Gold Nanodisk Arrays Research Articles

Plasmonic Coupling for High‐Sensitivity Detection of Low Molecular Weight Molecules

This article presents a novel plasmonic sensing platform designed for the detection of low molecular weight molecules, offering significant advancements in diagnostic applications. The platform features a periodic array of gold nanodisks on a 20 nm thin silica layer, supported by a 100 nm thick gold substrate. By leveraging the coupling between localized and propagating surface plasmon resonances, this design significantly enhances the sensitivity and specificity of molecular detection. Finite element method simulations are conducted to characterize the optical properties and reflectance response of the nanodisks array in the visible to near‐infrared range. Ellipsometric analysis is performed to measure the reflectance of the sample at various angles. Additionally, scanning near‐field optical microscopy in reflectance mode validates the design by revealing well‐defined plasmonic hot spots and interference patterns consistent with the simulated results. The findings demonstrate the platform's effectiveness in amplifying optical signals, achieving a limit of detection of 50 μM for molecules with a molecular weight of less than 1 KDa. This high sensitivity and specificity highlight the potential of the proposed plasmonic platform to advance the development of highly sensitive sensors for low molecular weight molecules, making it a valuable tool for diagnostics and precise molecular detection.

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.

Substrate-induced hybridization of plasmon modes in the composite nanostructure of nanodisk array/thin film for spectrum modulation

Abstract Hybridization coupling among plasmon modes is an effective approach to manipulate near-field properties thus optical spectral shapes of plasmonic nanostructures. Generally, mode hybridization coupling is achieved by modifying the topography and dimensions of nanostructures themselves, with few concerns about substrate-induced manipulation. Herein, we propose a composite nanostructure consisting of a gold (Au) nanodisk array and a thin Au film supported by a dielectric substrate. In this configuration, both the refractive index of the dielectric substrate and thin gold film’s thickness mediate the interaction of plasmon modes supported by upper and lower interfaces of the composite nanostructure, resulting in two hybridized plasmon modes. We systematically investigate the relationship between optical fields at the top surface of plasmon modes before and after the hybridization coupling. Specifically, the near-field amplitude at the top surface of the unhybridized modes is stronger than that of individual hybridized mode, and lower than the near-field summation of these two hybridized modes. This work not only provides a straightforward strategy for generating two plasmon modes in a nanostructure but also elucidates the variation of the optical field during the hybridization process, which is of crucial significance for applications, such as upconversion enhancement and multi-resonance sensing.

Tuning the plasmonic response of periodic gold nanodisk arrays for urea sensing

Tuning the plasmonic response of periodic gold nanodisk arrays for urea sensing

Period induced reflectance tuning in transparent gold metasurfaces

Gold (Au) nanodisk arrays are modeled and fabricated for tunable near infrared reflectance with high visible transmittance. Simulations are performed in the visible and near infrared regimes between 400 and 2000 nm with varying period of nanostructures. A progressive increase in period of up to three times the diameter of the nanodisks on polymer and quartz substrates is presented. As a result, an increase in visible transmittance with a decrease in near infrared reflectance peak intensity is observed. Modeling results exhibit a high average visible transmittance of 91.5% (400–700 nm) and average reflectance of 62.9% (750–2000 nm) with diameter (D) variations from D=120 nm to D=500 nm with a period (P) equal to twice the diameter, P=2D for a polymer substrate. Electric field intensities exhibit an increase with a decrease in the period between individual nanostructures for constant unit cell dimensions. Alongside simulations, electron beam lithography and evaporation processes are utilized for the patterning and fabrication. Optical characterizations including scanning electron microscopy (SEM) and atomic force microscopy (AFM) of selected metasurfaces on quartz are performed. For a specific period and diameter, the fabricated metasurfaces exhibit near infrared reflectance having a good agreement with the simulated results about plasmonic wavelengths. Such metasurfaces offer potential to be deployed for applications that require selective reflectance intensities in the near infrared range with high visible transmittance, such as controlled environment agriculture (CEA) or multi spectral near infrared sensing. With tunability over a broadband (750–2500 nm), this simple and fabrication-friendly model may be used as a theoretical guide in quantifying specific design parameters with varying light intensity configurations.

