Fano Resonant Aluminum Nanoclusters for Plasmonic Colorimetric Sensing

Tuning the modal coupling in three-dimensional Au@Cu2O@Au core-shell-satellite nanostructures for enhanced plasmonic photocatalysis

The fascinating optical properties of coupled plasmonic nanostructures have attracted great attention and emerged as a promising platform for applications in catalysis, sensing, and photonics. When two resonant modes interact strongly, the coupled modes are formed with mixed properties inherited from the basic modes. However, limitations still exist in the understanding of the coupling between optical modes in individual structures. In addition, how the coupling in hybrid plasmonic structures correlated with their efficiencies in promoting photocatalysis remains an important and challenging question. Here we demonstrate that coupling in individual Au@Cu2O@Au core-shell-satellite hybrid structures can be tuned through rationally designing the structural features for enhancing plasmonic photocatalysis. To further comprehend the optical phenomena of different coupled nanostructures, a model was developed based on the Mie theory, which provided a different perspective to analyze coupling qualitatively in individual nanoparticles. The insights gained from this work not only shed light on the underlying mechanisms of modal coupling in individual structures but also provide an important knowledge framework that guides the rational design of coupled plasmonic nanostructures for plasmonic photochemistry and photocatalysis.

Harnessing the Power of Plasmonics for in Vitro and in Vivo Biosensing

Plasmonic nanostructures exhibit localized surface plasmon resonances due to collective oscillation of conducting electrons that can be tuned by modulating the nanostructure size, shape, material composition, and local dielectric environment. The strong field confinement and enhancement provided by plasmonic nanostructures have been exploited over the years to enhance the sensitivity for analyte detection down to the single-molecule level, rendering these devices as potentially outstanding biosensors. Here, we summarize methods to detect biological analytes in vitro and in living cells, with a focus on plasmon-enhanced fluorescence, Raman scattering, infrared absorption, circular dichroism, and refractive index sensing. Given the tremendous advances in the field, we concentrate on a few recent examples toward biosensing under highly challenging detection conditions, including clinically relevant biomarkers in body fluids and nascent applications in living cells and in vivo. These emerging platforms serve as inspiration for exploring future directions of nanoplasmonics that can be further harnessed to advance real-world biosensing applications.

Manipulating Fano Coupling in an Opto-Thermoelectric Field

Fano resonances in photonics arise from the coupling and interference between two resonant modes in structures with broken symmetry. They feature an uneven and narrow and tunable lineshape and are ideally suited for optical spectroscopy. Many Fano resonance structures have been suggested in nanophotonics over the last ten years, but reconfigurability and tailored design remain challenging. Herein, an all-optical "pick-and-place" approach aimed at assembling Fano metamolecules of various geometries and compositions in a reconfigurable manner is proposed. Their coupling behavior by in situ dark-field scattering spectroscopy is studied. Driven by a light-directed opto-thermoelectric field, silicon nanoparticles with high-quality-factor Mie resonances (discrete states) and low-loss BaTiO3 nanoparticles (continuum states) are assembled into all-dielectric heterodimers, where distinct Fano resonances are observed. The Fano parameter can be adjusted by changing the resonant frequency of the discrete states or the light polarization. Tunable coupling strength and multiple Fano resonances by altering the number of continuum states and discrete states in dielectric heterooligomers are also shown. This work offers a general design rule for Fano resonance and an all-optical platform for controlling Fano coupling on demand.

Plasmonic nanoparticle sensors: current progress, challenges, and future prospects

Plasmonic nanoparticles (NPs) have played a significant role in the evolution of modern nanoscience and nanotechnology in terms of colloidal synthesis, general understanding of nanocrystal growth mechanisms, and their impact in a wide range of applications. They exhibit strong visible colors due to localized surface plasmon resonance (LSPR) that depends on their size, shape, composition, and the surrounding dielectric environment. Under resonant excitation, the LSPR of plasmonic NPs leads to a strong field enhancement near their surfaces and thus enhances various light-matter interactions. These unique optical properties of plasmonic NPs have been used to design chemical and biological sensors. Over the last few decades, colloidal plasmonic NPs have been greatly exploited in sensing applications through LSPR shifts (colorimetry), surface-enhanced Raman scattering, surface-enhanced fluorescence, and chiroptical activity. Although colloidal plasmonic NPs have emerged at the forefront of nanobiosensors, there are still several important challenges to be addressed for the realization of plasmonic NP-based sensor kits for routine use in daily life. In this comprehensive review, researchers of different disciplines (colloidal and analytical chemistry, biology, physics, and medicine) have joined together to summarize the past, present, and future of plasmonic NP-based sensors in terms of different sensing platforms, understanding of the sensing mechanisms, different chemical and biological analytes, and the expected future technologies. This review is expected to guide the researchers currently working in this field and inspire future generations of scientists to join this compelling research field and its branches.

Highly Ordered Nanoassemblies of Janus Spherocylindrical Nanoparticles Adhering to Lipid Vesicles

In recent years, there has been a heightened interest in the self-assembly of nanoparticles (NPs) that is mediated by their adsorption onto lipid membranes. The interplay between the adhesive energy of NPs on a lipid membrane and the membrane's curvature energy causes it to wrap around the NPs. This results in an interesting membrane curvature-mediated interaction, which can lead to the self-assembly of NPs on lipid membranes. Recent studies have demonstrated that Janus spherical NPs, which adhere to lipid vesicles, can self-assemble into well-ordered nanoclusters with various geometries, including a few Platonic solids. The present study explores the additional effect of geometric anisotropy on the self-assembly of Janus NPs on lipid vesicles. Specifically, the current study utilized extensive molecular dynamics simulations to investigate the arrangement of Janus spherocylindrical NPs on lipid vesicles. We found that the additional geometric anisotropy significantly expands the range of NPs' self-assemblies on lipid vesicles. The specific geometries of the resulting nanoclusters depend on several factors, including the number of Janus spherocylindrical NPs adhering to the vesicle and their aspect ratio. The lipid membrane-mediated self-assembly of NPs, demonstrated by this work, provides an alternative cost-effective route for fabricating highly engineered nanoclusters in three dimensions. Such structures, with the current wide range of material choices, have great potential for advanced applications, including biosensing, bioimaging, drug delivery, nanomechanics, and nanophotonics.

