ZnO nanorods decorated with Ag nanoflowers as a recyclable SERS substrate for rapid detection of pesticide residue in multiple-scenes.

Pesticide residues threaten the ecological environment and human health. Therefore, developing high performance SERS substrate to achieve highly sensitive detection of pesticide residues is meaningful. In this study, based on the strategy of combining "hot spots" engineering and material hybridization, we construct a novel hybrid SERS substrate by depositing Ag nanoflowers (NFs) on ZnO nanorods (NRs).

Refractive Index Sensor Based on the Fano Resonance in Metal-Insulator-Metal Waveguides Coupled with a Whistle-Shaped Cavity.

A plasmonic refractive index sensor based on surface plasmon polaritons (SPPs) that consist of metal-insulator-metal (MIM) waveguides and a whistle-shaped cavity is proposed. The transmission properties were simulated numerically by using the finite element method. The Fano resonance phenomenon can be observed in their transmission spectra, which is due to the coupling of SPPs between the transmission along the clockwise and anticlockwise directions.

Inverse design of structural color: finding multiple solutions conditional generative adversarial networks.

The "one-to-many" problem is a typical challenge that faced by many machine learning aided inverse nanophotonics designs where one target optical response can be achieved by many solutions (designs). Although novel training approaches, such as tandem network, and network architecture, such as the mixture density model, have been proposed, the critical problem of solution degeneracy still exists where some possible solutions or solution spaces are discarded or unreachable during the network training process. Here, we report a solution to the "one-to-many" problem by employing a conditional generative adversarial network (cGAN) that enables generating sets of multiple solution groups to a design problem.

Intraband hot-electron photoluminescence of a silver nanowire-coupled gold film high-order gap plasmons.

We report a strong one-photon photoluminescence (PL) behavior of a silver nanowire directly coupled gold film. The PL peak position of the silver nanowire-coupled gold film deviates from the intrinsic interband transition of gold materials and is not sensitive to the diameter change of the silver nanowire. We attribute this strong PL behavior to the intraband transition of hot electrons dominated by high-order gap plasmons, which are excited in the ultra-small gap formed by an ultra-thin polyvinyl pyrrolidone (PVP) layer coated on the silver nanowire.

Plasmonic metal nanostructures with extremely small features: new effects, fabrication and applications.

Surface plasmons in metals promise many fascinating properties and applications in optics, sensing, photonics and nonlinear fields. Plasmonic nanostructures with extremely small features especially demonstrate amazing new effects as the feature sizes scale down to the sub-nanometer scale, such as quantum size effects, quantum tunneling, spill-out of electrons and nonlocal states The unusual physical, optical and photo-electronic properties observed in metallic structures with extreme feature sizes enable their unique applications in electromagnetic field focusing, spectra enhancing, imaging, quantum photonics, In this review, we focus on the new effects, fabrication and applications of plasmonic metal nanostructures with extremely small features. For simplicity and consistency, we will focus our topic on the plasmonic metal nanostructures with feature sizes of sub-nanometers.

Periodic planar Fabry-Perot nanocavities with tunable interference colors based on filling density effects.

Structural colors of high performance and economically feasible fabrication are desired in various applications. Herein, we demonstrate that reflective full-color filters based on the interference effect can be realized in periodic Fabry-Perot (F-P) nanocavity arrays of the same thickness. Enabled by simply adjusting the nanocavity size and array period, the resonant wavelengths can be successively tuned in the whole visible light range, which is mainly attributed to the varied effective refractive index introduced by the different filling density of the F-P nanocavity.

Strongly coupled evenly divided disks: a new compact and tunable platform for plasmonic Fano resonances.

Plasmonic artificial molecules are promising platforms for linear and nonlinear optical modulation at various regimes including the visible, infrared and terahertz bands. Fano resonances in plasmonic artificial structures are widely used for controlling spectral lineshapes and tailoring of near-field and far-field optical response. Generation of a strong Fano resonance usually relies on strong plasmon coupling in densely packed plasmonic structures.

Double Fano resonances in hybrid disk/rod artificial plasmonic molecules based on dipole-quadrupole coupling.

Fano resonance can be achieved by the destructive interference between a superradiant bright mode and a subradiant dark mode. A variety of artificial plasmonic oligomers have been fabricated to generate Fano resonance for its extensive applications. However, the Fano resonance in plasmonic oligomer systems comes from the interaction of all metal particles, which greatly limits the tunability of the Fano resonance.

Huge field enhancement and high transmittance enabled by terahertz bow-tie aperture arrays: a simulation study.