Bound states in the continuum in a waveguide structure covered by a two-dimensional square array of gold nanodisks

Bound states in the continuum in a waveguide structure covered by a two-dimensional square array of gold nanodisks

Balancing detectivity and sensitivity of plasmonic sensors with surface lattice resonance

Abstract Resonators are at the core of optical sensors enhancing light–analyte interaction and leading to higher sensitivities. Maximizing the sensitivity is an obvious objective function for the resonator design. However, high sensitivity does not guarantee sufficient detectivity. When the optical energy budget is limited, as in sensors on mobile platforms, a higher sensitivity usually leads to lower detectivity for nanophotonic sensors. In such scenarios, resonator design requires balancing the trade-off between the sensitivity and detectivity of the resonant sensor. Here, we show the direct dependence of detectivity on the Q-factor and the trade-off between the Q-factor and sensitivity. We study this trade-off in an array of plasmonic resonators. We choose plasmonic resonators because of their high sensitivity arising from large local field enhancements. Then, we show that the detectivity of this sensor may be boosted for limited energy budget applications by making an array of resonators supporting a surface lattice resonance (SLR). We experimentally demonstrate sensing and detection of antimouse IgG protein in a gold nanodisk array–based SLR sensor for various energy budgets.

Efficient second harmonic generation at quasi-bound states in the continuum in hybrid nanostructures of 2D plasmonic array and waveguide

We numerically investigate the second harmonic generation (SHG) in hybrid plasmonic–photonic structures consisting of an array of periodic gold nanodisks covered by a nonlinear dielectric layer. In the designed hybrid systems, symmetry-protected bound states in the continuum (BICs) at normal incidence, and Friedrich–Wintgen BICs due to the coupling between localized surface plasmon resonances (LSPRs) and waveguide modes at oblique incidence, are observed under both transverse-electric (TE) and transverse-magnetic (TM) polarized illuminations. SHG near the BICs is studied, and the dramatically enhanced nonlinear optical response at quasi-BICs is obtained. Such enhancement is ascribed to the enhanced local field due to hybrid modes of LSPRs and photonic modes. The results indicate that hybrid plasmonic-dielectric systems supporting BICs at both TE- and TM-polarized incidences are promising platforms for linear and nonlinear photonic devices.

Imaging and Controlling Ultrafast Electron Pulses Emitted from Plasmonic Nanostructures.

We experimentally study photoemission from gold nanodisk arrays using space-, time-, and energy-resolved photoemission electron microscopy. When excited by a plasmonic resonant infrared (IR) laser pulse, plasmonic hotspots are generated owing to local surface plasmon resonance. Photoelectrons emitted from each plasmonic hotspot form a nanoscale and ultrashort electron pulse. When the system is excited by an extreme ultraviolet (EUV) laser pulse, a uniformly distributed photoelectron cloud is formed across the sample surface. When excited by the IR and EUV laser pulses together, both the photoemission image and kinetic energy vary significantly for the IR laser-generated electrons depending on the time delay between the two laser pulses. These observations are well explained by the Coulomb interaction with the EUV laser-generated electron cloud. Our study offers a feasible approach to manipulate the energy of electron pulse emitted from a plasmonic nanostructure on an ultrafast time scale.