Assessing the efficacy of aluminum metal clusters Al13 and Al15 in mitigating NO2 and SO2 pollutants: a DFT investigation

The present investigation delves into the adverse environmental impact of atmospheric pollutant gases, specifically nitrogen dioxide (NO2) and sulfur dioxide (SO2), which necessitates the identification and implementation of effective control measures. The central objective of this study is to explore the eradication of these pollutants through the utilization of aluminum Al13 and Al15 metal clusters, distinguished by their unique properties. The comprehensive evaluation of gas/cluster interactions is undertaken employing density functional theory (DFT). Geometric optimization calculations for all structures are executed using the ωB97XD functional and the Def2-svp basis set. To probe various interaction modalities, gas molecule distribution around the metal clusters is sampled using the bee colony algorithm. Frequency calculations employing identical model chemistry validate the precision of the optimization calculations. The quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) methodologies are applied for the analysis of intermolecular interactions. This research establishes the robust formation of van der Waals attractions between the investigated gas molecules, affirming aluminum metal clusters as viable candidates for the removal and control of these gases.

Dissipative coupling in a Bragg-grating-coupled single resonator with Fano resonance for anti-PT-symmetric gyroscopes

Anti-parity-time-symmetric Hamiltonians show an enhanced sensitivity to external perturbations that can be used for high-performance angular velocity sensing. Dissipative coupling is a valuable way for realizing anti-PT-symmetric Hamiltonians with optical resonators and is usually obtained by means of auxiliary waveguides. Here, we model and experimentally show the dissipative coupling between two counterpropagating modes of a single resonator, by means of a Bragg-grating placed in the feeding bus. The proposed solution enables the possibility of accurately designing the dissipative coupling strength in integrated non-Hermitian gyroscopes, thus providing high flexibility in the design of the proposed sensor. Moreover, we theoretically and experimentally demonstrate that the dissipative coupling between two counterpropagating modes of the same resonant cavity can give rise to an asymmetric Fano resonance.

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.

Plasmonic Color Switching by a Combination Device with Nematic Liquid Crystals and a Silver Nanocube Monolayer

We experimentally demonstrated electrical plasmonic color modulation by combining a nematic-phase liquid crystal (LC) layer and a silver nanocube (AgNC) monolayer. The color modulation LC/AgNC device was fabricated by filling LCs with negative dielectric anisotropy onto a densely assembled AgNC monolayer. The transmitted light color through the LC/AgNC device was modulated between green and magenta by applying voltages of 0-15 V. The peaks and dips in the transmission spectrum of the LC/AgNC device at wavelengths of 500-600 nm were switched with voltage. The switching effect of light transmission in the green region was achieved by overlapping the plasmon resonance of the AgNC monolayer and multiple transmittance peaks caused by the birefringence of the LC layer. In addition, the color inversion appeared at cross-Nicole and parallel-Nicole because the LC layer functioned like a half-wave plate due to birefringence. The electrical modulation of the plasmonic color with LCs has a high implementation capability in microdevices and is anticipated to be applied in display devices or color filters.

Full-Control and Switching of Optical Fano Resonance by Continuum State Engineering

Fano resonance, known for its unique asymmetric line shape, has gained significant attention in photonics, particularly in sensing applications. However, it remains difficult to achieve controllable Fano parameters with a simple geometric structure. Here, a novel approach of using a thin-film optical Fano resonator with a porous layer to generate entire spectral shapes from quasi-Lorentzian to Lorentzian to Fano is proposed and experimentally demonstrated. The glancing angle deposition technique is utilized to create a polarization-dependent Fano resonator. By altering the linear polarization between s- and p-polarization, a switchable Fano device between quasi-Lorentz state and negative Fano state is demonstrated. This change in spectral shape is advantageous for detecting materials with a low-refractive index. A bio-particle sensing experiment is conducted that demonstrates an enhanced signal-to-noise ratio and prediction accuracy. Finally, the challenge of optimizing the film-based Fano resonator due to intricate interplay among numerous parameters, including layer thicknesses, porosity, and materials selection, is addressed. The inverse design tool is developed based on a multilayer perceptron model that allows fast computation for all ranges of Fano parameters. The method provides improved accuracy of the mean validation factor (MVF = 0.07, q-q') compared to the conventional exhaustive enumeration method (MVF = 0.37).

Fano-Like Resonance from Disorder Correlation in Vacancy-Doped Photonic Crystals

By preparing colloidal crystals with random missing scatterers, crystals are created where disorder is embodied as vacancies in an otherwise perfect lattice. In this special system, there is a critical defect concentration where light propagation undergoes a transition from an all but perfect reflector (for the spectral range defined by the Bragg condition), to a metamaterial exhibiting an enhanced transmission phenomenon. It is shown that this behavior can be phenomenologically described in terms of Fano-like resonances. The results show that the Fano's parameter q experiences a sign change signaling the transition from a perfect crystal exhibiting a reflectance Bragg peak, through a state where background scattering is maximum and Bragg reflectance reaches a minimum to a point where the system reenters a low scattering state recovering ordinary Bragg diffraction. A simple dipolar model considering the correlation between scatterers and vacancies is proposed and the reported evolution of the Fano-like scattering is explained in terms of the emerging covariance between the optical paths and polarizabilities and the effect of field enhancement in photonic crystal (PhC) defects.

Nanostructured surface plasmon resonance sensors: Toward narrow linewidths

Surface plasmon resonance sensors have found wide applications in optical sensing field due to their excellent sensitivity to the slight refractive index change of surrounding medium. However, the intrinsically high optical losses in metals make it nontrivial to obtain narrow resonance spectra, which greatly limits the performance of surface plasmon resonance sensors. This review first introduces the influence factors of plasmon linewidths of metallic nanostructures. Then, various approaches to achieve narrow resonance linewidths are summarized, including the fabrication of nanostructured surface plasmon resonance sensors supporting surface lattice resonance/plasmonic Fano resonance or coupling with a photonic cavity, the preparation of surface plasmon resonance sensors with ultra-narrow resonators, as well as strategies such as platform-induced modification, alternating different dielectric layers, and the coupling with whispering-gallery-modes. Lastly, the applications and some existing challenges of surface plasmon resonance sensors are discussed. This review aims to provide guidance for the further development of nanostructured surface plasmon resonance sensors.

