold (Au) nanohole (NH) array via tip-based lithography is improved by incorporating a sacrificial layer-a tip-crash buffer layer. This inclusion mitigates scratches during the nano-indentation process by employing a 300 nm thick poly(methyl methacrylate) layer as a sacrificial layer on top of the Au film. Such a precaution ensures minimal scratches on the Au film, facilitating the creation of sub-50 nm Au NHs with a 15 nm gap between the Au NHs. The precision of this method exceeds that of fabricating Au NHs without a sacrificial layer. Demonstrating its versatility, this Au NH array is utilized in two distinct applications: as a dry etching mask to form a molybdenum disulfide hole array and as a catalyst in metal-assisted chemical etching, resulting in conical-shaped silicon nanostructures. Additionally, a significant electric field is generated when Au nanoparticles (NPs) are placed within the Au NHs. This effect arises from coupling electromagnetic waves, concentrated by the Au NHs and amplified by the Au NPs. A notable result of this configuration is the enhancement factor of surface-enhanced Raman scattering, which is an order of magnitude greater than that observed with just Au NHs and Au NPs alone.

High Q-Factor Plasmonic Surface Lattice Resonances in Colloidal Nanoparticle Arrays

Surface lattice resonances (SLRs) sustained by ordered metal arrays are characterized by their narrow spectral features, remarkable quality factors, and the ability to tune their spectral properties based on the periodicity of the array. However, the majority of these structures are fabricated using classical lithographic processes or require postannealing steps at high temperatures to enhance the quality of the metal. These limitations hinder the widespread utilization of these periodic metal arrays in various applications. In this work, we use the scalable technique of template-assisted assembly of metal colloids to produce plasmonic supercrystals over centimeter areas capable of sustaining SLRs with high Q factors reaching up to 270. Our approach obviates the need for any postprocessing, offering a streamlined and efficient fabrication route. Furthermore, our method enables extensive tunability across the entire visible and near-infrared spectral ranges, empowering the design of tailored plasmonic resonant structures for a wide range of applications.

Spike Growth on Patterned Gold Nanoparticle Scaffolds

This work reports a scaffold-templated, bottom-up synthesis of 3D anisotropic nanofeatures on periodic arrays of gold nanoparticles (AuNPs). Our method relies on substrate-bound AuNPs as large seeds with hemispherical shapes and smooth surfaces after the thermal annealing of as-fabricated particles. Spiky features were grown by immersing the patterned AuNPs into a growth solution consisting of a gold salt and Good's buffer; the number and length of spikes could be tuned by changing the solution pH and buffer concentration. Intermediate structures that informed the growth mechanism were characterized as a function of time by correlating the optical properties and spike features. Large-area (cm2) spiky AuNP arrays exhibited surface-enhanced Raman spectroscopy enhancement that was associated with increased numbers of high-aspect-ratio spikes formed on the AuNP seeds.

Quasi-Random Multimetallic Nanoparticle Arrays

This paper describes a nanofabrication procedure that can generate multiscale substrates with quasi-random microregions of nanoparticle arrays having different periodicities and metals. We combine cycles of large-area nanoparticle array fabrication with solvent-assisted wrinkle lithography to mask and etch quasi-random areas of prefabricated nanoparticles to control the fill factors of the arrays. The approach is highly flexible, and parameters, including nanoparticle size and material, array geometry, and fill factor, can be tailored independently. Multimetallic nanoparticle arrays can support surface lattice resonances at fill factors as low as 20% and can function as nanoscale cavities for lasing action with as few as 10% of the nanoparticles in an array. We demonstrated that multimetallic nanoparticle substrates that combine two or three arrays with different periodicities can exhibit lasing responses over visible and near-infrared wavelengths. Our work showcases the robust optical responses of multimetallic and periodic devices for broadband light manipulation.

