Electromagnetic Field Enhancement Research Articles

Reusable SERS Platform of Femtosecond Laser Processed Substrate for Detection of Malachite Green.

This study presents the development of highly efficient Surface-Enhanced Raman Scattering (SERS) substrates through femtosecond (fs) laser processing of crystalline silicon (Si), resulting in mountain-like microstructures. These microstructures, when decorated with gold nanoparticles (Au NPs), exhibit remarkable SERS performance due to the creation of concentrated hotspots. The enhanced Raman signals originate from the excitation of localized surface plasmon resonance (LSPR) of the Au NPs and the multi-scale rough morphology of the Si substrates. Finite-element method simulations confirm the electromagnetic field enhancement in narrow gaps, supporting the experimental observations. The fabricated substrates show high uniformity, oxidation resistance, long-term stability, and exceptional reproducibility, making them ideal for molecular detection, especially in food safety applications. A remarkable enhancement factor (EF) of 1010 is attained in the detection of Malachite Green (MG), boasting a limit of detection (LoD) as low as 10-14 M. This underscores the immense potential of this technique for achieving highly sensitive and dependable SERS-based sensing capabilities.

Versatile Metamaterial: Exploring the Resonances of Symmetry‐Protected Modes for Multitasking Functionality

AbstractIn this work, an experimental study supported by numerical modeling that demonstrates the possibility of exciting Symmetry Protected‐Bound states In the Continuum (SP‐BICs) in a 1D silicon grating fabricated on a lithium niobate substrate is presented. bBoth transverse electric and magnetic polarization states are investigated, leading to the excitation of four quasi‐ Bound states In the Continuum (quasi‐BIC) resonances, exhibiting distinct behaviors. Under standard illumination conditions (plane of incidence perpendicular to the 1D grating lines), two of these resonances are highly sensitive to illumination conditions, while the other two resonances involving unconventional illumination directions (plane of incidence parallel to the grating lines) are more robust to the angle of incidence, but just as sensitive to external stresses in terms of resonance wavelength and quality factor. Additionally, temperature detection is experimentally demonstrated with a Sensitivity of ST = 0.81∼nm °C−1, a state‐of‐the‐art value achieved due to significant electromagnetic field enhancement inside the lithium niobate substrate at the quasi‐BIC resonance. These findings pave the way for their use in various sensing applications (such as biology, electromagnetic, and temperature sensing), as well as nonlinear applications like second harmonic generation, and electro‐ and acousto‐optic modulation.

A novel controllable nanocyclic plasma coupled array in SERS trace detection of multi-component pollutants

The development and design of a novel, uniform and highly active local electromagnetic field enhanced structure is crucial for expanding Surface-enhanced Raman Scattering (SERS) applications. In this study, we developed Ag ring-coupled nanoarrays (Ag RCNAs) with controllable nanogaps using a substrate rotary evaporation coating technique with self-assembled polystyrene (PS) microspheres as templates. This straightforward and cost-effective method efficiently prepares plasma-coupled nanoarrays. Ag RCNAs demonstrated high sensitivity in detecting organic dyes, our prepared Ag RCNAs showed high sensitivity (with the limit of detection of 10−8 M), high signal reproducibility (with the relative standard deviation of 6.73 %). Furthermore, Ag RCNAs showed remarkable sensitivity to a broad spectrum of dyes in river water, indicating the large-area uniform and highly active circular-ring-shaped nanogaps can realize highly sensitive detection of various pollutants. This approach offers advantages in electromagnetic field enhancement, tunable nanogaps, uniformity, reproducibility, and recyclability, making it promising for applications in environmental monitoring, bioassays, food safety, and medical diagnostics.