Light-enhanced transparent hydrogel for uric acid and glucose detection by four different analytical platforms

The lack of solid-phase media limits the portability of colorimetric sensing platforms. In this study, a series of transparent polyvinyl alcohol (PVA) hydrogels encapsulated antimony tin oxide nanoparticles (ATO NPs) and 3,3',5,5'-tetramethylbenzidine (TMB) were developed as the solid-phase sensing media for glucose and uric acid. Under the conditions of H₂O₂ and UV light, the hydrogel presented a multicatalytic ability (photo Fenton-like and peroxidase-like activities), which accelerated the oxidation of TMB, turning the hydrogel from colorless to blue and finally enhancing the detection signal. The plasticity of the hydrogel allowed it to be designed into various shapes (membrane, microsphere etc.) to adapt multiple detection platforms (a liquid/solid-phase UV spectrophotometer, a NanoPhotometer, and smartphone spectroscopy). The hydrogel sensing media exhibited excellent tunability and enhanced the photocatalytic ability. The proposed material was successfully applied to detect glucose and uric acids in real samples by four detection platforms to evaluate its practicability.

Nanoscale mapping of shifts in dark plasmon modes in sub 10 nm aluminum nanoantennas

In this work, we report the fabrication and spectroscopic characterization of subwavelength aluminum nanocavities-consisting of hexamer or tetramer clusters of sub 10 nm width Al nanorods-with tunable localized surface plasmon resonance (LSPR) energies on suspended SiN_xmembranes. Here the volume plasmon (VP) and LSPR modes of lithographically-fabricated Al nanocavities are revealed by low-loss electron energy-loss spectroscopy (EELS) in an aberration corrected scanning transmission electron microscope (STEM). We show that the existence of grain boundaries (GBs) in these nanocavities results in shifts in the VP energy and a reduction in the VP lifetime. We map the VP energy and lifetime across GBs and we observe a decrease in VP energy and lifetime at GBs that is consistent with a reduction in free carrier density and increased plasmon scattering at these locations. Dipolar LSPR modes resonant in the UV and blue regions of the electromagnetic spectrum as well as higher-energy optically dark quadrupolar and hexapolar LSPR modes are also observed and mapped by STEM and EELS. All LSPR modes are confirmed via electromagnetic simulations based on the boundary element method. Both tetramer and hexamer structures support the excitation of dipolar bright and dipolar dark modes. Finally, we find that asymmetries in fabricated nanorod hexamer and tetramer nanocavities result in a mode mixing leading to a shift in dipolar dark LSPR modes.

Electrically switchable structural colors based on liquid-crystal-overlaid aluminum anisotropic nanoaperture arrays

Actively tunable or reconfigurable structural colors are highly promising in future development for high resolution imaging and displaying applications. To this end, we demonstrate switchable structural colors covering the entire visible range by integrating aluminum nanoaperture arrays with nematic liquid crystals. The geometrically anisotropic design of the nanoapertures provides strong polarization-dependent coloration. By overlaying a nematic liquid crystal layer, we further demonstrate switchable ability of the structural colors by either changing the polarization of the incident light or applying an external voltage. The switchable structural colors have a fast response time of 28 ms at a driving voltage of 6.5 V. Furthermore, colorful patterns are demonstrated by coding the colors with various dimensions of nanoaperture arrays with dual switching modes. Our proposed technique in this work provides a dual-mode switchable structural colors, which is highly promising for polarimetric displays, imaging sensors, and visual cryptography.

Large-scale vivid metasurface color printing using advanced 12-in. immersion photolithography

Nanostructures exhibiting optical resonances (so-called nanoantennas) have strong potential for applications in color printing and filtering with sub-wavelength resolution. While small scale demonstrations of these systems are interesting as a proof-of-concept, their large scale and volume fabrication requires deeper analysis and further development for industrial adoption. Here, we evaluate the color quality produced by large size nanoantenna arrays fabricated on a 12-in. wafer using deep UV immersion photolithography and dry etching processes. The color reproduction and quality are analyzed in context of the CIE color diagram, showing that a vivid and vibrant color palette, almost fully covering the sRGB color space, can be obtained with this mass-manufacturing-ready fabrication process. The obtained results, thus, provide a solid foundation for the potential industrial adoption of this emerging technology and expose the limits and challenges of the process.

Three-Dimensional Plasmonic Nanocluster-Driven Light-Matter Interaction for Photoluminescence Enhancement and Picomolar-Level Biosensing

Plasmonic nanoparticle clusters promise to support unique engineered electromagnetic responses at optical frequencies, realizing a new concept of devices for nanophotonic applications. However, the technological challenges associated with the fabrication of three-dimensional nanoparticle clusters with programmed compositions remain unresolved. Here, we present a novel strategy for realizing heterogeneous structures that enable efficient near-field coupling between the plasmonic modes of gold nanoparticles and various other nanomaterials via a simple three-dimensional coassembly process. Quantum dots embedded in the plasmonic structures display ∼56 meV of a blue shift in the emission spectrum. The decay enhancement factor increases as the total contribution of radiative and nonradiative plasmonic modes increases. Furthermore, we demonstrate an ultracompact diagnostic platform to detect M13 viruses and their mutations from femtoliter volume, sub-100 pM analytes. This platform could pave the way toward an effective diagnosis of diverse pathogens, which is in high demand for handling pandemic situations.

Hot carrier extraction from plasmonic-photonic superimposed heterostructures

Plasmonic nanostructures have been exploited in photochemical and photocatalytic processes owing to their surface plasmon resonance characteristics. This unique property generates photoinduced potentials and currents capable of driving chemical reactions. However, these processes are hampered by low photon conversion and utilization efficiencies, which are issues that need to be addressed. In this study, we integrate plasmonic photochemistry and simple tunable heterostructure characteristics of a dielectric photonic crystal for the effective control of electromagnetic energy below the diffraction limit of light. The nanostructure comprises high-density Ag nanoparticles on nanocavity arrays of SrTiO₃ and TiO₂, where two oxides constitute a chemical heterojunction. Such a nanostructure is designed to form intense electric fields and a vectorial electron flow channel of Ag → SrTiO₃ → TiO₂. When the plasmonic absorption of Ag nanoparticles matched the photonic stopband, we observed an apparent quantum yield of 3.1 × 10^-4 e^- per absorbed photon. The contributions of light confinement and charge separation to the enhanced photocurrent were evaluated.