Impact of thermal annealing and laser treatment on the morphology and optical responses of mono- and bi-metallic plasmonic honeycomb lattice

Plasmonic nanoparticle arrays with a specific lattice arrangement can support surface lattice resonances (SLRs). SLR exhibits a sharp spectral peak and finds many applications including optical sensing and plasmonic lasers. To optimize SLR for application, a robust method that allows the mass production of plasmonic nanoparticle arrays with refined particle morphology and well-defined lattice arrangement is required. In this work, we combine nanosphere lithography (NSL) with thermal annealing or nanosecond-pulsed laser treatment to refine plasmonic nanoparticles in a honeycomb lattice. We comparatively study the effects of the two treatment methods on the particle morphology and lattice arrangement of mono (Ag and Pd) and bi-metallic (Ag-Pd) nanoparticle lattices. In general, thermal annealing preserves the lattice arrangement but fairly changes the particle roundness, while laser treatment produces particles with varying morphologies and spatial distribution. We also theoretically and experimentally investigate the optical responses of Ag nanoparticle lattices produced by different treatment methods. The observed difference in spectra can be attributed to the varying particle morphology, which shifts the localized surface plasmon resonance differently, resulting in a significant change in SLR. These findings provide valuable insights for optimizing plasmonic nanoparticle arrays for various applications.

Multiple surface lattice resonances of overlapping nanoparticle arrays with different lattice spacing

Multiple surface lattice resonances generated with nanoparticle arrays are promising to enhance light-matter interactions at different spectral positions simultaneously, and it is important to tailor these resonances to desired frequencies for practical applications such as multi-modal nanolasing. To this end, this study proposes to generate multiple surface lattice resonances using overlapping nanoparticle arrays with different lattice spacing. Both full-wave numerical simulations and analytical coupled dipole approximation calculations reveal that for the overlapping structures composed with two different gold nanosphere arrays, both surface lattice resonances for the element structures are effectively excited. Considering that the optical responses are governed by the dipole-dipole interactions between the nanoparticles, it is interesting to find that the multiple surface lattice resonances are almost invariant by adjusting the relative shifts between the two arrays, which can be useful to tailor the high-quality factor resonances to desired spectral positions. In addition, due to the same reason, it is also shown that the multiple surface lattice resonances can be further finely tuned by selectively removing specific nanoparticles in the array. We anticipate that the tolerance to generate multiple surface lattice resonances and the flexible tunability make the overlapping nanoparticle arrays useful to design high performance linear and nonlinear nanophotonic devices.

Superlattice Surface Lattice Resonances in Plasmonic Nanoparticle Arrays with Patterned Dielectrics

This paper describes how two-dimensional plasmonic nanoparticle lattices covered with microscale arrays of dielectric patches can show superlattice surface lattice resonances (SLRs). These optical resonances originate from multiscale diffractive coupling that can be controlled by the periodicity and size of the patterned dielectrics. The features in the optical dispersion diagram are similar to those of index-matched microscale arrays of metal nanoparticle lattices, having the same lateral dimensions as the dielectric patches. With an increase in nanoparticle size, superlattice SLRs can also support quadrupole excitations with distinct dispersion diagrams. The tunable optical band structure enabled by patterned dielectrics on plasmonic nanoparticle arrays offers prospects for enhanced nonlinear optics, nanoscale lasing, and engineered parity-time symmetries.

Low-Loss Plasmonics with Nanostructured Potassium and Sodium-Potassium Liquid Alloys

Alkali metals have low optical losses in the visible to near-infrared (NIR) compared with noble metals. However, their high reactivity prohibits the exploration of their optical properties. Recently sodium (Na) has been experimentally demonstrated as a low-loss plasmonic material. Here we report on a thermo-assisted nanoscale embossing (TANE) technique for fabricating plasmonic nanostructures from pure potassium (K) and NaK liquid alloys. We show high-quality-factor resonances from K as narrow as 15 nm in the NIR, which we attribute to the high material quality and low optical loss. We further demonstrate liquid Na-K plasmonics by exploiting the Na-K eutectic phase diagram. Our study expands the material library for alkali metal plasmonics and liquid plasmonics, potentially enabling a range of new material platforms for active metamaterials and photonic devices.

