Field Enhancement Research Articles

Unveiling the Influence of Line Shape of Surface Plasmon Resonances on Exciton–Plasmon Strong Coupling

AbstractStrong coupling between surface plasmon resonance (SPR) modes and excitons holds great potential for many chemical and physical applications. However, a clear understanding on how far‐field spectral characteristics of SPR (linewidths and intensity) affect the coupling strength remains unclear because these parameters are difficult to be individually tuned. These spectral characteristics are associated with the damping rate and field enhancement that depend on the morphology of the plasmonic nanostructure. In this work, the influence of the SPR line shape on exciton‐plasmon coupling is systematically investigated by delicately tuning the linewidths and resonance intensities of the plasmonic arrays. It is found that the SPR modes with narrow linewidths or large intensity are favorable for the exciton–plasmon system to achieve strong coupling. The obtained clear relationships between the SPR line shape and coupling strength provide guidance for the design and optimization of exciton‐plasmon coupling systems, favoring the highly efficient manipulation of exciton polaritons and enabling specific applications driven by strong coupling.

An Easy-to-Integrate Embedded Cascade Microspheres for Efficient Local Field Enhancement

Surface-enhanced Raman spectroscopy (SERS) is a non-destructive and ultra-sensitive tool for optical trace detection. However, the complex manufacturing process hinders the application of SERS. For this reason, an embedded cascaded microsphere structure is proposed in this paper, which is easy to integrate and process. Such a structure is composed of large microspheres and small microspheres with micropores cascaded. More importantly, the micropore diameter at the bottom of the large microspheres is the same as that of the small microspheres. Through the finite element analysis, it is concluded that compared with a single microsphere, the photonic nanojet generated by this structure is stronger, which can achieve local electric field enhancement more effectively. Meanwhile, different materials of the cascade microspheres used in the embedded cascade have various reinforcement effects. In addition, compared with the manual physical alignment of two microspheres in the traditional cascade structure, this structure is easier to process and integrate, convenient for future integrated applications.

Plasmon dephasing time and optical field enhancement in a plasmonic nanobowl substrate studied by scanning near-field optical microscopy.

Plasmonic substrates have been extensively investigated due to their potential applications in fluorescence microscopy, chemical sensing, and photochemical reactions. The optical properties of the substrate depend on the spatial and temporal features of the plasmon excited. Hence, the ability to directly visualize plasmon dynamics is crucial. In this study, we investigated the spatial and temporal properties of plasmon excitation in a plasmonic nanobowl substrate consisting of a periodic hexagonal array of nanoscale bowl-like structures developed with self-assembly. Near-field transmission imaging revealed that multiple plasmon resonance bands are observed from visible to near-infrared spectral region, and the optical contrast of the image is dependent on the observed band. Near-field two-photon photoluminescence microscopy revealed that the probability of excitation inside each nanoscale bowl-like structure is greater than that in the surrounding area. Near-field time-resolved imaging revealed that the nanobowl substrate exhibited a substantially long plasmon dephasing time, exceeding 12fs. Based on the spectral features of the near-field and far-field spectra, we found that optically dark plasmon mode is excited by the near-field illumination and only partly contributes to the long dephasing time observed. This fact indicates that the dephasing time is extended by some other mechanism in the periodic substrate. We revealed from this study that the enhanced optical fields induced in the nanobowl structure originate from the photosynergetic effect of the cavity mode and plasmon mode excited.

Machine learning-assisted label-free colorectal cancer diagnosis using plasmonic needle-endoscopy system

Early and accurate detection of colorectal cancer (CRC) is critical for improving patient outcomes. Existing diagnostic techniques are often invasive and carry risks of complications. Herein, we introduce a plasmonic gold nanopolyhedron (AuNH)-coated needle-based surface-enhanced Raman scattering (SERS) sensor, integrated with endoscopy, for direct mucus sampling and label-free detection of CRC. The thin and flexible stainless-steel needle is coated with polymerized dopamine, which serves as an adhesive layer and simultaneously initiates the nucleation of gold nanoparticle (AuNP) seeds on the needle surface. The AuNP seeds are further grown through a surface-directed reduction using Au ions-hydroxylamine hydrochloride solution, resulting in the formation of dense AuNHs. The formation mechanism of AuNHs and the layered structure of the plasmonic needle-based SERS (PNS) sensor are thoroughly analyzed. Furthermore, a strong field enhancement of the PNS sensor is observed, amplified around the edges of the polyhedral shapes and at nanogap sites between AuNHs. The feasibility of the PNS sensor combined with endoscopy system is further investigated using mouse models for direct colonic mucus sampling and verifying noninvasive label-free classification of CRC from normal controls. A logistic regression-based machine learning method is employed and successfully differentiates CRC and normal mice, achieving 100% sensitivity, 93.33% specificity, and 96.67% accuracy. Moreover, Raman profiling of metabolites and their correlations with Raman signals of mucus samples are analyzed using the Pearson correlation coefficient, offering insights for identifying potential cancer biomarkers. The developed PNS-assisted endoscopy technology is expected to advance the early screening and diagnosis approach of CRC in the future.