Design of highly perceptible dual-resonance all-dielectric metasurface colorimetric sensor via deep neural networks

Colorimetric sensing, which provides effective detection of bio-molecular signals with one's naked eye, is an exceptionally promising sensing technique in that it enables convenient detection and simplification of entire sensing system. Though colorimetric sensors based on all-dielectric nanostructures have potential to exhibit distinct color variations enabling manageable detection due to their trivial intrinsic loss, there is crucial limitation that the sensitivity to environmental changes lags behind their plasmonic counterparts because of relatively small region of near field-analyte interaction of the dielectric Mie-type resonator. To overcome this challenge, we proposed all-dielectric metasurface colorimetric sensor which exhibits dual-resonance in the visible region. Thereafter, we confirmed with simulation that, in the elaborately designed dual-Lorentzian-type spectra, highly perceptible variations of structural color were manifested even in minute change of peripheral refractive index. In addition to verifying physical effectiveness of the superior colorimetric sensing performance appearing in the dual-resonance type sensor, by combining advanced optimization technique utilizing deep neural networks, we attempted to maximize sensing performance while obtaining dramatic improvement of design efficiency. Through well-trained deep neural network that accurately simulates the input target spectrum, we numerically verified that designed colorimetric sensor shows a remarkable sensing resolution distinguishable up to change of refractive index of 0.0086.

Self-driving dynamic plasmonic colors based on needle steering for simultaneous control of transition direction and time on metallic nanogroove metasurfaces

Dynamically tunable plasmonic colors hold great promise for a wide range of applications including color displays, colorimetric sensing, and information encryption. However, dynamic control speed of plasmonic colors is still slow to date. Herein, we propose to use a needle to direct the flow of water and gas pressure to drive water, realizing a simultaneous direction-controllable and fast plasmonic color transition. The highly reflected background light of the metallic nanogroove metasurface is suppressed to generate high-purity plasmonic colors through the cross-polarized input and output configuration. When the environment is changed from air to water, a giant color change from cyan to red (a wavelength shift of 156 nm) is experimentally observed. More importantly, by utilizing a needle to steer the flow of water, direction-controllable and fast plasmonic color transition is achieved by controlling gas pressure to drive water. Compared with current state-of-the-art plasmonic color scanning technology, the color transition time via water driven by gas pressure decreases by three orders of magnitude for the same scanning length. The multi-degrees of freedom dynamic structural colors could have potential applications in dynamic displays, anti-counterfeiting, and information security.

Synthesis, Assembly, Optical Properties, and Sensing Applications of Plasmonic Gap Nanostructures

Plasmonic gap nanostructures (PGNs) have been extensively investigated mainly because of their strongly enhanced optical responses, which stem from the high intensity of the localized field in the nanogap. The recently developed methods for the preparation of versatile nanogap structures open new avenues for the exploration of unprecedented optical properties and development of sensing applications relying on the amplification of various optical signals. However, the reproducible and controlled preparation of highly uniform plasmonic nanogaps and the prediction, understanding, and control of their optical properties, especially for nanogaps in the nanometer or sub-nanometer range, remain challenging. This is because subtle changes in the nanogap significantly affect the plasmonic response and are of paramount importance to the desired optical performance and further applications. Here, recent advances in the synthesis, assembly, and fabrication strategies, prediction and control of optical properties, and sensing applications of PGNs are discussed, and perspectives toward addressing these challenging issues and the future research directions are presented.

Hybrid Plasmonic/Photonic Nanoscale Strategy for Multilevel Anticounterfeit Labels

Innovative goods authentication strategies are of fundamental importance considering the increasing counterfeiting levels. Such a task has been effectively addressed with the so-called physical unclonable functions (PUFs), being physical properties of a system that characterize it univocally. PUFs are commonly implemented by exploiting naturally occurring non-idealities in clean-room fabrication processes. The broad availability of classic paradigm PUFs, however, makes them vulnerable. Here, we propose a hybrid plasmonic/photonic multilayered structure working as a three-level strong PUF. Our approach leverages on the combination of a functional nanostructured surface, a resonant response, and a unique chromatic signature all together in one single device. The structure consists of a resonant cavity, where the top mirror is replaced with a layer of plasmonic Ag nanoislands. The naturally random spatial distribution of clusters and nanoparticles formed by this deposition technique constitutes the manufacturer-resistant nanoscale morphological fingerprint of the proposed PUF. The presence of Ag nanoislands allows us to tailor the interplay between the photonic and plasmonic modes to achieve two additional security levels. The first one is constituted by the chromatic response and broad iridescence of our structures, while the second by their rich spectral response, accessible even through a common smartphone light-emitting diode. We demonstrate that the proposed architectures could also be used as an irreversible and quantitative temperature exposure label. The proposed PUFs are inexpensive, chip-to-wafer-size scalable, and can be deposited over a variety of substrates. They also hold a great promise as an encryption framework envisioning morpho-cryptography applications.

Dynamic Bifunctional Metasurfaces for Holography and Color Display

Metasurfaces have shown their unprecedented ability in wavefront shaping and triggered various applications with state-of-the-art performances, e.g., color nanoprinting and metaholograms. Recently, these two functions have been combined into a single metasurface to further expand its capabilities. Despite the progress, the current dual-mode metasurfaces are mostly static and strongly hinder their practical applications. Herein, the realization of dynamic bifunctional metasurfaces is reported. Five metaholograms at two different wavelengths are multiplexed with structural colors by controlling the spectral and phase response of metasurface. Owing to the destructive interference and the resonance on external environment, the light diffraction at particular wavelengths can be switched between "ON" and "OFF" states, or remain unchanged with the change of surrounding refractive index. Consequently, the encoded metaholograms are selectively turned on and off, making the overall holographic image dynamically switchable. This concept paves a solid step toward practical applications of all-dielectric metasurfaces.