Rapid genetic screening with high quality factor metasurfaces

Genetic analysis methods are foundational to advancing personalized medicine, accelerating disease diagnostics, and monitoring the health of organisms and ecosystems. Current nucleic acid technologies such as polymerase chain reaction (PCR) and next-generation sequencing (NGS) rely on sample amplification and can suffer from inhibition. Here, we introduce a label-free genetic screening platform based on high quality (high-Q) factor silicon nanoantennas functionalized with nucleic acid fragments. Each high-Q nanoantenna exhibits average resonant quality factors of 2,200 in physiological buffer. We quantitatively detect two gene fragments, SARS-CoV-2 envelope (E) and open reading frame 1b (ORF1b), with high-specificity via DNA hybridization. We also demonstrate femtomolar sensitivity in buffer and nanomolar sensitivity in spiked nasopharyngeal eluates within 5 minutes. Nanoantennas are patterned at densities of 160,000 devices per cm², enabling future work on highly-multiplexed detection. Combined with advances in complex sample processing, our work provides a foundation for rapid, compact, and amplification-free molecular assays.

Chiral Lattice Resonances in 2.5-Dimensional Periodic Arrays with Achiral Unit Cells

Lattice resonances are collective electromagnetic modes supported by periodic arrays of metallic nanostructures. These excitations arise from the coherent multiple scattering between the elements of the array and, thanks to their collective origin, produce very strong and spectrally narrow optical responses. In recent years, there has been significant effort dedicated to characterizing the lattice resonances supported by arrays built from complex unit cells containing multiple nanostructures. Simultaneously, periodic arrays with chiral unit cells, made of either an individual nanostructure with a chiral morphology or a group of nanostructures placed in a chiral arrangement, have been shown to exhibit lattice resonances with different responses to right- and left-handed circularly polarized light. Motivated by this, here, we investigate the lattice resonances supported by square bipartite arrays in which the relative positions of the nanostructures can vary in all three spatial dimensions, effectively functioning as 2.5-dimensional arrays. We find that these systems can support lattice resonances with almost perfect chiral responses and very large quality factors, despite the achirality of the unit cell. Furthermore, we show that the chiral response of the lattice resonances originates from the constructive and destructive interference between the electric and magnetic dipoles induced in the two nanostructures of the unit cell. Our results serve to establish a theoretical framework to describe the optical response of 2.5-dimensional arrays and provide an approach to obtain chiral lattice resonances in periodic arrays with achiral unit cells.

Molecular-Induced Chirality Transfer to Plasmonic Lattice Modes

Molecular chirality plays fundamental roles in biology. The chiral response of a molecule occurs at a specific spectral position, determined by its molecular structure. This fingerprint can be transferred to other spectral regions via the interaction with localized surface plasmon resonances of gold nanoparticles. Here, we demonstrate that molecular chirality transfer occurs also for plasmonic lattice modes, providing a very effective and tunable means to control chirality. We use colloidal self-assembly to fabricate non-close packed, periodic arrays of achiral gold nanoparticles, which are embedded in a polymer film containing chiral molecules. In the presence of the chiral molecules, the surface lattice resonances (SLRs) become optically active, i.e., showing handedness-dependent excitation. Numerical simulations with varying lattice parameters show circular dichroism peaks shifting along with the spectral positions of the lattice modes, corroborating the chirality transfer to these collective modes. A semi-analytical model based on the coupling of single-molecular and plasmonic resonances rationalizes this chirality transfer.

Attosecond electron microscopy of sub-cycle optical dynamics

The primary step of almost any interaction between light and materials is the electrodynamic response of the electrons to the optical cycles of the impinging light wave on sub-wavelength and sub-cycle dimensions¹. Understanding and controlling the electromagnetic responses of a material^2-11 is therefore essential for modern optics and nanophotonics^12-19. Although the small de Broglie wavelength of electron beams should allow access to attosecond and ångström dimensions²⁰, the time resolution of ultrafast electron microscopy²¹ and diffraction²² has so far been limited to the femtosecond domain^16-18, which is insufficient for recording fundamental material responses on the scale of the cycles of light^1,2,10. Here we advance transmission electron microscopy to attosecond time resolution of optical responses within one cycle of excitation light²³. We apply a continuous-wave laser²⁴ to modulate the electron wave function into a rapid sequence of electron pulses, and use an energy filter to resolve electromagnetic near-fields in and around a material as a movie in space and time. Experiments on nanostructured needle tips, dielectric resonators and metamaterial antennas reveal a directional launch of chiral surface waves, a delay between dipole and quadrupole dynamics, a subluminal buried waveguide field and a symmetry-broken multi-antenna response. These results signify the value of combining electron microscopy and attosecond laser science to understand light-matter interactions in terms of their fundamental dimensions in space and time.

