Gold Arrays Research Articles

Active infrared tuning of metal–insulator-metal resonances by VO2 thin film

VO2 is a promising phase change material offering a large contrast of electric, thermal, and optical properties when transitioning from semiconductor to metallic phase. Here we show that a hybrid metamaterial obtained by proper combination of a VO2 layer and a nanodisk gold array provides a tunable plasmonic gap resonance in the infrared range. Specifically, we have designed and fabricated a metal–insulator-metal gap resonance by inserting sub-wavelength VO2 film between a flat gold layer and a gold nanodisk resonator array. The resonance of the hybrid metamaterial is centered in the useful 3–5 μm range when VO2 is in its semiconductor state. The experimental study highlights a monotonical spectral tuning of the resonance when increasing temperature up to 50 °C above the room temperature, providing a continuous resonance shift of almost 1 μm in the mid-infrared range. Wavelength range and intensity tunability can be further optimized by modifying the thicknesses of the layers and metamaterial parameters.

Tunability of plasmonic resonances in stratified hyperbolic metamaterials

We numerically and experimentally demonstrate the tunability of plasmonic resonances in the spectral range from 550 nm to 900 nm for type II hyperbolic metamaterials, as a function of their number of layers, period, and filling fraction. The type II hyperbolic metamaterial consisted of stratified periodic arrays of gold and titanium dioxide thin layers on top of a glass substrate. Through numerical and experimental characterization of the totally internally reflected spectra as a function of incidence angle, we show that the tunability of the plasmonic resonance mainly depends on the filling fraction, while the number of layers and the period of the structure only affect the contrast of attenuated reflected spectra. In addition to demonstrating the capability of tuning plasmonic resonances in simplistic stratified mediums, this work contributes to the understanding of light interaction with hyperbolic metamaterials for the improvement of optical devices for sensing applications.

Magnetic quadrupolar resonance in a symmetric three-layered plasmonic metasurface

Dark plasmon mode with even symmetry presents an effective way for manipulating light-matter interactions due to its attractive properties of small radiative loss and strongly intensified near-field, which opens up space for both fundamental explorations and optoelectronic device developments. In this paper, we demonstrate magnetic quadrupolar resonance in a symmetric plasmonic metasurface under oblique light incidence. In a three-layered metasurface made of a cross-shaped gold array on top of a magnesium fluoride film backed with a copper ground plane, a magnetic quadrupolar resonance is excited at 37.3 THz with a quality factor of 34, which is four times of the regular magnetic dipolar resonance excited in the same structure. By increasing the incident angle, asymmetry perturbation on the magnetic quadrupolar mode becomes more pronounced as evidenced from its decreased quality factor. In addition, due to the structure symmetry of the used metal cross, the metasurface exhibits polarization-selective excitation of two separate magnetic quadrupolar modes at 37.3 THz and 42.3 THz for the electric field along four symmetry axes of the structure. Our demonstrated distinctive magnetic quadrupolar resonance offers an alternative mode design in plasmonic metamaterials with an improved quality factor, which could be beneficial for metamaterial applications in sensing, optical filtering and infrared emission manipulation etc.

Graphene-assisted metamaterial absorber for refractive index sensor at lower optical frequencies

Numerical studies on a graphene-assisted metamaterial absorber operating at lower optical frequencies are presented. The metamaterial absorber comprises one-dimensional gold grating with a gold disk array in grooves on a monolayer graphene supported by a glass substrate. Numerical simulations are conducted using Computer Simulation Technology Microwave Studio to investigate the underlying absorption mechanisms in the system. The absorber shows a very sharp absorption band with almost unity absorptance and nearly 20 nm full width at half maximum at a wavelength of 766 nm. The simulated absorption spectrum of the absorber is theoretically validated using coupled mode theory, showing close agreement with the theoretical predictions. The absorption occurs due to the simultaneous excitation of localized and propagating surface plasmons under the transverse electric mode (where the electric field is aligned parallel to the length of the grating ridge) excitation in the system. The sensing performance of the absorber has been evaluated, demonstrating a sensitivity as high as 667 nm/RIU. Due to the robustness of the design, this absorber can be used to sense biological, chemical, gaseous, and even acidic samples since it is not comprised of any photoresist. The absorber holds potential for diverse applications, including graphene-based devices, narrowband filters, thermal emitters, solar cells, imaging apparatus, etc.

Metasurface of Strongly Coupled Excitons and Nanoplasmonic Arrays.