Surface Plasmonic Sensors: Sensing Mechanism and Recent Applications

Surface plasmonic sensors have been widely used in biology, chemistry, and environment monitoring. These sensors exhibit extraordinary sensitivity based on surface plasmon resonance (SPR) or localized surface plasmon resonance (LSPR) effects, and they have found commercial applications. In this review, we present recent progress in the field of surface plasmonic sensors, mainly in the configurations of planar metastructures and optical-fiber waveguides. In the metastructure platform, the optical sensors based on LSPR, hyperbolic dispersion, Fano resonance, and two-dimensional (2D) materials integration are introduced. The optical-fiber sensors integrated with LSPR/SPR structures and 2D materials are summarized. We also introduce the recent advances in quantum plasmonic sensing beyond the classical shot noise limit. The challenges and opportunities in this field are discussed.

Effect of alkali atom doping on the electronic structure and aromatic character of planar and quasi-planar Al13+ clusters

A set of three Al₁₃⁺ clusters, one perfectly planar, and two quasi-planar structures, have been recently reported by our group (Guin et al. Journal of Molecular Graphics and Modeling, 2020, 97, 107544). All three clusters possess bilateral symmetry with identical structural features-a set of ten aluminum atoms encircle a triangular core. The symmetry axis passes through one of the Al atoms of the central triangular core and two Al atoms located on the periphery at two opposite ends of the cluster. This set of three aluminum clusters is an example of a rare metallo-aromatic system where highly anti-aromatic islands are embedded within an aromatic sea. In the present study, we have explored the effect of doping alkali atoms (Li, Na, and K) at the positions of the Al atoms that lie on the symmetry axis of the cluster intending to understand the structural stability and the effect on the aromatic character as compared to the undoped parent clusters. Besides the electronic structural analysis, NICS and ELF studies have also been carried out to characterize the aromatic nature of the doped clusters. Interestingly, it has been found that even with the incorporation of the alkali atoms, the bilateral symmetry of the clusters remains intact, but the alkali atoms are pushed out of the original location toward the edge of the cluster, whereas the aluminum atoms remain grouped. The dipole moment of the clusters systematically increases, and the overall aromaticity of the cluster systematically decreases with the increase in the atomic number of the doped alkali atoms. Effect of alkali atom doping to Al₁₃⁺ cluster.

Linewidth narrowing of aluminum breathing plasmon resonances in Bragg grating decorated nanodisks

Plasmon resonances with high-quality are of great importance in light emission control and light-matter interactions. Nevertheless, inherent ohmic and radiative losses usually hinder the plasmon performance of metallic nanostructures, especially for aluminum (Al). Here we demonstrate a Bragg grating decorated nanodisk to narrow the linewidth of breathing plasmon resonances compared with a commensurate nanodisk. Two kinds of plasmon resonant modes and the corresponding mode patterns are investigated in cathodoluminescence (CL) depending on the different electron bombardment positions, and the experimental results agree well with full wave electromagnetic simulations. Linewidth narrowing can be clearly understood using an approximated magnetic dipole model. Our results suggest a feasible mechanism for linewidth narrowing of plasmon resonances as well as pave the way for in-depth analysis and potential applications of Al plasmon systems.

High-sensitivity plasmonic sensor by narrowing Fano resonances in a tilted metallic nano-groove array

Plasmonic sensors exhibit enormous potential in the areas of environmental monitoring, biomedical diagnostics, healthcare, food safety, security, and chemical reactions. However, the large bandwidths of surface-plasmon response spectra greatly reduce the sensitivities and detection limits of plasmonic sensors. Herein, we propose to tilt a metallic nano-groove array to reduce linewidths of Fano resonances, and the figure of merit (FOM) of a refractive index sensor is greatly increased. The Fano resonances stem from interference between narrow SPP resonant modes and a broad LSP mode in the metallic nano-groove array. When tilting the metallic nano-groove array, new Fano resonances emerge, greatly compressing the linewidth of Fano resonance of interest to ∼1.1 nm in the simulation. Experimentally, a narrow Fano resonance with a linewidth of Δλ≈2.5 nm is achieved, and a high-FOM (FOM ≈ 263) plasmonic sensor is demonstrated. This value of FOM is more than 4.7 times that (FOM ≤ 55) of Fano sensors based on SPP modes, and it is even approximately twice that (FOM ≈ 140) of the previous Fano sensor based on Wood's Anomaly.

Electrically Excited Plasmonic Ultraviolet Light Sources

The emission of photons from metal-insulator-metal (MIM) nanojunctions through inelastic tunneling of electrically driven electrons is a well-acknowledged approach to develop miniaturized light sources and ultradense photonic instruments. Generally, the existing research in the optimization of electromigrated tunneling junctions is principally centered on the generation of visible and near-infrared lights. This study reports on the near-ultraviolet (NUV, λ ≈ 355 nm) light emission from enhanced tunneling of electrons using aluminum nanoelectrodes. Compared to conventional noble metals, the high electron density and low screening of aluminum enable supporting of pronounced local fields at high energies (i.e, ultraviolet (UV)). As the color of light can be straightforwardly determined by the properties of tunneling structures, the exquisite features of aluminum have empowered the fashioning of tunneling devices that are able to effectively sustain plasmons at short wavelengths and emit UV light with high photon yield. This demonstration is a breakthrough in the generation of high-energy beams using electrically excited aluminum tunneling platforms, which promisingly accelerates the implementation of electrically tunable and ultradense UV light sources.

First-Principles Insights into Plasmon-Induced Catalysis

The size- and shape-controlled enhanced optical response of metal nanoparticles (NPs) is referred to as a localized surface plasmon resonance (LSPR). LSPRs result in amplified surface and interparticle electric fields, which then enhance light absorption of the molecules or other materials coupled to the metallic NPs and/or generate hot carriers within the NPs themselves. When mediated by metallic NPs, photocatalysis can take advantage of this unique optical phenomenon. This review highlights the contributions of quantum mechanical modeling in understanding and guiding current attempts to incorporate plasmonic excitations to improve the kinetics of heterogeneously catalyzed reactions. A range of first-principles quantum mechanics techniques has offered insights, from ground-state density functional theory (DFT) to excited-state theories such as multireference correlated wavefunction methods. Here we discuss the advantages and limitations of these methods in the context of accurately capturing plasmonic effects, with accompanying examples.