Far-field coupling between moiré photonic lattices

Superposing two or more periodic structures to form moiré patterns is emerging as a promising platform to confine and manipulate light. However, moiré-facilitated interactions and phenomena have been constrained to the vicinity of moiré lattices. Here we report on the observation of ultralong-range coupling between photonic lattices in bilayer and multilayer moiré architectures mediated by dark surface lattice resonances in the vertical direction. We show that two-dimensional plasmonic nanoparticle lattices enable twist-angle-controlled directional lasing emission, even when the lattices are spatially separated by distances exceeding three orders of magnitude of lattice periodicity. Our discovery of far-field interlattice coupling opens the possibility of using the out-of-plane dimension for optical manipulation on the nanoscale and microscale.

d-Band Hole Dynamics in Gold Nanoparticles Measured with Time-Resolved Emission Upconversion Microscopy

The performance of photocatalysts and photovoltaic devices can be enhanced by energetic charge carriers produced from plasmon decay, and the lifetime of these energetic carriers greatly affects overall efficiencies. Although hot electron lifetimes in plasmonic gold nanoparticles have been investigated, hot hole lifetimes have not been as thoroughly studied in plasmonic systems. Here, we demonstrate time-resolved emission upconversion microscopy and use it to resolve the lifetime and energy-dependent cooling of d-band holes formed in gold nanoparticles by plasmon excitation and by following plasmon decay into interband and then intraband electron-hole pairs.

Field-Pulse-Induced Annealing of 2D Colloidal Polycrystals

Two-dimensional colloidal crystals are of considerable fundamental and practical importance. However, their quality is often low due to the widespread presence of domain walls and defects. In this work, we explored the annealing process undergone by monolayers of superparamagnetic colloids adsorbed onto fluid interfaces in the presence of magnetic field pulses. These systems present the extraordinary peculiarity that both the extent and the character of interparticle interactions can be adjusted at will by simply varying the strength and orientation of the applied field so that the application of field pulses results in a sudden input of energy. Specifically, we have studied the effect of polycrystal size, pulse duration, slope and frequency on the efficiency of the annealing process and found that (i) this strategy is only effective when the polycrystal consists of less than approximately 10 domains; (ii) that the pulse duration should be of the order of magnitude of the time required for the outer particles to travel one diameter during the heating step; (iii) that the quality of larger polycrystals can be slightly improved by applying tilted pulses. The experimental results were corroborated by Brownian dynamics simulations.

Fabrication of Centimeter-Scale Plasmonic Nanoparticle Arrays with Ultranarrow Surface Lattice Resonances

Plasmonic surface lattice resonances (SLRs) supported by metallic nanoparticle (NP) arrays show diverse applications including nanolasers, sensors, photocatalysis, and nonlinear optics. However, to rationally fabricate high-quality plasmonic NP arrays with ultranarrow SLR line widths over large areas remains challenging. This article describes a general approach for the efficient fabrication of centimeter-scale inorganic NP arrays with precisely controlled NP size, composition, position, and lattice geometry. This method combines the processes of solvent-assisted soft lithography and in situ site-specific NP growth to reproducibly create many replicates of NP arrays without utilizing cleanroom and specialized equipment. For demonstration, we show that Au NP arrays exhibit ultranarrow SLRs with a line width of 4 nm and a quality factor of 218 toward the theoretical limit.