Sub-wavelength aperture arrays featuring small gaps have an extraordinary significance in enhancing the interactions of terahertz (THz) waves with matters. But it is difficult to obtain large light-substance interaction enhancement and high optical response signal detection capabilities at the same time. Here, we propose a simple terahertz bow-tie aperture arrays structure with a large electric field enhancement factor and high transmittance at the same time.

Notched terahertz Bowtie metamaterials with strongly enhanced near-field and narrowed resonance linewidth.

Enhanced near-field and quality factor of resonance are key issues in plasmonic structures. Here, we demonstrate a kind of notched bowtie metamaterials in the terahertz (THz) regime with narrow linewidth and extremely enhanced near field. The notched bowtie is a variation of common bowtie structure created by introducing symmetric notches on the two sides of the triangular metallic structure.

Tunable reflective color filters based on asymmetric Fabry-Perot cavities employing ultrathin GeSbTe as a broadband absorber.

We demonstrated a tunable structural color filter based on an asymmetric Fabry-Perot cavity employing germanium antimony tellurium alloy GeSbTe (GST) as a switchable ultrathin lossy layer. The color tunability and switch mechanism of our designed structure were investigated by both simulation and analytical approaches. Both numerical simulations and analytical results show that the tunable reflective colors can be generated through the reversible phase transition of GST from amorphous to crystalline.

Three-Dimensional-Stacked Gold Nanoparticles with Sub-5 nm Gaps on Vertically Aligned TiO Nanosheets for Surface-Enhanced Raman Scattering Detection Down to 10 fM Scale.

Seeking for ultrasensitive and low-cost substrates is highly demandable for practical applications of surface-enhanced Raman scattering (SERS) technology. In this work, we report an ultrasensitive SERS-active substrate based on wet-chemistry-synthesized vertically aligned large-area TiO nanosheets (NSs) decorated by densely packed gold nanoparticles (Au NPs) with sub-5 nm gaps. Via a multistep successive deposition process, three-dimensional-stacked Au NPs sandwiched by a 3 nm SiO layer were assembled onto the TiO NS, enabling numerous hotspots due to the formation of both ultratiny plasmonic gaps and semiconductor/metal interfaces.

Transmissive structural color filters using vertically coupled aluminum nanohole/nanodisk array with a triangular-lattice.

We demonstrate a configuration to generate transmissive structural colors through triangular-lattice square nanohole arrays in aluminum (Al) film with Al nanodisks on the bottom of the nanoholes. By using a simple nanofabrication process, colors covering the entire visible light with different brightness and saturation are achieved by tuning both the period of arrays and the size of nanoholes. The optical behaviors of the structures are systematically investigated by both experimental and theoretical methods.

Isotopic graphene-isolated-Au-nanocrystals with cellular Raman-silent signals for cancer cell pattern recognition.

For cancer diagnosis, technologies must be capable of molecular recognition, and they must possess a built-in pattern recognition component for efficient imaging and discrimination of targeted cancer cells. Surface enhanced Raman scattering (SERS) tags based on plasmonically active nanoparticles hold promise for accurate and efficient cancer cell recognition, owing to ultra-narrow peak and sensitive optical properties. However, a complex fingerprint spectrum increases data analysis difficulty, making it necessary to develop multicolor SERS tags with a simple fingerprint spectrum.

Uniform Gold-Nanoparticle-Decorated {001}-Faceted Anatase TiO Nanosheets for Enhanced Solar-Light Photocatalytic Reactions.

The {001}-faceted anatase TiO micro-/nanocrystals have been widely investigated for enhancing the photocatalysis and photoelectrochemical performance of TiO nanostructures, but their practical applications still require improved energy conversion efficiency under solar-light and enhanced cycling stability. In this work, we demonstrate the controlled growth of ultrathin {001}-faceted anatase TiO nanosheets on flexible carbon cloth for enhancing the cycling stability, and the solar-light photocatalytic performance of the synthesized TiO nanosheets can be significantly improved by decorating with vapor-phase-deposited uniformly distributed plasmonic gold nanoparticles. The fabricated Au-TiO hybrid system shows an 8-fold solar-light photocatalysis enhancement factor in photodegrading Rhodamine B, a high photocurrent density of 300 μA cm under the illumination of AM 1.

Sensitive Surface-Enhanced Raman Scattering Detection Using On-Demand Postassembled Particle-on-Film Structure.

Highly sensitive and low-cost surface-enhanced Raman scattering (SERS) substrates are essential for practical applications of SERS. In this work, we report an extremely simple but effective approach to achieve sensitive SERS detection of molecules (down to 10 M) by using a particle/molecule/film sandwich configuration. Compared to conventional SERS substrates which are preprepared to absorb analyte molecules for detection, the proposed sandwich configuration is achieved by postassembling a flexible transparent gel tape embedded with plasmonic nanoparticles onto an Au film decorated with to-be-detected analyte molecules.