Metal Nanowire‐Induced Enhancement of Surface Emission in Luminescent Glass

AbstractLuminescent glass, as an important component of photonic materials, is extensively utilized in applications such as lasers and optical fiber amplifiers, which demand high quantum efficiency. To further enhance the surface emission performance of luminescent glass, this study introduces a novel approach by employing custom‐made metal nanowires, applying them as coatings on the surface of Erbium‐doped glass, rather than embedding traditional metal nanoparticles. Utilizing experimental techniques and Finite‐Difference Time‐Domain (FDTD) simulations, it is demonstrated that the surface‐emission from Erbium‐doped glass coated with silver nanowires or copper nanowire‐graphene hybrids is significantly increased, showing an enhancement rate of up to 117.27% compared to intrinsic glass. Extended FDTD analysis reveals that variations in the size and density of the nanowires can optimize emission characteristics, thereby improving the emission performance of luminescent glass. The enhancement of the local electromagnetic field at the interface underscores the efficacy of metal nanowires in boosting photonic materials. This study not only highlights the practicality of metal nanowires in photonics but also introduces a rapid deployment pathway for customizing photonic interactions through nano‐engineering, potentially extendable to other photonic materials.

Super-Resolution Surface-Enhanced Raman Scattering: Perspectives on the Past, Present, and Future.

Super-resolution surface-enhanced Raman scattering (SERS) allows researchers to overcome the resolution limit of far field optical microscopy and peer into electromagnetic hot spots with nanoscale resolution. By localizing the signal from single (or few) molecules on the surface of plasmonic nanoparticle aggregates, relationships between the spatial origin of the SERS signal, local electromagnetic field enhancements, and SERS intensity can be determined. This Perspective describes the successes and challenges of super-resolution SERS, from the earliest mapping of single-molecule SERS hot spots to the current state-of-the-art, while highlighting open questions and future opportunities to advance the field. Comparisons with fluorescence-based super-resolution imaging are discussed to help frame the unique challenges associated with performing SERS in the super-resolution regime.

De-wetted gold nanostructures for SERS-based sensing of static and dynamic targets

De-wetting of metal thin film is one of the simplest fabrication techniques to produce metallic nanostructures for Surface-Enhanced Raman Spectroscopy (SERS)-based sensing. In this manuscript, we explore the sensitivity of a de-wetted metal SERS substrate by testing it with the organic dye Rhodamine 6G (R6G) as well as with the single amino acids (Glycine, Serine, and Histidine) and a binary mixture of Glycine and Serine mixture. We observe that repeated de-wetting of the metal thin film provides a more than ten times higher SERS enhancement factor compared to a single de-wetting. This is attributed to the presence of more closely spaced metal nanoclusters, enabling a higher electromagnetic field enhancement at the hotspots. We characterized the sensing capability of our SERS substrates by detecting R6G dye drop-casted onto the substrates at sub-femtomolar concentrations. We were also able to detect R6G in a dynamic state while it was transported across a 500 nm diameter pore with a nanomolar detection limit. Finally, we were also able to detect amino acids with a micromolar detection limit using our SERS substrate. These results indicate the promise in the integration of our SERS substrate with solid-state nanopores for SERS-based signal readout where concentrations higher than nanomolar exist in the pore due to high confinement.

Impact of Graphene Oxide on SERS Enhancement of Arylated Gold Nanospheres: Mechanistic Insight.

The performance of gold nanospheres as substrates for surface-enhanced Raman spectroscopy (SERS) investigation has been compromised by their low adsorption efficiency, high colloidal dispersibility, and diminishing hot spots. However, gold nanosphere substrates modified using aryldiazonium gold(III) chemistry via durable gold-carbon bonds are promising for SERS enhancement due to their controlled organic layer density. In this study, arylated gold nanospheres AuNSs-COOH have shown SERS enhancement when incorporated into graphene oxide (GO) to form nanocomposites (NCs) labeled AuNSs-COOH/GO (AuNCs). Our investigation using X-ray photoelectron spectroscopy (XPS) surface analysis showed that the gold-aryl nanospheres reached their maximum SERS enhancement with an optimal coating. The evaluation included the Au 4f chemical environment and compact graphitic layers for the SERS substrate optimization. The fabricated AuNC substrates demonstrated superior efficiency and reproducibility. A broad linear range of 10-3-10-7 M 4-nitrophenol detection was obtained with exceptional repeatability, as evidenced by the relative standard deviation (RSD) of 9.32%. A detailed investigation of the energy profiles, particularly the valence band maximum (VBM) and band gap values of the substrate and analyte, depicted the electromagnetic (EM) and charge-transfer-induced enhancement and the role of GO inclusion in substrate efficiency in SERS enhancement mechanisms. The finite-difference time domain (FDTD) simulation results revealed that AuNCs incorporated with graphitic nanostructures exhibited the most substantial SERS effect through an EM field enhancement mechanism. This study demonstrated significant SERS enhancement using gold-aryl nanospheres when modified with GO, in contrast to the typical reliance on anisotropic nanostructures.