Engineering Electromagnetic Field Distribution and Resonance Quality Factor Using Slotted Quasi-BIC Metasurfaces

Dielectric metasurfaces governed by bound states in the continuum (BIC) are actively investigated for achieving high-quality factors and strong electromagnetic field enhancements. Traditional approaches reported for tuning the performance of quasi-BIC metasurfaces include tuning the resonator size, period, and structure symmetry. Here we propose and experimentally demonstrate an alternative approach through engineering slots within a zigzag array of elliptical silicon resonators. Through analytical theory, three-dimensional electromagnetic modeling, and infrared spectroscopy, we systematically investigate the spectral responses and field distributions of the slotted metasurface in the mid-IR. Our results show that by introducing slots, the electric field intensity enhancement near the apex and the quality factor of the quasi-BIC resonance are increased by a factor of 2.1 and 3.3, respectively, in comparison to the metasurface without slots. Furthermore, the slotted metasurface also provides extra regions of electromagnetic enhancement and confinement, which holds enormous potential in particle trapping, sensing, and emission enhancement.

High-Q out-of-plane Mie electric dipole surface lattice resonances in silicon metasurfaces

Metasurfaces supporting surface lattice resonances (SLRs) with narrow linewidths and high quality factors have become an exciting platform for diverse applications. Here we numerically show, for the first time, that narrowband out-of-plane Mie electric dipole SLRs (ED-SLRs) can be excited together with the in-plane ED-SLRs and magnetic-dipole SLRs in periodic silicon disks under oblique incidence with TM polarization. Simulation results show that the out-of-plane ED-SLR can have four times larger quality factors than the in-plane one under the same excitation conditions, and can have distinct near-field distributions and dispersion relationships compared with the plasmonic counterpart in periodic metallic nanodisks. We further show that the out-of-plane ED-SLR can define a symmetry-protected bound state in the continuum (BIC) at normal incidence, which transits into a quasi-BIC when the excitation field symmetry is slightly broken by the small incidence angle. We expect this work will advance the engineering of Mie SLRs for applications in metasurface-based nanolasers, nonlinear optics, and optical sensing.

Lattice Resonances Excited by Finite-Width Light Beams

Periodic arrays of metallic nanostructures support collective lattice resonances, which give rise to optical responses that are, at the same time, stronger and more spectrally narrow than those of the localized plasmons of the individual nanostructures. Despite the extensive research effort devoted to investigating the optical properties of lattice resonances, the majority of theoretical studies have analyzed them under plane-wave excitation conditions. Such analysis not only constitutes an approximation to realistic experimental conditions, which require the use of finite-width light beams, but also misses a rich variety of interesting behaviors. Here, we provide a comprehensive study of the response of periodic arrays of metallic nanostructures when excited by finite-width light beams under both paraxial and nonparaxial conditions. We show how as the width of the light beam increases, the response of the array becomes more collective and converges to the plane-wave limit. Furthermore, we analyze the spatial extent of the lattice resonance and identify the optimum values of the light beam width to achieve the strongest optical responses. We also investigate the impact that the combination of finite-size effects in the array and the finite width of the light beam has on the response of the system. Our results provide a solid theoretical framework to understand the excitation of lattice resonances by finite-width light beams and uncover a set of behaviors that do not take place under plane-wave excitation.

Theoretical Quantum Model of Two-Dimensional Propagating Plexcitons

When plasmonic excitations of metallic interfaces and nanostructures interact with electronic excitations in semiconductors, new states emerge that hybridize the characteristics of the uncoupled states. The engendered properties make these hybrid states appealing for a broad range of applications, ranging from photovoltaic devices to integrated circuitry for quantum devices. Here, through quantum modeling, the coupling of surface plasmon polaritons and mobile two-dimensional excitons such as those in atomically thin semiconductors is examined with emphasis on the case of strong coupling. Our model shows that at around the energy crossing of the dispersion relationships of the uncoupled species, they strongly interact and polariton states --propagating plexcitons -- emerge. The temporal evolution of the system where surface plasmon polaritons are continuously injected into the system is simulated to gain initial insight on potential experimental realizations of these states. The results show a steady state that is dominated by the lower-energy polariton. The study theoretically further establishes the possible existence of propagating plexcitons in atomically thin semiconductors and provides important guidance for the experimental detection and characterization of such states for a wide range of optoelectronic technologies.