Effect of Zr, Sm and Gd Doped CoFe2O4 on Structural, Spectral and Magnetic Properties

In this work, cobalt composite nano ferrites with chemical formula CoZr0.05-xRExFe1.95O4 (x = 0 and 0.025 and RE = Sm, Gd) were prepared using sol-gel synthesis method and studied structural, optical and magnetic properties of them. The Rietveld assessment of XRD data validated the emergence of a single-phase cubic spinel configuration for all compounds. SEM has been used to study the surface morphology of the compounds. Energy gap has been estimated and the values are 2 eV, 1.8 eV and 1.6 eV for CoZr0.05Fe2O4, CoZr0.025Sm0.025 Fe1.95O4, and CoZr0.025Gd0.025 Fe1.95O4 compounds respectively. FTIR and Raman spectra confirmed the structure with the appearance of standard modes. The RE replacement in CoFe2O4 ferrites has an intense impact on magnetic properties. With the substitution of Sm3+ and Gd3+ ions in cobalt-zirconium ferrite, there is a reduction in saturation magnetization (55 emu g−1 of CoZr0.05Fe2O4 reduced to 25 emu g−1 for Gd3+ and to 29 emu gm−1 for Sm3+) as the size of crystallites decreases. A decrease in crystallite size correlates with an increase in the number of spin disorder occurrences in rare Earth-substituted cobalt-zirconium ferrite nanoparticles Further, the decrease in coercivity (2440 Oe of CoZr0.05Fe2O4 reduced to 720 Oe for Gd3+ and to 29 Oe for Sm3+ with rare Earth element doping is due to the crystallite size and large lattice distortion showed an enhancement of magnetocrystalline anisotropy field which effects a shift of resonance frequencies from higher frequencies which is good for the memory storage devices.

An Infrared Nanospectroscopy Technique for the Study of Electric-Field-Induced Molecular Dynamics.

Static electric fields play a considerable role in a variety of molecular nanosystems as diverse as single-molecule junctions, molecules supporting electrostatic catalysis, and biological cell membranes incorporating proteins. External electric fields can be applied to nanoscale samples with a conductive atomic force microscopy (AFM) probe in contact mode, but typically, no structural information is retrieved. Here we combine photothermal expansion infrared (IR) nanospectroscopy with electrostatic AFM probes to measure nanometric volumes where the IR field enhancement and the static electric field overlap spatially. We leverage the vibrational Stark effect in the polymer poly(methyl methacrylate) for calibrating the local electric field strength. In the relevant case of membrane protein bacteriorhodopsin, we observe electric-field-induced changes of the protein backbone conformation and residue protonation state. The proposed technique also has the potential to measure DC currents and IR spectra simultaneously, insofar enabling the monitoring of the possible interplay between charge transport and other effects.

Modeling the Magnetic Vectors of Interplanetary Coronal Mass Ejections at Different Heliocentric Distances with INFROS

The INterplanetary Flux ROpe Simulator (INFROS) is an observationally constrained analytical model dedicated to forecasting the strength of the southward component (Bz) of the magnetic field embedded in interplanetary coronal mass ejections (ICMEs). In this work, we validate the model for six ICMEs sequentially observed by two radially aligned spacecraft positioned at different heliocentric distances. The six selected ICMEs in this study comprise cases associated with isolated coronal mass ejection (CME) evolution as well as those interacting with high-speed streams (HSSs) and high-density streams (HDSs). For the isolated CMEs, our results show that the model outputs at both spacecraft are in good agreement with in situ observations. However, for most of the interacting events, the model correctly captures the CME evolution only at the inner spacecraft. Due to the interaction with HSSs and HDSs, which in most cases occurred at heliocentric distances beyond the inner spacecraft, the ICME evolution no longer remains self-similar. Consequently, the model underestimates the field strength at the outer spacecraft. Our findings indicate that constraining the INFROS model with inner-spacecraft observations significantly enhances the prediction accuracy at the outer spacecraft for the three events undergoing self-similar expansion, achieving a 90% correlation between observed and predicted Bz profiles. This work also presents a quantitative estimation of the ICME magnetic field enhancement due to interaction which may lead to severe space weather. We conclude that the assumption of self-similar expansion provides a lower limit to the magnetic field strength estimated at any heliocentric distance, based on the remote-sensing observations.