(Invited) Ultrafast Spectroscopy of Nanostructured Plasmonic and Energy Conversion Materials

Ultrafast processes that occur upon the absorption of photons by a material frequently play important roles in the efficiency of a desired energy conversion or photocatalytic outcome. For example, energetic carriers produced upon photoexcitation of plasmonic systems can quickly dissipate energy through electron scattering processes on femtosecond timescales, followed by electron-phonon coupling on picosecond timescales. Once the carriers are thermalized, less energy is available to drive a charge separation or photocatalytic process. At the same time, there are number of handles with which to improve the efficiency of photoinduced processes, and ultrafast spectroscopy can be used to characterize that improvement. Nanostructuring, for example, can produce higher efficiency of photocatalytic processes as free charges are more likely to reach a surface following photoexcitation, and local anisotropies at the surface can drive electromagnetic field enhancements and the greater production of “hot” electrons. Hybridization of differing materials is also a powerful handle with which to manipulate or improve charge separation and photocatalytic outcomes. New materials can lead to greater robustness under illumination, as well as lead to changes in dissipation processes. In this talk, I describe recent efforts to characterize novel nanostructures of interest for plasmonics and energy conversion via ultrafast spectroscopy, as well as explore the ultrafast dissipation processes in refractory plasmonic nanomaterials. Particular attention is paid to dissipation of energy to the local environment of the nanoparticles as well as to ultrafast electron scattering processes. Hybrid nanostructures are also explored. Work performed at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility, was supported by the U.S. DOE, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

Novel biomimetic hybrid plasmonic nanocavity-based ECL strategy for the detection of extracellular vesicles

Tumor-derived extracellular vesicles detection has emerged as an important clinical liquid biopsy approach for cancer diagnosis. In this work, we developed a novel hybrid plasmonic nanocavity consisting of hexagonal Au nanoplates nanoarray, SnS2/Au nanosheet layer and biomimetic lipid bilayer. Firstly, the hybrid plasmonic nanocavity combined the optical confinement for the ECL regulation and the biological recognition for the detection of extracellular vesicles. Secondly, MXene-derived Ti2N QDs have been prepared as ECL nanoprobe to label extracellular vesicles. Moreover, biomimetic lipid bilayer with specific aptamer was used to identify extracellular vesicles and integrate Ti2N QDs into the nanocavity with membrane fusion strategy. Due to the significant electromagnetic field enhancement at the cavity region, the hybrid plasmonic nanocavity provided strong field confinement to concentrate and redistribute the ECL emission of QDs with a 9.3-fold enhancement. The hybrid plasmonic nanocavity-based ECL sensing system improved the spatial controllability of EVs analysis and the accurate resolution of specific protein. It achieved the sensitive detection of extracellular vesicles in ascites and successfully distinguished the peritoneal metastasis of gastric cancer.

Atomic-precision control of plasmon-induced single-molecule switching in a metal–semiconductor nanojunction

Atomic-scale control of photochemistry facilitates extreme miniaturisation of optoelectronic devices. Localised surface plasmons, which provide strong confinement and enhancement of electromagnetic fields at the nanoscale, secure a route to achieve sub-nanoscale reaction control. Such local plasmon-induced photochemistry has been realised only in metallic structures so far. Here we demonstrate controlled plasmon-induced single-molecule switching of peryleneanhydride on a silicon surface. Using a plasmon-resonant tip in low-temperature scanning tunnelling microscopy, we can selectively induce the dissociation of the O–Si bonds between the molecule and surface, resulting in reversible switching between two configurations within the nanojunction. The switching rate can be controlled by changing the tip height with 0.1-Å precision. Furthermore, the plasmon-induced reactivity can be modified by chemical substitution within the molecule, suggesting the importance of atomic-level design for plasmon-driven optoelectronic devices. Thus, metal–single-molecule–semiconductor junctions may serve as a prominent controllable platform beyond conventional nano-optoelectronics.