Metasurfaces allow light to be manipulated at the nanoscale. Integrating metasurfaces with transition metal dichalcogenide monolayers provides additional functionality to ultrathin optics, including tunable optical properties with enhanced light-matter interactions. In this work, we demonstrate the realization of a polaritonic metasurface utilizing the sizable light-matter coupling of excitons in monolayer WSe2 and the collective lattice resonances of nanoplasmonic gold arrays. We developed a novel fabrication method to integrate gold nanodisk arrays in hexagonal boron nitride and thus simultaneously ensure spectrally narrow exciton transitions and their immediate proximity to the near-field of array surface lattice resonances. In the regime of strong light-matter coupling, the resulting van der Waals metasurface exhibits all key characteristics of lattice polaritons, with a directional and linearly polarized far-field emission profile dictated by the underlying nanoplasmonic lattice. Our work can be straightforwardly adapted to other lattice geometries, establishing structured van der Waals metasurfaces as means to engineer polaritonic lattices.

The Difference between Plasmon Excitations in Chemically Heterogeneous Gold and Silver Atomic Clusters.

Weak doping can broaden, shift, and quench plasmon peaks in nanoparticles, but the mechanistic intricacies of the diverse responses to doping remain unclear. In this study, we used the time-dependent density functional theory (TD-DFT) to compute the excitation properties of transition-metal Pd- or Pt-doped gold and silver atomic arrays and investigate the evolution characteristics and response mechanisms of their plasmon peaks. The results demonstrated that the Pd or Pt doping of the off-centered 10 × 2 atomic arrays broadened or shifted the plasmon peaks to varying degrees. In particular, for Pd-doped 10 × 2 Au atomic arrays, the broadened plasmon peak significantly blueshifted, whereas a slight red shift was observed for Pt-doped arrays. For the 10 × 2 Ag atomic arrays, Pd doping caused almost no shift in the plasmon peak, whereas Pt doping caused a substantial red shift in the broadened plasmon peak. The analysis revealed that the diversity in these doping responses was related to the energy positions of the d electrons in the gold and silver atomic clusters and the positions of the doping atomic orbitals in the energy bands. The introduction of doping atoms altered the symmetry and gap size of the occupied and unoccupied orbitals, so multiple modes of single-particle transitions were involved in the excitation. An electron transfer analysis indicated a close correlation between excitation energy and the electron transfer of doping atoms. Finally, the differences in the symmetrically centered 11 × 2 doped atomic array were discussed using electron transfer analysis to validate the reliability of this analytical method. These findings elucidate the microscopic mechanisms of the evolution of plasmon peaks in doped atomic clusters and provide new insights into the rational control and application of plasmons in low-dimensional nanostructures.

A Gold Needle Tip Array Ultrafast Electron Source with High Beam Quality.

Electron sources are crucial elements in diverse applications such as electron microscopes, synchrotrons, and free-electron lasers. Nanometer-sharp needle tips are electron emitters with the highest beam quality, yet for a single needle the current is limited. Combining the emission of multiple needles promises large current yields while preserving the individual emitters' favorable properties. We present an ultrafast electron source consisting of a lithographically fabricated array of sharp gold tips illuminated with 25 fs laser pulses. The source provides up to 2000 electrons per pulse for moderate laser peak intensities of 1011 W/cm2 and a narrow energy width of 0.5 ± 0.05 eV at low current. The electron beam has a well-behaved Gaussian profile and is highly collimated, yielding a small normalized emittance on the order of nm·rad. These properties are well suited for applications requiring both high current and spatial resolution, such as free-electron light sources and chip-based particle accelerators.

A novel ratiometric aptasensor based on SERS for accurate quantification of cardiac troponin I

Cardiac troponin I (cTnI) as a biomarker of acute myocardial infarction (AMI) is directly related to myocardial damage. Here, we proposed a ratiometric aptamer-sensor (aptasensor) for quantifying cTnI based on surface-enhanced Raman scattering (SERS), in which the uniform and high-density gold array was decorated with highly specific aptamer Tro4 to form the capture substrate for target cTnI. Firstly, gold array was labeled with 5, 5’-dithio bis-(2-nitrobenzoic acid) (DTNB) as one SERS signal to effectively realize signal self-calibration. In addition, aptamer Tro6 targeting cTnI and 4-mercaptobenzoic acid (4-MBA) as another SERS signal were anchored to porous gold (pAu) to form a pAu signal probe. In the presence of cTnI, the capture substrate and pAu signal probes could build a typical sandwich structure, thus the ratio of SERS peak intensity (I4-MBA/IDTNB) could be used to quantify cTnI. The ratiometric aptasensor showed a good linearity from 0.001 to 100 ng mL−1 and a low detection limit down to 0.27 pg mL−1. Own to dual signal output and the gold array substrate, the aptasensor exhibited good reproducibility, high sensitivity and accuracy. When the aptasensor was applied to the quantification of cTnI in human serum samples, the detection performance was comparable to that of enzyme-linked immunosorbent assay (ELISA), proving that the sensor has a good prospect in the diagnosis of AMI disease.