Coupled plasmonic systems: controlling the plasmon dynamics and spectral modulations for molecular detection

This review describes recent studies on coupled plasmonic systems for controlling plasmon dynamics and molecular detection using spectral modulations. The plasmon dephasing time can be controlled by weak and strong coupling regimes between the plasmonic nanostructures or localized surface plasmon resonances (LSPRs) and the other optical modes such as microcavities. The modal coupling induces near-field enhancement by extending the plasmon dephasing time to increase the near-field enhancement at certain wavelengths resulting in the enhancement of molecular detection. On the other hand, the interaction between LSPR and molecular excited or vibrational states also modulates the resonance spectrum, which can also be used for detecting a small number of molecules with a subtle change in the spectrum. The spectral modulation is induced by weak and strong couplings between LSPRs and the electronic or vibrational states of molecules, and this method is sensitive enough to measure a single molecule.

Periodic arrays of plasmonic crossed-bowtie nanostructures interspaced with plasmonic nanocrosses for highly sensitive LSPR based chemical and biological sensing

In this paper, we present novel localized surface plasmon resonance (LSPR) sensors based on periodic arrays of gold crossed-bowtie nanostructures interspaced with gold nanocross pillars. Finite difference time domain (FDTD) numerical simulations were carried out to model bulk sensors as well as localized sensors based on the plasmonic nanostructures being proposed. The geometrical parameters of the plasmonic nanostructures are varied to obtain the best possible sensing performance in terms of sensitivity and figure of merit. A very high bulk sensitivity of 1753 nm per unit change in refractive index (nm RIU-1), with a figure of merit for bulk sensing (FOMbulk) of 3.65 RIU-1, is obtained for these plasmonic nanostructures. This value of bulk sensitivity is higher in comparison to previously proposed LSPR sensors based on plasmonic nanopillars and nanocrosses. Moreover, the optimized LSPR sensors being proposed in this paper provide maximum sensitivity of localized refractive index sensing of 70 nm/nm with a FOMlocalized of 0.33 nm-1. This sensitivity of localized refractive index sensing is the highest reported thus far in comparison with previously reported LSPR sensors. It is also demonstrated that the operating resonance wavelengths of these LSPR sensors can be controllably tuned for specific applications by changing the dimensions of the plasmonic nanostructures.

Hollow aluminum microspheres with high mass extinction coefficients in the long wave infrared

Previous electromagnetic computations of multilayered dielectric/metallic spheres identified the ideal dimensions and composition for achieving optimized mass extinction coefficients (m²/g). A hollow metallic sphere, with a thin metallic shell, is one such example of a spherical structure that can theoretically achieve high mass extinction coefficients in the long wave infrared (LWIR) region (8-12 µm). To this end, we endeavored to demonstrate a cost-effective and scalable manufacturing approach for synthesizing and experimentally validating the mass extinction coefficients of hollow metallic spheres. Specifically, we detail a novel approach for fabricating hollow aluminum spheres using radio frequency (RF) magnetron sputter deposition. Sacrificial high-density polyethylene polymer microspheres were used as substrates for the deposition of thin layers of aluminum. The core shell structures were subsequently thermally processed to form the hollow micron sized aluminum shells. The mass extinction coefficients of the hollow aluminum spheres were subsequently measured and compared to computational results. A strong agreement between experimental and theoretical predictions was observed. Finally, the LWIR mass extinction coefficients of the hollow spheres were compared to high aspect ratio brass flakes, a common pigment used for LWIR attenuation, and other materials and geometries that are used for LWIR filtering applications. This comparison of both performance and availability revealed that the fabricated hollow aluminum spheres exhibited competitive LWIR properties using a more scalable and cost-effective manufacturing approach.

Narrowband Light Reflection Resonances from Waveguide Modes for High-Quality Sensors

Designing various nanostructures to achieve narrowband light reflection resonances is desirable for optical sensing applications. In this work, we theoretically demonstrate two narrowband light reflection resonances resulting from the excitations of the zero-order transverse magnetic (TM) and transverse electric (TE) waveguide modes, in a waveguide structure consisting of an Au sphere array on an indium tin oxide (ITO) spacer on a silica (SiO₂) substrate. The positions of the light reflection resonances can be tuned easily, by varying the array periods of gold (Au) spheres or by changing the thickness of the ITO film. More importantly, the light reflection resonances have a very narrow bandwidth, the full width at half maximum (FWHM) of which can be reduced to only several nanometers for the zero-order TM and TE waveguide modes. The conventionally defined performance parameters of sensors, sensitivity (S) and figure of merit (FOM), have quite high values of about 80 nm/RIU and 32, respectively, in the visible wavelength range.

Full-color based on bismuth core-shell nanoparticles in one-step fabrication

Plasmonic resonances in metallic nanostructures are promising for the structure-dependent color-rendering effect. In this study, bismuth is selected as an alternative plasmonic material due to its large tunable range from near-ultraviolet to near-infrared. Various sizes of core-shell bismuth nanoparticles are fabricated on a large-area silicon substrate using a one-step thermal evaporation deposition process. Particle diameters, cross-sections, and arrangement are characterized at 12 featured sections, which reveal spectral shifts and full visible colors in a hue order with a color gamut that is close to sRGB. Color palettes on the chromaticity coordinates rendered from both measured and simulation reflection spectra are in very good accordance with the microscopic image colors of all sections.

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.

Monolithic Metal Dimer-on-Film Structure: New Plasmonic Properties Introduced by the Underlying Metal

Dimers, two closely spaced metallic nanostructures, are one of the primary nanoscale geometries in plasmonics, supporting high local field enhancements in their interparticle junction under excitation of their hybridized "bonding" plasmon. However, when a dimer is fabricated on a metallic substrate, its characteristics are changed profoundly. Here we examine the properties of a Au dimer on a Au substrate. This structure supports a bright "bonding" dimer plasmon, screened by the metal, and a lower energy magnetic charge transfer plasmon. Changing the dielectric environment of the dimer-on-film structure reveals a broad family of higher-order hybrid plasmons in the visible region of the spectrum. Both of the localized surface plasmons resonances (LSPR) of the individual dimer-on-film structures as well as their collective surface lattice resonances (SLR) show a highly sensitive refractive index sensing response. Implementation of such all-metal magnetic-resonant nanostructures offers a promising route to achieve higher-performance LSPR- and SLR-based plasmonic sensors.