Lasing Action from Quasi-Propagating Modes

Band edges at the high symmetry points in reciprocal space of periodic structures hold special interest in materials engineering for their high density of states. In optical metamaterials, standing waves found at these points have facilitated lasing, bound-states-in-the-continuum, and Bose-Einstein condensation. However, because high symmetry points by definition are localized, properties associated with them are limited to specific energies and wavevectors. Conversely, quasi-propagating modes along the high symmetry directions are predicted to enable similar phenomena over a continuum of energies and wavevectors. Here, quasi-propagating modes in 2D nanoparticle lattices are shown to support lasing action over a continuous range of wavelengths and symmetry-determined directions from a single device. Using lead halide perovskite nanocrystal films as gain materials, lasing is achieved from waveguide-surface lattice resonance (W-SLR) modes that can be decomposed into propagating waves along high symmetry directions, and standing waves in the orthogonal direction that provide optical feedback. The characteristics of the lasing beams are analyzed using an analytical 3D model that describes diffracted light in 2D lattices. Demonstrations of lasing across different wavelengths and lattice designs highlight how quasi-propagating modes offer possibilities to engineer chromatic multibeam emission important in hyperspectral 3D sensing, high-bandwidth Li-Fi communication, and laser projection displays.

Light-Matter Interactions in Hybrid Material Metasurfaces

This Review focuses on the integration of plasmonic and dielectric metasurfaces with emissive or stimuli-responsive materials for manipulating light-matter interactions at the nanoscale. Metasurfaces, engineered planar structures with rationally designed building blocks, can change the local phase and intensity of electromagnetic waves at the subwavelength unit level and offers more degrees of freedom to control the flow of light. A combination of metasurfaces and nanoscale emitters facilitates access to weak and strong coupling regimes for enhanced photoluminescence, nanoscale lasing, controlled quantum emission, and formation of exciton-polaritons. In addition to emissive materials, functional materials that respond to external stimuli can be combined with metasurfaces to engineer tunable nanophotonic devices. Emerging metasurface designs including surface-functionalized, chemically tunable, and multilayer hybrid metasurfaces open prospects for diverse applications, including photocatalysis, sensing, displays, and quantum information.

Engineering van der Waals Materials for Advanced Metaphotonics

The outstanding chemical and physical properties of 2D materials, together with their atomically thin nature, make them ideal candidates for metaphotonic device integration and construction, which requires deep subwavelength light-matter interaction to achieve optical functionalities beyond conventional optical phenomena observed in naturally available materials. In addition to their intrinsic properties, the possibility to further manipulate the properties of 2D materials via chemical or physical engineering dramatically enhances their capability, evoking new science on light-matter interaction, leading to leaped performance of existing functional devices and giving birth to new metaphotonic devices that were unattainable previously. Comprehensive understanding of the intrinsic properties of 2D materials, approaches and capabilities for chemical and physical engineering methods, the resulting property modifications and novel functionalities, and applications of metaphotonic devices are provided in this review. Through reviewing the detailed progress in each aspect and the state-of-the-art achievement, insightful analyses of the outstanding challenges and future directions are elucidated in this cross-disciplinary comprehensive review with the aim to provide an overall development picture in the field of 2D material metaphotonics and promote rapid progress in this fast emerging and prosperous field.

Interfacial engineering of plasmonic nanoparticle metasurfaces

SignificanceMolecules interacting with metallic nanostructures can show tunable exciton-plasmon coupling, ranging from weak to strong. One factor that influences the interactions is the spatial organization of the molecules relative to the localized plasmon-enhanced electromagnetic fields. In this work, we show that the arrangement of aromatic dye molecules can be tuned within plasmonic hotspots by interfacial engineering of nanoparticle surfaces. By controlling the local chemical and physical interactions, we could modulate lasing thresholds. Surface-functionalized plasmonic metasurfaces open prospects for programmable light-matter interactions at the nanoscale.