Gold nanoclusters-based fluorescence sensor array for herbicides qualitative and quantitative analysis

Herbicides have been extensively used around the world, which poses a potential hazard to humans and wildlife. Accurate detection of herbicides is crucial for the environment and human health. Herein, a simple and sensitive fluorescence sensor array was constructed for discrimination and identification of herbicides. Fluorescent gold nanoclusters modified with 11-mercaptoundecanoic acid or reduced glutathione were prepared, respectively. Metal ions quenched the fluorescence of nanoclusters through coordination and leading to the aggregation of gold nanoclusters. The addition of auxin herbicides (2,4-dichlorophenoxyacetic acid, 2-methyl-4-chlorophenoxyacetic acid, decamba, picloram, quinclorac) restored the fluorescence of nanoclusters with different degrees. The mechanism study showed auxin herbicides can bind with metal ions and re-disperse the gold nanoclusters from the aggregation state. The "on-off-on" fluorescent sensor array was constructed basic on above detection mechanism. Combined with principal component analysis (PCA) and hierarchical cluster analysis (HCA) methods, auxin herbicides are well separated on 2D/3D PCA score plots and HCA dendrogram in the range of 40–500 μm. In addition, the fluorescence sensor array performed successful in detecting real samples and blind samples. The developed sensor system shows a promising in practical detection of herbicides.

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.

Simulation research of localized surface plasmon sensor based on gold nanocone hexagonal array structure

The sensor based on gold nanocone hexagonal array structure was designed. Gold nanocone array was positioned above a gold film and separated by silica dielectric spacer layer. Finite-difference time-domain simulation was used to discuss theoretically the influence of gold film and dielectric layer thickness on the structure performance. The formation mechanism of three resonance peaks was studied through the electric field distribution. With the increase of the dielectric thickness, the absorption went upwards first and then went downwards. The refractive index sensitivity of the structure and figure of merit (FOM) at the dipole resonance peak can reach 450 nm/RIU and 4.3, respectively. At the peak caused by the plasmon resonance between the gold nanocone array and the gold film, the sensitivity is 275 nm/RIU while the FOM value is 18.3. The results provide a valuable insight into the application of metal nanoarrays in sensors.

Dual-signal SERS biosensor based on spindle-shaped gold array for sensitive and accurate detection of miRNA 21

MicroRNAs (miRNAs) play an important regulatory role in tumor progression and have been emerging as promising biomarkers for early diagnosis of breast cancer. Herein, we developed a dual-signal surface-enhanced Raman scattering (SERS) biosensor for the quantitative detection of miRNA 21 (miR-21) by utilizing a tightly ordered spindle-shaped gold (SAu) array with 5, 5′-dithio-bis-(2-nitrobenzoic acid) (DTNB) molecule as an internal standard. Meanwhile, 6-carboxyl-Xrhodamine (ROX) labeled hairpin DNA H1 was linked to the SAu array by Au–S bonds generating another SERS signal. In the presence of miR-21, H1 was turned on with a decrease in SERS signal of ROX, while the SERS signal of DTNB remained stable. The SAu array as substrate provides a large area of hot spots, and the dual-signal can avoid background interference and achieve signal self-calibration. By the dual-signal SERS biosensor, target miR-21 could be detected over a broad linear range (0.1 −105 pM) with a low detection limit (0.046 pM) without signal amplification strategy. Thanks to the SAu array and dual-signal output, the sensor showed high sensitivity, good signal reproducibility, excellent specificity, and high detection accuracy. When this sensor was applied to the miR-21 assay in human serum samples, its performance was comparable to quantitative reverse transcription polymerase chain reaction (qRT-PCR), indicating promising applications of the developed sensor for cancer diagnostics.