Stretchable All-Dielectric Metasurfaces with Polarization-Insensitive and Full-Spectrum Response

Mechanical stretching has been an effective way to achieve widely tunable optical response in artificial nanostructures. However, the typical stretchable optical devices produce exactly the reverse effects for two orthogonal linear polarizations, significantly hindering their practical applications in many emerging systems. Herein, we demonstrate an approach for a mechanically tunable all-dielectric metasurface with polarization insensitivity and full-spectrum response in the visible range from 450 to 650 nm. By embedding a TiO₂ metasurface in a polydimethylsiloxane substrate and stretching it in one direction, we find that the distinct reflection colors of two orthogonal linear polarizations can be tuned across the entire visible spectrum simultaneously. Encryption and display of information have also been realized with the same technique. The corresponding calculations show that the spectral responses of light with polarizations perpendicular and parallel to the strain are determined by two different mechanisms, that is, the near-field mutual interaction and the grating effects. This research shall shed light on stretchable and wearable photonics.

Strengthening Fano resonance on gold nanoplates with gold nanospheres

Plasmonic Fano resonance has attracted extensive attention due to its many applications, including plasmonic sensing, electromagnetically induced transparency, light trapping and stopping, due to its narrow linewidth and asymmetric spectral shape. However, many metal nanostructures are designed with complex geometries to generate Fano resonance and few of them can support a deep Fano dip. Herein we report on the strengthening of the Fano resonance on silicon-supported Au nanoplates through the formation of (Au nanosphere)-(Au nanoplate) heterodimers. The deposition of the Au nanosphere on the top can greatly strengthen the substrate-induced Fano resonance of the Au nanoplate with a deep dip. We also observe that the replacement of the Au nanosphere with a Au nanocube can suppress the excitation of the Fano resonance in the heterodimer. When the sharp corners and edges of the nanocubes gradually become rounded, the Fano resonance appears again with increasing asymmetry. Both the dip depth and wavelength of the Fano resonance can be independently tailored by varying the nanosphere diameter and the nanoplate thickness, respectively. We believe that our results provide an attractive and facile platform for modulating Fano dips and constructing Fano resonance-based devices.

Exceptional cones in 4D parameter space

The notion of synthetic dimensions has expanded the realm of topological physics to four dimensional (4D) space lately. In this work, non-Hermiticity is used as a synthetic parameter in PT-symmetric photonic crystals to study the topological physics in 4D non-Hermitian synthetic parameter space. We realize a 3D exceptional hypersurface (EHS) in such 4D parameter space, and the degeneracy points emerge due to the symmetry of synthetic parameters. We further demonstrate the existence of exceptional degenerate points (EDPs) on the EHS that originates from the chirality of exceptional points (EPs), and the exceptional surface near EDPs behaves like a Dirac cone. We further show that a very narrow reflection plateau can be found near these EDPs, and their sensitivity towards the PT-symmetry breaking environmental perturbation can make these degeneracy points useful in optical sensing and many other nonlinear and quantum optical applications.

Full color generation with Fano-type resonant HfO2 nanopillars designed by a deep-learning approach

In contrast to lossy plasmonic metasurfaces (MSs), wideband dielectric MSs comprising subwavelength nanostructures supporting Mie resonances are of great interest in the visible wavelength range. Here, for the first time to our knowledge, we experimentally demonstrate a reflective MS consisting of a square-lattice array of hafnia (HfO₂) nanopillars to generate a wide color gamut. To design and optimize these MSs, we use a deep-learning algorithm based on a dimensionality reduction technique. Good agreement is observed between simulation and experimental results in yielding vivid and high-quality colors. We envision that these structures not only empower the high-resolution digital displays and sensitive colorimetric biosensors but also can be applied to on-demand applications of beaming in a wide wavelength range down to deep ultraviolet.

Fabrication and Characterization of a Metallic-Dielectric Nanorod Array by Nanosphere Lithography for Plasmonic Sensing Application

In this paper, a periodic metallic–dielectric nanorod array which consists of Si nanorods coated with 30 nm Ag thin film set in a hexagonal configuration is fabricated and characterized. The fabrication procedure is performed by using nanosphere lithography with reactive ion etching, followed by Ag thin-film deposition. The mechanism of the surface and gap plasmon modes supported by the fabricated structure is numerically demonstrated by the three-dimensional finite element method. The measured and simulated absorptance spectra are observed to have a same trend and a qualitative fit. Our fabricated plasmonic sensor shows an average sensitivity of 340.0 nm/RIU when applied to a refractive index sensor ranging from 1.0 to 1.6. The proposed substrates provide a practical plasmonic nanorod-based sensing platform, and the fabrication methods used are technically effective and low-cost.

All-Dielectric Terahertz Plasmonic Metamaterial Absorbers and High-Sensitivity Sensing

Two types of plasmonic metamaterial absorbers (PMAs) formed from patterned all-dielectric resonators are designed and demonstrated experimentally in the terahertz (THz) range. Both PMAs use a simple grating design on highly N-doped silicon. The first shows broadband absorption with near-perfect peak absorbance at 1.45 THz and a bandwidth of 1.05 THz for 90% absorbance, while the second is a dual-band absorber. Experiments show that the second absorber has two distinct absorption peaks at 0.96 and 1.92 THz with absorption rates of 99.7 and 99.9%, respectively. A fundamental cavity mode coupled to coaxial surface plasmon polaritons is responsible for the characteristics of both PMAs. Additionally, the optically tunable responses of these all-dielectric absorbers demonstrate that the absorption behavior can be modified. The quality factor (Q) values of the dual-band resonances are 4.6 and 7.8 times larger than those of the broadband PMAs, respectively, which leads to a better sensing performance. As an example, the two proposed PMAs act as high-sensitivity sensors and demonstrate considerable potential for chlorpyrifos detection. These results show that these PMAs can be used as sensors that can detect the presence of trace pesticides in adsorption analyses, among other practical applications.