Ti3C2T x MXene as surface-enhanced Raman scattering substrate

The electromagnetic field enhancement mechanisms leading to surface-enhanced Raman scattering (SERS) of R6G molecules near Ti3C2T x MXene flakes of different shapes and sizes are analyzed theoretically in this paper. In COMSOL simulations for the enhancement factor (EF) of SERS, the dye molecule is modeled as a small sphere with polarizability spectrum based on experimental data. It is demonstrated, for the first time, that in the wavelength range of 500 nm – 1000 nm , the enhancement of Raman signals is largely conditioned by quadrupole surface plasmon (QSP) oscillations that induce a strong polarization of the MXene substrate. We show that the vis-NIR spectral range quadrupole SP resonances are strengthened due to interband transitions (IBTs), which provide EF values of the order of 105–107 in agreement with experimental data. The weak sensitivity of the EF to the shape and size of MXene nanoparticles (NPs) is interpreted as a consequence of the low dependence of the absorption cross-section of QSP oscillations and IBT on the geometry of the flakes. This reveals a new feature: the independence of EF on the geometry of MXene substrates, which allows to avoid the monitoring of the shape and size of flakes during their synthesis. Thus, MXene flakes can be advantageous for the easy manufacturing of universal substrates for SERS applications. The electromagnetic SERS enhancement is determined by the ‘lightning rod’ and ‘hot-spot’ effects due to the partial overlapping of the absorption spectrum of the R6G molecule with these MXene resonances.

In-situ growth of silver nanoparticles on 3D cellulose nanofibrous network for SERS sensing of pesticide residues on uneven surfaces

In this work, 3D nanofibrous network of bacterial nanocellulose (BNC) was employed as matrix template material in a composite system. The plasmonic silver nanoparticles (AgNPs) were in-situ grown onto the nanofibrous network of BNC to form AgNPs@BNC composite nanofibers. Flexible 3D SERS substrates of BNC nanofibrous network with various AgNPs loadings were manufactured by a simple and rapid vacuum-filtration route. Silver nanoparticles were uniformly and firmly embedded into nanocellulose supporting substrate to form 3D high-density SERS hot spots, resulting in a significant enhancement of electromagnetic field. In addition, the hydrophilic BNC demonstrates exceptional adsorption and permeation characteristics, allowing for the capture of target molecules in hotspot regions to amplify detection sensitivity. Consequently, the AgNPs decorated BNC SERS substrate achieves a notable detection sensitivity of 10−14 M, prominent signal homogeneity (RSD=7.3 %) and remarkable storage stability (over a month) for malachite green molecules. A lowest distinguishable level of 10−9 M for carbendazim molecules is also achieved. Moreover, carbendazim residues on uneven fruit surfaces can be directly and rapidly recognized by flexible AgNPs@BNC SERS sensor using a facile adhere-and-read method.

A novel three-dimensional porous Ag/TiO2 hybrid aerogels with high dense hot spot as effective SERS substrate for ultrasensitive detection

This research focuses on preparing a series of new TiO2/Ag hybrid aerogels with varying TiO2 contents, and demonstrates their application as ultrasensitive SERS substrates. The synthesized TiO2/Ag hybrid aerogels exhibited excellent SERS behavior when detecting 4-Mercaptobenzoic acid (4-MBA), and the calculated SERS enhancement factor (EF) was 6.34 × 106. 3D structured aerogels can create more hot spots and adsorption sites, and multiple interband chemical transfer (CT) pathways emerged and enhanced CT efficiency because of the large number of surface oxygen vacancies of meso-TiO2 NPs. Therefore, the synergy of electromagnetic field enhancement and chemical enhancement leads to SERS enhancement. In addition, the composite SERS substrate has high sensitivity, and the detection limit of adsorbed 4-MBA probe molecules reaches 10-11 M. Furthermore, the TiO2/Ag hybrid aerogels demonstrate good reproducibility with minimal standard deviation in terms of SERS signals. In addition, even after standing for 6 months, there is almost no attenuation in the SERS signal intensity, which highlights the excellent stability of this substrate. Therefore, these highly sensitive TiO2/Ag hybrid aerogels SERS substrates have important practical value in environmental monitoring, medical inspection and food supervision.