Electrically tunable and switchable perfect infrared absorber based on ENZ material

In this paper, a dynamically tunable and switchable perfect infrared absorber is put forward, which is composed of a gold ring array, an indium tin oxide (ITO) layer, an alumina (Al2O3) spacer and a continuous Au film from top to bottom. Epsilon-near-zero (ENZ) mode in ITO layer is excited by localized surface plasmon resonance (LSPR) mode of the gold ring. With the increase of the biasing voltage V, the carrier concentration of ITO material decreases, and the absorption peak caused by ENZ resonance blue shifts obviously. Notice that our infrared absorber can realize bidirectional switching of “ON” and “OFF” states at 1210 nm, with modulation depth of 70 % and switching contrast (SC) of 241 %, manifesting its excellent electrical regulation performance. After verification, the switching performance of our absorption structure has a high precision tolerate of the manufacturing process. A clear Fano type absorption curve appears in the wavelength range 1000 nm–1300 nm, under the biasing voltage V of 3.4 V. Moreover, a series of small absorption peaks are produced by Fabry-Pérot (F–P) resonance in the horizontal plane caused by the standing wave in the ENZ layer, and this phenomenon is unexpected. We propose a theoretical interpretation for the absorption mechanism in both cases based on multiple interference model. It is also proved that the proposed infrared absorber is polarization insensitive and exhibits high absorptance (>90 %) over an incident range of 0 deg–60 deg. The electrically tunable absorption switch function makes our absorber have potential in selective thermal emitters, infrared sensors, infrared stealth, beam focusing and other fields.

Controlling Plasmon Resonance of Gold and Silver Nanoparticle Arrays with Help of Liquid Crystal

The tunability of plasmonic resonances in gold and silver nanosphere arrays on a glass substrate, embedded in a liquid crystal matrix, was explored. The calculations involving the finite element method revealed that the optical properties of these arrays can be modulated by reorienting the liquid crystal. When the liquid crystal director was reoriented between planar and homeotropic configurations in the plane containing the incident wave polarization vector, the plasmonic resonance wavelength shifted within an approximately 100 nm range. A reduced shift of about 40 nm was observed when the reorientation occurred in the plane perpendicular to the polarization. Both metal nanosphere arrays showed notable near-field amplification. Gold achieved up to 18 times the amplification of the incident wave electric field, while silver reached 16 times but showed a remarkable 40 times amplification at the inter-band transition resonance wavelength. This research underscores the potential of using liquid crystal reorientation for controlling the plasmonic lattice resonance in metal nanosphere arrays, opening up new possibilities for adaptable plasmonic devices.

Dynamic tunable narrow-band perfect absorber for fiber -optic communication band based on liquid crystal

At present, most of the reported metasurface structure absorbers show that its working band cannot be regulated actively. In this study, a dynamic tunable narrow-band perfect absorption structure for fiber-optic communication band based on liquid crystal (LC) is proposed and studied. The structure is mainly composed of two effective tiers. The top tier gold array and the bottom tier reflective gold film, which are separated by a SiO2-LC dielectric medium interlayer to form a metal–dielectric–metal structure. Due to the unique optical properties of LCs, its index of refraction can be changed by adjusting the bias voltage and temperature, so as to adjust the resonance wavelength actively. The designed structure is analyzed by finite element method and the coupled mode theory is used to verify the analysis results. The designed structure has a 99.92% absorption effect in the most commonly used band of fiber-optic communication. Due to the symmetry of the absorber structure, the device is not sensitive to the polarization state of the excitation source. Moreover, the absorber exhibits an unusual dependence on the incident angle, which can be attributed to the anisotropy of the LC. Based on the dependence of incident angle, a plasma optical switch with large ON/OFF ratio (η) of 27.395 dB and nearly flawless modulation depth of 99.818% can be realized. It is believed that this structure can provide a method for the dynamic control of near infrared electromagnetic waves, and to be applied in electromagnetic energy absorption, filtering and plasma optical switch system.

2D TiS2-Nanosheet-Coated Concave Gold Arrays with Triple-Coupled Resonances as Sensitive SERS Substrates.