Polariton Dynamics in Two-Dimensional Ruddlesden-Popper Perovskites Strongly Coupled with Plasmonic Lattices

Strong coupling between light and matter can produce hybrid eigenstates known as exciton-polaritons. Although polariton dynamics are important photophysical properties, the relaxation pathways of polaritons in different coupling regimes have seen limited attention. This paper reports the dynamics of hybridized states from 2D Ruddlesden-Popper perovskites coupled to plasmonic nanoparticle lattices. The open cavity architecture of Al lattices enables the coupling strength to be modulated by varying either the lead halide perovskite film thickness or the superstrate refractive index. Both experiments and finite-difference time-domain simulations of the optical dispersion diagrams showed avoided crossings that are a signature of strong coupling. Our analytical model also elucidated the correlation between the exciton/plasmon mixing ratio and polariton coupling strength. Using fs-transient absorption spectroscopy, we found that both the upper and lower polaritons have shorter lifetimes than the excitons and that polaritons can show faster excited-state dynamics when they have access to additional energy transfer channels.

Finite-size and disorder effects on 1D unipartite and bipartite surface lattice resonances

Optical resonances in bipartite metal nanostructure lattices are more resilient to finite size-effects than equivalent unipartite lattices, but the complexities of their behaviour in non-ideal settings remain relatively unexplored. Here we investigate the quality factor and extinction efficiency of 1D Ag and Au unipartite and bipartite lattices. By modelling finite size lattices over a range of periods we show that the quality factor of Ag bipartite lattices is significantly better than unipartite lattices. This improvement is less pronounced for Au bipartite lattices. We also show that bipartite lattices are dramatically affected by structure size variations at scales that are typically seen in electron beam lithography fabrication in contrast to unipartite lattices, which are not as sensitive.

Metallic Plasmonic Array Structures: Principles, Fabrications, Properties, and Applications

The vast development of nanofabrication has spurred recent progress for the manipulation of light down to a region much smaller than the wavelength. Metallic plasmonic array structures are demonstrated to be the most powerful platform to realize controllable light-matter interactions and have found wide applications due to their rich and tunable optical performance through the morphology and parameter engineering. Here, various light-management mechanisms that may exist on metallic plasmonic array structures are described. Then, the typical techniques for fabrication of metallic plasmonic arrays are summarized. Next, some recent applications of plasmonic arrays are reviewed, including plasmonic sensing, surface-enhanced spectroscopies, plasmonic nanolasing, and perfect light absorption. Lastly, the existing challenges and perspectives for metallic plasmonic arrays are discussed. The aim is to provide guidance for future development of metallic plasmonic array structures.

Hybrid anisotropic plasmonic metasurfaces with multiple resonances of focused light beams

Plasmonic metasurfaces supporting collective lattice resonances have attracted increasing interest due to their exciting properties of strong spatial coherence and enhanced light-matter interaction. Although the focusing of light by high-numerical-aperture (NA) objectives provides an essential way to boost the field intensities, it remains challenging to excite high-quality resonances by using high-NA objectives due to strong angular dispersion. Here, we address this challenge by employing the physics of bound states in the continuum (BICs). We design a novel anisotropic plasmonic metasurface combining a two-dimensional lattice of high-aspect-ratio pillars with a one-dimensional plasmonic grating, fabricated by a two-photon polymerization technique and gold sputtering. We demonstrate experimentally multiple resonances with absorption amplitudes exceeding 80% at mid-IR using an NA = 0.4 reflective objective. This is enabled by the weak angular dispersion of quasi-BIC resonances in such hybrid plasmonic metasurfaces. Our results suggest novel strategies for designing photonic devices that manipulate focused light with a strong field concentration.