Hue- and Chromaticity-Based Exploration of Surface Complexation-Induced Tunable Emission from Non-Luminescent Quantum Dots

Herein we report the use of a hue parameter of HSV (Hue, Saturation and Value) color space-in combination with chromaticity color coordinates-for exploring the complexation-induced luminescence color changes, ranging from blue to green to yellow to white, from a non-luminescent Fe-doped ZnS quantum dot (QD). Importantly, the surface complexation reaction helped a presynthesized non-luminescent Fe-doped ZnS QD to glow with different luminescence colors (such as blue, cyan, green, greenish-yellow, yellow) by virtue of the formation of various luminescent inorganic complexes (using different external organic ligands), while the simultaneous blue- and yellow-emitting complex formation on the surface of non-luminescent Fe-doped ZnS QD led to the generation of white light emission, with a hue mean value of 85 and a chromaticity of (0.28,0.33). Furthermore, the surface complexation-assisted incorporation of luminescence properties to a non-luminescent QD not only overcomes their restricted luminescence-based applications such as light-emitting, biological and sensing applications but also bring newer avenues towards unravelling the surface chemistry between QDs and inorganic complexes and the advantage of having an inorganic complex with QD for their aforementioned useful applications.

Preventing Aluminum Photocorrosion for Ultraviolet Plasmonics

Aluminum can sustain plasmonic resonances down into the ultraviolet (UV) range to promote surface-enhanced spectroscopy and catalysis. Despite its natural alumina passivating layer, we find here that under 266 nm pulsed UV illumination, aluminum can undergo a dramatic photocorrosion in water within a few tens of seconds and even at low average UV powers. This aluminum instability in water environments is a critical limitation. We show that the aluminum photocorrosion is related to the nonlinear absorption by water in the UV range leading to the production of hydroxyl radicals. Different corrosion protection approaches are tested using scavengers for reactive oxygen species and polymer layers deposited on top of the aluminum structures. Using optimized protection, we achieve a 10-fold increase in the available UV power range leading to no visible photocorrosion effects. This technique is crucial to achieve stable use of aluminum nanostructures enabling UV plasmonics in aqueous solutions.

Designable Spectrometer-Free Index Sensing Using Plasmonic Doppler Gratings

Typical nanoparticle-based plasmonic index sensors detect the spectral shift of localized surface plasmon resonance (LSPR) upon the change of the environmental index. Therefore, they require broadband illumination and spectrometers. The sensitivity and flexibility of nanoparticle-based index sensors are usually limited because LSPR peaks are usually broad and the spectral position cannot be freely designed. Here, we present a fully designable index sensing platform using plasmonic Doppler gratings (PDGs), which provide broadband and azimuthal angle dependent grating periodicity. Different from LSPR sensors, PDG index sensors are based on the momentum matching between photons and surface plasmons via the lattice momentum of the grating. Therefore, the index change is translated into the variation of the in-plane azimuthal angle for photon-to-plasmon coupling, which manifests as directly observable dark bands in the reflection image. The PDG can be freely designed to optimally match the range of index variation for specific applications. In this work, we demonstrate PDG index sensors for large (n = 1.00-1.52) and small index variations (n = 1.3330-1.3650). The tiny and nonlinear index change of the water-ethanol mixture has been clearly observed and accurately quantified. Since the PDG is a dispersive device, it enables on-site and single-color index sensing without a spectrometer and provides a promising spectroscopic platform for on-chip analytical applications.

Ultrahighly Saturated Structural Colors Enhanced by Multipolar-Modulated Metasurfaces

Colors with high saturation are of prime significance for display and imaging devices. So far, structural colors arising from all-dielectric metasurfaces, particularly amorphous silicon and titanium oxide, have exceeded the gamut of standard RGB (sRGB) space. However, the excitation of higher-order modes for dielectric materials hinders the further increase of saturation. Here, to address the challenge, we propose a new design strategy of multipolar-modulated metasurfaces with multi-dielectric stacked layers to realize the deep modulation of multipolar modes. Index matching between layers can suppress the multipolar modes at nonresonant wavelength, resulting in the dramatic enhancement in the monochromaticity of reflection spectra. Ultrahigh-saturation colors ranging from 70% to 90% with full hue have been theoretically and experimentally obtained. The huge gamut space can be realized in an unprecedented way, taking up 171% sRGB space, 127% Adobe RGB space, and 57% CIE space. More interestingly, the coverage for Recommendation 2020 (Rec. 2020) space, which almost has not been successfully realized so far, can reach 90%. We anticipate that the proposed multipolar-modulated metasurfaces are promising for the enlargement of the color range for high-end and advanced display applications.

Directional Fano Resonance in an Individual GaAs Nanospheroid

Fano resonance has been observed in a wide variety of nanophotonic structures such as photonic crystals, plasmonic structures, and metamaterials. It arises from the interference of discrete resonance states with broadband continuum states. As an emerging nanophotonic material, high-index all-dielectric nanomaterials provide a new platform to achieve Fano resonance by virtue of the simultaneous excited electric and magnetic resonances. However, to date, Fano resonance in the visible region has not been observed in individual high-index all-dielectric nanoparticles. Here, for the first time, the experimental observation of the directional Fano resonance is reported in an individual GaAs nanospheroid. The special geometry enables GaAs nanospheroids to generate spectrally overlapped electric and magnetic dipole resonances, which enhances their spectral coupling, giving rise to asymmetric-shaped backward scattering spectrum. This directional Fano resonance can be tuned by the aspect ratio of nanospheroids as well as excitation polarization. In addition, efficient directional light scattering is realized at the total scattering peak of the GaAs nanospheroid. The forward-to-backward scattering ratio can be largely enhanced due to Fano dip in the backward scattering spectrum. These findings suggest that high-index all-dielectric nanospheroid is a promising candidate for directional sources and optical switches.

Plasmonic mode conversion in individual tilted 3D nanostructures

We investigate mode conversion in 3D asymmetric nanocones using angle-dependent linear optical spectroscopy and second-harmonic generation microscopy supported by corresponding simulations. The results prove the efficient excitation of the plasmonic out-of-plane mode that enhances the electric near-field at the sharp tip. Furthermore, we introduce two advanced fabrication processes including either etch mask transfer by tilted etching into a metallic layer or tilted electron-beam lithography followed by tilted evaporation and lift-off. These processes enable the fabrication of tilted nanostructures which can be optimized for a given purpose. The combination of the optical properties and the introduced fabrication processes enables a new design of plasmonic nanostructures for ultra-compact sensors or photon sources.