Broadband Quantum Efficiency Enhancement of Al0.3InAsSb p–i–n Photodiodes with All‐Dielectric Amorphous Germanium Metasurfaces

Transparent amorphous germanium (a‐Ge) has emerged as a promising material for engineering nanostructures and metasurfaces, offering significant potential for enhancing the performance of photonic devices in the short‐wavelength infrared (SWIR) spectrum. Herein, the successful application of a‐Ge metasurfaces with a truncated pyramid profile to enhance the external quantum efficiency (EQE) of a digital alloy Al0.3InAsSb p–i–n photodiode across a broad‐wavelength range in the SWIR is presented. The experimental findings demonstrate a broadband enhancement in EQE. Two metasurface samples are designed to emphasize different‐wavelength ranges. Notably, 51% improvement in EQE at 1550 nm and 125% enhancement at 2000 nm is achieved. Finite‐difference time domain simulations show that the observed EQE improvement originates from the reduction of reflection and electromagnetic field enhancement. This study underscores the promising role of a‐Ge metasurfaces in advancing the capabilities of SWIR photodetectors. It lays the groundwork for further exploration in optoelectronic device enhancements.

Shaping sub-10 nm metallic nanogaps by laser shock-induced extreme plasticity

The nanoscale metallic gaps generate the unique local electromagnetic field enhancement effect due to the strong electromagnetic coupling between closely spaced nanostructures. Besides the conventional material deposition or removal strategy, laser shock reshaping (LSR), based on the ultrahigh-strain-rate deformation of the nanometals, provides a viable process for ultrafine-gap fabrication. However, the extreme deformation behavior of nanometals during the high-speed forming process, which significantly determines the forming precision and quality, is largely unexplored. In this study, the scalable fabrication of the nanogaps was achieved using the LSR process; the extreme deformation behaviors of the nanometals were comprehensively investigated. The results indicated that the geometry of the primary structure and the LSR process will signally affect the microstructure evolution and deformation behavior of the sidewall during the LSR reshaping processes, and further affect the final geometrical morphology and optical properties of the generated nanogaps. The discovery of geometry-dependent deformation behaviors of the metallic structures is of great significance to the understanding of the mechanical deformation based nano-manufacturing processes.

Tunable quasi-bound states in the continuum in magneto-optical metasurfaces

The enhancement of electromagnetic field with high Q factor in metasurfaces has attracted extensive attention of researchers. Magneto-optical metasurfaces (MOMS) provide approaches with controllable magnetic fields to modulate the optical response, which contributes towards the magneto-optical Kerr effect in nanophotonic devices. However, it is challenging for MOMS to obtain narrow spectra peak with high Q factors. Here, we propose MOMS-supported quasi-bound states in the continuum (quasi-BIC) and theoretically investigate the evolution process from BIC to quasi-BIC. By applying an external magnetic field, the BIC in our proposed metasurface can be transformed into a quasi-BIC with finite Q factor up to 33 620. Meanwhile, the quasi-BIC shows magnetization-related circular dichroism with a vortex point in the polarization graph, based on time-reversal symmetry breaking in the magneto-optical material under the condition of external magnetic field. The quasi-BIC also maintains nearly unity reflection and almost zero absorption, showing a promising future in sensing. Our results enrich the light control mechanisms of MOMS and provide a unique opportunity for applications requiring flexible tunability and high Q factors, such as sensors, laser sources, filters and chiral-related elements.

Design and optimization of a long-range surface plasmon resonance-based plasmonic SERS biosensor

In this paper, we designed and studied a novel type of hybrid chip based on long-range surface plasmon resonance (LRSPR) and surface-enhanced Raman scattering (SERS). The substrate has periodic arrangement gold nanoring cavity arrays structure, which can obtain a uniform and stable electromagnetic field distribution. In addition, due to the clever design of the long-range configuration (a “sandwich” structure with symmetric refractive index), the chip has a significant extended electromagnetic field enhancement ability. In numerous existing studies focusing on SERS performance, the consideration of plasmonic structure is limited to its shape and size, while neglecting the significance of spacer layer parameters. Thus, we use the Finite Difference Time Domain (FDTD) software to design and optimize the chip substrate configuration by investigating the impact of various parameters such as refractive index, spacer layer thickness, plasmonic structure's inner and outer radius, and period of array on the long-range effect. The presented geometrically-tunable long-range plasmonic chip provides a way to identify the governing factors for SERS and presents a design principle for optimizing SERS substrates.