Plasmonic gold disk nanoarrays-based surface lattice resonance sensor with high figure of merit and its near-field and far-field coupling characteristics

In the field of localized surface plasmon resonance sensing, narrow linewidth, high-quality factor, and high absorptivity have always been the goals that researchers are committed to achieving. The primary methods for achieving this goal are near-field coupling and far-field coupling. Due to the limitations of nanomanufacturing processes, current research often only focuses on single coupling methods such as far-field coupling or near-field coupling, with few relevant reports combining the two coupling methods. Based on this, this paper proposes a single-layer gold nano-disk array structure, which is simple and easy to manufacture. Through modal field analysis and structural parameter adjustment, some significant results have been achieved. On the one hand, the suggested gold nano-disk array structure yields a resonant peak based on far-field coupling with a full width at half maximum (FWHM) of less than 0.05 nm and a figure of merit of more than 15 000 (the quality factor is more than 20 000). On the other hand, it has been successfully accomplished to combine near-field coupling and far-field coupling, leading to a composite coupling peak that incorporates the benefits of both types of coupling. Moreover, this coupling peak has excellent characteristics such as narrow FWHM (less than 3 nm) and high absorptivity (up to 80.5%), which is superior to most reported sensing structures and more in line with practical sensing application requirements. In this paper, the potential of the gold disk array structure as a high-performance sensing structure is further explored, providing reference and new ideas for the design of easy-to-prepare and high-performance electromagnetic resonance devices.

Multiphysics Modeling of Plasmon-Enhanced All-Optical Helicity-Dependent Switching.

In this work, we propose a multiphysics approach to simulate all-optical helicity-dependent switching induced by the local hot spots of plasmonic nanostructures. Due to the plasmonic resonance of an array of gold nanodisks, strong electromagnetic fields are generated within the magnetic recording media underneath the gold nanodisks. We construct a multiphysics framework considering the opto-magnetic and opto-thermal effects, and then model the magnetization switching using the Monte Carlo method. Our approach bridges the gap between plasmonic nanostructure design and magnetization switching modeling, allowing for the simulation of helicity-dependent, nanoscale magnetization switching in the presence of localized surface plasmons.

Plasmonic Resonance Coupling of Nanodisk Array/Thin Film on the Optical Fiber Tip for Integrated and Miniaturized Sensing Detection.

Fiber-optic surface plasmon resonance (FOSPR) sensing technology has become an appealing candidate in biochemical sensing applications due to its distinguished capability of remote and point-of-care detection. However, FOSPR sensing devices with a flat plasmonic film on the optical fiber tip are seldom proposed with most reports concentrating on fiber sidewalls. In this paper, we propose and experimentally demonstrate the plasmonic coupled structure of a gold (Au) nanodisk array and a thin film integrated into the fiber facet, enabling the excitation of the plasmon mode on the planar gold film by strong coupling. This plasmonic fiber sensor is fabricated by the ultraviolet (UV) curing adhesive transferring technology from a planar substrate to a fiber facet. The experimental results demonstrate that the fabricated sensing probe has a bulk refractive index sensitivity of 137.28 nm/RIU and exhibits moderate surface sensitivity by measuring the spatial localization of its excited plasmon mode on Au film by layer-by-layer self-assembly technology. Furthermore, the fabricated plasmonic sensing probe enables the detection of bovine serum albumin (BSA) biomolecule with a detection limit of 19.35 μM. The demonstrated fiber probe here provides a potential strategy to integrate plasmonic nanostructure on the fiber facet with excellent sensing performance, which has a unique application prospect in the detection of remote, in situ, and in vivo invasion.

Plasmon-Enhanced Raman Scattering by Multilayered Graphene at the Micro- and Nanoscale: SERS and TERS Analysis

This work is devoted to the study of plasmon-enhanced Raman scattering by the fundamental vibrational modes of multilayered graphene films. The film thickness was ∼3.5 nm, which corresponds to ∼10 monolayers. Multilayered graphene films were placed on a plasmonic substrate consisting of arrays of gold nanodisks (50–250 nm in diameter). Surface-enhanced Raman scattering by the main vibrational modes of multilayered graphene film placed on an array of Au nanodisks of various sizes was implemented. The measurements were performed at excitation wavelengths of 532, 638, and 785 nm. A resonant SERS enhancement of the main vibrational modes of multilayered graphene by a factor of 25 was achieved for nanodisks with a diameter of 103 nm upon excitation at 638 nm. A stronger local enhancement of Raman scattering in multilayered graphene (by a factor of 50) placed on Au nanodisk array is achieved using gap-mode tip-enhanced Raman scattering (gap-mode TERS). Nanofolds in the graphene film appeared due to the corrugated surface of the plasmonic substrate were visualized with nanometer spatial resolution. It is shown that the frequency positions of G and 2D modes of nanofolds decrease with respect to the corresponding values in flat multilayered graphene manifesting mechanical stresses in the nanofolds up to 0.7%. The results obtained shed light on the effects of the interaction of multilayered graphene with metal nanostructures and are important in creating hybrid metal/graphene plasmonic substrates.