Ultra-narrow-band metamaterial perfect absorber based on surface lattice resonance in a WS2 nanodisk array

Metamaterial perfect absorbers (MPAs) are attractive platforms for the unique manipulation of electromagnetic waves from near-field to far-field. Narrow-band MPAs are particularly intriguing for their potential applications as thermal emitters or biosensors. In this work, we proposed ultra-narrow-band MPAs based on surface lattice resonance (SLR) modes of WS₂ nanodisk arrays on gold films. The SLR modes stem from the coupling between the magnetic dipole modes of individual nanodisks and the Rayleigh anomaly of the array giving rise to high quality-factor resonances. With proper design of the nanodisk array, an ultra-narrow-band of 15 nm is achieved in the near infrared wavelength range. The underneath gold film provides the loss channel converting the incident light within the narrow band into heat in the gold film, effectively creating a perfect absorber. Systematic numerical simulations were performed to investigate the effects of the geometrical parameters on their optical properties, demonstrating the great tunability of this type of MPAs as well as their potential for engineering light-matter interactions.

Multipolar Lattice Resonances in Plasmonic Finite-Size Metasurfaces

Collective lattice resonances in regular arrays of plasmonic nanoparticles have attracted much attention due to a large number of applications in optics and photonics. Most of the research in this field is concentrated on the electric dipolar lattice resonances, leaving higher-order multipolar lattice resonances in plasmonic nanostructures relatively unexplored. Just a few works report exceptionally high-Q multipolar lattice resonances in plasmonic arrays, but only with infinite extent (i.e., perfectly periodic). In this work, we comprehensively study multipolar collective lattice resonances both in finite and in infinite arrays of Au and Al plasmonic nanoparticles using a rigorous theoretical treatment. It is shown that multipolar lattice resonances in the relatively large (up to 6400 nanoparticles) finite arrays exhibit broader full width at half maximum (FWHM) compared to similar resonances in the infinite arrays. We argue that our results are of particular importance for the practical implementation of multipolar lattice resonances in different photonics applications.

High-Q quadrupolar plasmonic lattice resonances in horizontal metal-insulator-metal gratings

We propose a plasmonic platform for achieving out-of-plane quadrupolar plasmonic surface lattice resonances (SLRs) with large quality factors. The proposed platform is composed of a horizontal metal-insulator-metal (MIM) grating embedded in a homogeneous dielectric environment. Numerical results based on rigorous coupled-wave analysis show that under oblique incidences, high-Q out-of-plane quadrupolar SLRs can be excited at wavelengths of 1242 nm over a wide range of insulator widths, and the quality factor can reach 1036. As a comparison, under the same conditions, only dipolar SLRs with much lower quality factors of ∼300 can be excited in a vertical MIM grating, which has the same period and a quarter-turned unit cell. We expect that the proposed high-Q quadrupolar SLR platform will find applications in light-matter interactions on the nanoscale.

Ultra-high-Q resonances in plasmonic metasurfaces

Plasmonic nanostructures hold promise for the realization of ultra-thin sub-wavelength devices, reducing power operating thresholds and enabling nonlinear optical functionality in metasurfaces. However, this promise is substantially undercut by absorption introduced by resistive losses, causing the metasurface community to turn away from plasmonics in favour of alternative material platforms (e.g., dielectrics) that provide weaker field enhancement, but more tolerable losses. Here, we report a plasmonic metasurface with a quality-factor (Q-factor) of 2340 in the telecommunication C band by exploiting surface lattice resonances (SLRs), exceeding the record by an order of magnitude. Additionally, we show that SLRs retain many of the same benefits as localized plasmonic resonances, such as field enhancement and strong confinement of light along the metal surface. Our results demonstrate that SLRs provide an exciting and unexplored method to tailor incident light fields, and could pave the way to flexible wavelength-scale devices for any optical resonating application.

Plasmonic Photoelectrocatalysis in Copper-Platinum Core-Shell Nanoparticle Lattices

This paper reports that strongly coupled bimetallic core-shell nanoparticle arrays show photoelectrocatalytic activity for hydrogen evolution reactions (HER). We fabricated large-area Cu-Pt nanoparticle lattices by combining top-down lithography and solution-based chemistry. These coupled lattices support two different types of plasmon modes, localized surface plasmons from individual particles and surface lattice resonances (SLRs) from the 2D lattice, that increased HER catalytic activity under white-light illumination up to 60%. Comparing photoelectrocatalytic performances of the two plasmon modes at different wavelength ranges, we found that SLRs had two-fold activity enhancement over that from localized surface plasmons.