Silver-decorated three-dimensional ZnO nanoflowers enhancing the electromagnetic coupling for ultrasensitive SERS detection of multi-pesticide residues

—The development of sensitive analytical methods is of prime importance for the detection of pesticide residues on the surfaces of fruits and vegetables at extremely low concentrations. Here, we synthesized Ag-decorated three-dimensional ZnO nanoflowers (ZnO@Ag) composed of ultrathin nanosheets as surface-enhanced Raman spectroscopy (SERS) substrates. Combined with the finite-difference time-domain (FDTD) method, it is proven that the ultrathin nanosheets greatly improve the electromagnetic coupling between the two metal nanoparticles due to the high refractive index of ZnO and generate new “hot spots”. The nanosheets also endow the incident light with multiple reflections to motivate the local electromagnetic field. The substrate presents high sensitivity and the enhancement factor was as high as 2.98 × 107 utilizing R6G as probe molecules. The detection concentration of thiram and malachite green (MG) solutions can reach 10−10 M and 10−9 M, respectively. For the real-life sample assays, the substrate can simultaneously detect the two pesticides on apple skin with high sensitivity. The research also reveals the importance of semiconductors toward the local electromagnetic field enhancement in noble metal-semiconductor substrates, benefiting the realization of highly sensitive SERS substrates and the wide application of the SERS technology in real-world scenarios.

3D Superstructures Consisting of Intersecting Gold Lamellae Formed by a Micelle‐Mediated Anisotropic Growth Approach

3D superstructures (3DSs) have attracted increasing interest because of the collective synergistic effects of individual building units, but their customization relies on tedious multistep strategy or high‐end nanofabrication technology. Herein, for the first time, a facile block copolymer micelle‐mediated anisotropic growth approach is reported to fabricate gold 3DSs consisting of tunable and intersecting lamellae with sawtooth‐like edges. The preparation of the 3DSs depends on the mediation of reduction kinetics of gold precursors and adsorption of block copolymer micelles on gold crystal surfaces using disulfiram as ligands. The thickness of lamellae in the 3DSs is controllable from ≈21 to 102 nm by adjusting the weight fraction of the micellar hydrophobicity blocks and the composed lamellar number is regulated from ≈3 to ≈30. Additional morphologies, such as a dendritic mesoporous structure and meatball‐like shapes, are obtained through controlling the extent of micelle swelling. Finite‐difference time‐domain simulations demonstrate that the unique 3DSs of gold lamellae with sawtooth‐like edges form abundant hotspots giving rise to surface‐enhanced Raman scattering (SERS). The 3DSs exhibit strong electromagnetic field enhancement and excellent performance as SERS substrates for detecting 4‐mercaptobenzoic acid.

Femtosecond laser-induced nanoparticle implantation into flexible substrate for sensitive and reusable microfluidics SERS detection

Surface-enhanced Raman spectroscopy (SERS) microfluidic system, which enables rapid detection of chemical and biological analytes, offers an effective platform to monitor various food contaminants and disease diagnoses. The efficacy of SERS microfluidic systems is greatly dependent on the sensitivity and reusability of SERS detection substrates to ensure repeated use for prolonged periods. This study proposed a novel process of femtosecond laser nanoparticle array (NPA) implantation to achieve homogeneous forward transfer of gold NPA on a flexible polymer film and accurately integrated it within microfluidic chips for SERS detection. The implanted Au-NPA strips show a remarkable electromagnetic field enhancement with the factor of 9 × 108 during SERS detection of malachite green (MG) solution, achieving a detection limit lower than 10 ppt, far better than most laser-prepared SERS substrates. Furthermore, Au-NPA strips show excellent reusability after several physical and chemical cleaning, because of the robust embedment of laser-implanted NPA in flexible substrates. To demonstrate the performance of Au-NPA, a SERS microfluidic system is built to monitor the online oxidation reaction between MG/NaClO reactants, which helps infer the reaction path. The proposed method of nanoparticle implantation is more effective than the direct laser structuring technique. It provides better performance for SERS detection, robustness of detection, and substrate flexibility and has a wider range of applications for microfluidic systems without any negative impact.