Tuning Electrogenerated Chemiluminescence Intensity Enhancement Using Hexagonal Lattice Arrays of Gold Nanodisks.

Electrogenerated chemiluminescence (ECL) microscopy shows promise as a technique for mapping chemical reactions on single nanoparticles. The technique's spatial resolution is limited by the quantum yield of the emission and the diffusive nature of the ECL process. To improve signal intensity, ECL dyes have been coupled with plasmonic nanoparticles, which act as nanoantennas. Here, we characterize the optical properties of hexagonal arrays of gold nanodisks and how they impact the enhancement of ECL from the coreaction of tris(2,2'-bipyridyl)dichlororuthenium(II) hexahydrate and tripropylamine. We find that varying the lattice spacing results in a 23-fold enhancement of ECL intensity because of increased dye-array near-field coupling as modeled using finite element method simulations.

Graphene-assisted infrared plasmonic metamaterial absorber for gas detection

In this paper, we propose a graphene-assisted plasmonic metamaterial absorber to operate as an ultra-compact optical gas sensor in the infrared (IR) region. The metamaterial absorber is comprised of a thin silicon-dioxide layer that is middling between a gold film and an array of graphene-coated gold nano-disks. Using the finite element method, we investigate the resonance characteristics of the proposed plasmonic metamaterial and show that the metamaterial has three distinct resonances in visible and infrared regions. We demonstrate that the strongest absorption peak of the proposed metamaterial is attributed to a fundamental localized surface plasmon resonance in IR, which has also the highest spectral sensitivity to the refractive index (RI) variations of the surrounding medium. The sensing mechanism is based on evaluation of gaseous medium RI surrounding the nano-disks, by illuminating the metamaterial with IR radiation and subsequently measuring the reflection or absorption spectra of the structure. We show that graphene coating of the gold nano-disks improves the sensitivity of the metamaterial to the gas RI by more than two times. Using the optimized design parameters, an average linear spectral sensitivity of 720 nm/RIU and an approximate detection limit of 1 × 10−5 RIU has been obtained for gas RI variations between 1 and 1.05. We also show that the proposed sensor has a linear response even for extremely small variations in gas RI in the order of 10−5 RIU. The proposed metamaterial configuration is relatively easy to fabricate and can be used to monitor low concentrations of various gases in different applications ranging from environmental monitoring to home safety monitoring systems.

Nanoplasmonic Biosensing Approach for Endotoxin Detection in Pharmaceutical Field

The outer membrane of Gram-negative bacteria contains bacterial endotoxins known as Lipopolysaccharides (LPS). Owing to the strong immune responses induced in humans and animals, these large molecules have a strong toxic effect that can cause severe fever, hypotension, shock, and death. Endotoxins are often present in the environment and medical implants and represent undesirable contaminations of pharmaceutical preparations and medical devices. To overcome the limitations of the standard technique, novel methods for early and sensitive detection of LPS will be of crucial importance. In this work, an interesting approach for the sensitive detection of LPS has been realized by exploiting optical features of nanoplasmonic transducers supporting Localized Surface Plasmon Resonances (LSPRs). Ordered arrays of gold nano-prisms and nano-disks have been realized by nanospheres lithography. The realized transducers have been integrated into a simple and miniaturized lab-on-a-chip (LOC) platform and functionalized with specific antibodies as sensing elements for the detection of LPS. Interactions of specific antibodies anchored on protein A-modified sensor chips with the investigated analyte resulted in a spectral shift in the plasmonic resonance peak of the transducers. A good linear relationship between peak shifts and the LPS concentration has been demonstrated for the fabricated nano-structures with a detection limit down to 5 ng/mL. Integration with a proper microfluidic platform demonstrates the possibility of yielding a prototypal compact device to be used as an analytical test for quality determination of pharmaceutical products.