Herein, a hybrid substrate for surface-enhanced Raman scattering (SERS) is fabricated, which couples localized surface plasmon resonance (LSPR), charge transfer (CT) resonance, and molecular resonance. Exfoliated 2D TiS2 nanosheets with semimetallic properties accelerate the CT with the tested analytes, inducing a remarkable chemical mechanism enhancement. In addition, the LSPR effect is coupled with a concave gold array located underneath the thin TiS2 nanosheet, providing a strong electromagnetic enhancement. The concave gold array is prepared by etching silicone nanospheres assembled on larger polystyrene nanospheres, followed by depositing a gold layer. The LSPR intensity near the gold layer can be adjusted by changing the layer thickness to couple the molecular and CT resonances, in order to maximize the SERS enhancement. The best SERS performance is recorded on TiS2-nanosheet-coated plasmonic substrates, with a detectable methylene blue concentration down to 10-13m and an enhancement factor of 2.1 × 109 and this concentration is several orders of magnitude lower than that of the TiS2 nanosheet (10-11m) and plasmonic substrates (10-9m). The present hybrid substrate with triple-coupled resonance further shows significant advantages in the label-free monitoring of curcumin (a widely applied drug for treating multiple cancers and inflammations) in serum and urine.

Chromatic Plasmonic Polarizer-Based Synapse for All-Optical Convolutional Neural Network.

Emerging memory devices have been demonstrated as artificial synapses for neural networks. However, the process of rewriting these synapses is often inefficient, in terms of hardware and energy usage. Herein, we present a novel surface plasmon resonance polarizer-based all-optical synapse for realizing convolutional filters and optical convolutional neural networks. The synaptic device comprises nanoscale crossed gold arrays with varying vertical and horizontal arms that respond strongly to the incident light's polarization angle. The presented synapse in an optical convolutional neural network achieved excellent performance in four different convolutional results for classifying the Modified National Institute of Standards and Technology (MNIST) handwritten digit data set. After training on 1,000 images, the network achieved a classification accuracy of over 98% when tested on a separate set of 10,000 images. This presents a promising approach for designing artificial neural networks with efficient hardware and energy consumption, low cost, and scalable fabrication.

Continuously Activated Function of Aluminum in Galvanic Micro Arrays in Contact with Water

Galvanic coupled arrays of Aluminum (Al) and Gold (Au) aligned in microgap has displayed high accuracy and sensitivity towards detection of invisibly small water droplets. With Al in moisture sensing chip, 1) formation of Al2O3 layer around Al and 2) decay of Al electrode with time, may interfere its overall activity. In this study, a continuous activation of Al electrode on the sensor chip was performed and investigated through electrochemical evaluation, output current measurement, and SEM observations. Further pH in its vicinity was simulated for ∼2 years. The results showed that a sensing chip exhibits almost steady output current throughout the period. At relatively high current density of 1.25 × 10−6 A.cm−2, obtained in the short circuit, anodic reaction on the Al electrode generates protons. This production of protons reduces the pH to ∼4, where Al2O3 and Al3+ formed on its surface could get dissolved. In addition, Al consumption in 100 ms, i.e. the time for one closed circuit corresponds to 2.26 × 10−17 g.A−1 s−1, which is negligible. The SEM images confirmed unchanged morphology of Al electrodes. The results support the long-term stability of accurate and sensitive sensor chip with galvanic array of Al and Au electrodes.

Mid-infrared plasmonic filter arrays using nanoholes in gold and silver films.

Plasmonic filters based on subwavelength nanohole arrays are an attractive solution for creating arrays of filters with varying passbands in a single lithography step. In this work, we have developed a fabrication method which allows fabrication of nanohole arrays in silver by use of a thin layer of aluminum oxide, which serves the dual purpose of both capping layer and hardmask for metal patterning. We demonstrate arrays of gold and silver mid-infrared plasmonic filters, fabricated on silicon, intended for use in optical filter blocks or for future integration with infrared imagers. The filter arrays are designed for the wavelength range 2-7 µm, and exhibit peak filter transmission efficiencies around 70%.

Effect of praseodymium coating on electron emission from a nanoscale gold field emitter array

Although field emission devices are inherently robust to high temperature and radiation environments as well as have high switching speeds, their development has been hindered by high voltages that are typically required for their operation. In this work, we investigate the effect of thin-film praseodymium (Pr) coating on the emission characteristics of a lateral gold (Au) field emitter array. Because Pr has a significantly lower work function than Au, it is expected to increase the field emission measured current. Pr is deposited onto the device via thermal angled evaporation in a custom-built vacuum chamber with in situ electrical characterization capability. Our experiments demonstrate that a 10 nm-thick Pr layer reduces the turn-on voltage by almost half compared to the noncoated Au structure. These results are promising for the development of power-efficient, low voltage field emission electronics.