Broad-band and high-efficiency polarization converters around 1550 nm based on composite structures.

Broad-band and high-efficiency polarization converter is an imperative component in communication systems, but its functionality often clashes with the constraint of materials. Herein we theoretically and numerically demonstrate that a broad-band and high-efficiency 90° polarization rotator around 1550 nm can be realized using an ultrathin and geometry-optimized composite structure. Based on simulation results, the reflection efficiency and operation bandwidth is up to ≈80% and ≈300 nm, respectively, for the 90° polarization rotator.

Sub-diffraction-limit localization imaging of a plasmonic nanoparticle pair with wavelength-resolved dark-field microscopy.

In this work, with wavelength-resolved dark-field microscopy, the center-of-mass localization information from nanoparticle pairs (i.e., spherical (45 nm in diameter) and rod (45 × 70 nm) shaped gold nanoparticle pairs with different gap distances and orientations) was explored and compared with the results determined by scanning electron microscopy (SEM) measurements.

Constructive-interference-enhanced Fano resonance of silver plasmonic heptamers with a substrate mirror: a numerical study.

Plasmonic nanostructures with strong Fano resonance are of fundamental interest. Here, our systematic simulations show that rational positioning of a silver plasmonic heptamer above a highly reflective substrate mirror can significantly enhance its intrinsic Fano-resonance intensity. The silver nanodisk heptamer positioned at an appropriate distance above the reflective substrate enables 2.

Pronounced Fano Resonance in Single Gold Split Nanodisks with 15 nm Split Gaps for Intensive Second Harmonic Generation.

Single metallic nanostructures supporting strong Fano resonances allow more compact nanophotonics integration and easier geometrical control in practical applications such as enhanced spectroscopy and sensing. In this work, we designed a class of plasmonic split nanodisks that show pronounced Fano resonance comparable to that observed in widely studied plasmonic oligomer clusters. Using our recently developed "sketch and peel" electron-beam lithography, split nanodisks with varied diameter and split length were fabricated over a large area with high uniformity.

Three-dimensional donut-like gold nanorings with multiple hot spots for surface-enhanced raman spectroscopy.

Seeking for the best possible substrates for surface-enhanced Raman spectroscopy (SERS) is of great interest for single-molecule-level detection applications. Lithographic plasmonic nanostructures are supposed to enable uniform enhancement and thus have attracted extensive interest in the past decade. In this work, we propose and demonstrate a lithographic three-dimensional (3D) donut-like gold nanoring array as a SERS substrate with an enhancement factor (EF) up to 3.

Fabrication of single-crystal silicon nanotubes with sub-10 nm walls using cryogenic inductively coupled plasma reactive ion etching.

Single-crystal silicon nanostructures have attracted much attention in recent years due in part to their unique optical properties. In this work, we demonstrate direct fabrication of single-crystal silicon nanotubes with sub-10 nm walls which show low reflectivity. The fabrication was based on a cryogenic inductively coupled plasma reactive ion etching process using high-resolution hydrogen silsesquioxane nanostructures as the hard mask.

Orientational Imaging of a Single Gold Nanorod at the Liquid/Solid Interface with Polarized Evanescent Field Illumination.

Understanding the mechanistic information on many kinetic processes requires the exploration of dynamic rotational information on the target object at the single particle (or molecule) level. In this work, we developed a new strategy, total internal reflection scattering (TIRS) microscopy, to determine the full three-dimensional (3D) angular information on a single gold nanorod (GNR) close to the liquid/solid interface. It was found that the 3D orientational information on individual GNR could be readily elucidated by using p-polarized TIRS illumination through deciphering the orientation-coded intensity distribution pattern in a single TIRS image.

Surface enhanced Raman scattering of gold nanoparticles supported on copper foil with graphene as a nanometer gap.

Gaps with single-nanometer dimensions (<10 nm) between metallic nanostructures enable giant local field enhancements for surface enhanced Raman scattering (SERS). Monolayer graphene is an ideal candidate to obtain a sub-nanometer gap between plasmonic nanostructures. In this work, we demonstrate a simple method to achieve a sub-nanometer gap by dewetting a gold film supported on monolayer graphene grown on copper foil.

Vapor-phase preparation of gold nanocrystals by chloroauric acid pyrolysis.

We report that gold nanocrystals can be prepared from vapor phase using chloroauric acid (HAuCl4) as the precursor. By tuning the vapor-phase deposition parameters, the size and space distribution of the gold nanocrystals can be well controlled on substrates. Systematic control experiments demonstrate that intermediate AuCl and AuCl3 products pyrolyzed from HAuCl4 play an essential role in this vapor-phase deposition process.