Fourier-Imaging Spectroscopy of Two-Dimensional Gold Nanodisk Array on Photoluminescent Layer

In this paper, we theoretically and experimentally study the reflection and photoluminescence spectra angular dependencies of the two-dimensional gold nanodisk array on top of thin luminescent layer. Hydrogenated amorphous silicon-carbon alloy was used as a luminescent material with a high refractive index in the visible spectral range. Angle-resolved optical spectra were measured by a Fourier imaging spectroscopy in p- and s-polarizations. Both transverse electric (TE) and transverse magnetic (TM) quasiguided modes are clearly seen in the reflection and photoluminescence spectra in both polarizations simultaneously.

Photothermal Intracellular Delivery Using Gold Nanodisk Arrays.

Local heating using pulsed laser-induced photothermal effects on plasmonic nanostructured substrates can be used for intracellular delivery applications. However, the fabrication of plasmonic nanostructured interfaces is hampered by complex nanomanufacturing schemes. Here, we demonstrate the fabrication of large-area plasmonic gold (Au) nanodisk arrays that enable photothermal intracellular delivery of biomolecular cargo at high efficiency. The Au nanodisks (350 nm in diameter) were fabricated using chemical lift-off lithography (CLL). Nanosecond laser pulses were used to excite the plasmonic nanostructures, thereby generating transient pores at the outer membranes of targeted cells that enable the delivery of biomolecules via diffusion. Delivery efficiencies of >98% were achieved using the cell impermeable dye calcein (0.6 kDa) as a model payload, while maintaining cell viabilities at >98%. The highly efficient intracellular delivery approach demonstrated in this work will facilitate translational studies targeting molecular screening and drug testing that bridge laboratory and clinical investigations.

Harvesting the lost photon by plasmonic enhanced hematite-upconversion nanocomposite for water splitting.

Converting solar energy to chemical energy in the form of hydrogen via water splitting is one of the promising strategies to solve the global energy crisis. Hematite, a traditional semiconducting oxide photoelectrode, can only absorb UV and visible parts of the solar spectrum, losing 40% infrared energy. In this paper, we report a novel plasmonic enhanced water splitting photoanode based on hematite-lanthanide upconversion nanocomposites to harvest lost photons below the bandgap of hematite. NaYF4:Er, Yb upconversion nanoparticles can upconvert photons from 980 nm to 510 nm-570 nm within the bandgap of hematite. More importantly, a gold nanodisk array with a plasmonic peak centered ∼1000 nm can further boost the photocurrent by 93-fold. It is demonstrated that the excitation process of lanthanide upconversion nanoparticles can be significantly enhanced by plasmonic nanostructures and can thus improve the water oxidation activity via plasmonic enhanced upconversion and hot electron injection, respectively. This new promising strategy will pave the way for plasmonic enhanced lost photon harvesting for applications in solar energy conversion.

Characterizing the non-paraxial Talbot effect of two-dimensional periodic arrays of plasmonic gold nanodisks by differential interference contrast microscopy.

Exploiting the working principle of conventional differential interference contrast (DIC) microscopy, we experimentally investigate the non-paraxial Talbot effect of two-dimensional periodic arrays of gold nanodisks (AuNDs) with a periodicity ao comparable to the excitation wavelength λ. In the non-paraxial regime, strongly contrasting self-image patterns at the Talbot and fractional Talbot distances appeared perpendicularly to the AuND array. The experimental self-image distances were comparable to the calculated non-paraxial Talbot distances at two excitation wavelengths (540 nm and 600 nm). Beyond the paraxial limit, Talbot distances were observed at positions smaller than the paraxial Talbot distance.