Photon-assisted tunnel rectenna for solar energy harvesting

In this work, we introduce a solar rectenna that is combined with a periodic subwavelength spiral antenna and a metal-oxide-semiconductor photon-assisted tunneling diode. We obtain spectral absorption and electromagnetic field distributions by numerical simulations and calculate the tunneling current densities generated by solar rectenna. The results indicate that broadband absorption with a peak value over 0.99 is achieved in the range of 400-2500 nm. Additionally, the figure of merit for tunneling electric field enhancement reaches over 104. Under AM1.5 solar irradiation, the calculated zero-bias tunneling current densities are up to 13.93 mA/cm2. This study is expected to advance the development of ultra-high frequency rectennas and pave the way for efficient solar energy harvesting.

Application of Gap Mode Ultrasensitive P-GERTs in SERS-Based Rapid Detection

In surface-enhanced Raman scattering (SERS) detection research, the shape, structure, surface modification, and material selection of nanoparticles can significantly impact the SERS intensity. Petal-like gap-enhanced Raman tags (P-GERTs) possess numerous sharp tips and edges, which generate localized electric field enhancements, further amplifying the electric field enhancement effect on neighboring molecules and enhancing the SERS signal. Additionally, the surface of P-GERTs can be modified with functional molecules, enabling their application in the detection of disease biomarkers. Using COVID-19 as an example, the performance of P-GERTs in disease biomarker detection was validated, demonstrating that the signal intensity of this probe can reach 55 times that of regular gold nanoparticles and 36.7 times that of smooth shell gap-enhanced Raman tags (S-GERTs). Furthermore, in combination with magnetically retrievable magnetic bead substrates, the N-protein antigen was specifically detected in a one-step process. N-protein was detected within 15 min using a portable Raman spectrometer. The limit of detection (LOD) for the standard sample was 4.28 pg/mL, and the LOD for the actual throat swab sample system was 25.4 pg/mL. This workflow can be extended to the detection of other biomarkers, making it widely applicable.

Highly sensitive near-field leakage-enhanced optical fiber SPR sensor based on gold nanoarrays modulation

A novel near-field leakage-enhanced optical fiber surface plasmon resonance (NLF-SPR) sensor based on the synergistic sensitization of cuboid gold nanoarrays and WSe2 film is proposed for the first time to our knowledge. The sensor is composed of a side-polished multimode fiber, a continuous gold film, a continuous tungsten selenide (WSe2) film, and well-arranged cuboid gold nanoarrays. The sensor leverages the high carrier mobility characteristic of the WSe2 film, the near-field leakage enhancement arising from the nanoarrays structure, and the localized electric field enhancement property introduced by nanotips to enhance the refractive index sensitivity. In order to optimize the spectral characteristic, the modulation of electric field distribution and spectral characteristic by the structural parameters of gold nanoarrays is investigated through three-dimensional (3D) finite element simulation. Results show that in the refractive index range of 1.3332–1.3432 refractive index unit (RIU), the average sensitivity of the NLF-SPR sensor can reach up to 10108 nm/RIU, and the highest sensitivity can reach up to 14692.46 nm/RIU. The average sensitivity and the highest sensitivity are 1.87 times and 2.56 times higher, respectively, than those of the side-polished fiber SPR sensor with a monolayer gold film. Furthermore, the experimental results demonstrate that the near-field leakage enhancement introduced by gold nanoarrays improves the performance of the fiber SPR sensor, resulting in a 49.3 % increment in sensitivity. The experimental results are in agreement with the trend of the simulation results. Even better performance improvements have been achieved compared to existing methods and a more flexible approach to performance tuning is provided. With the maturity and development of various nanoarray processing techniques such as focused ion beam, electron beam lithography, and nanoimprint lithography, the proposed NLF-SPR sensor has the potential to be fabricated on a large scale and is expected to be widely employed in biomedical, clinical applications, and other fields.

Au Nanoparticles@Si Nanowire Oligomer Arrays for SERS: Dimers Are Best.

We report the synthesis of vertically aligned silicon nanowire (VA-SiNW) oligomer arrays coated with Au nanoparticle (NP) monolayers via a combination of colloidal lithography, metal-assisted chemical etching, and directed NP assembly. Arrays of SiNW monomers (i.e., isolated NWs), dimers, and tetramers are synthesized, decorated with AuNPs, and tested for their performance in surface-enhanced Raman spectroscopy. The ∼20 nm AuNPs easily enter within the ca. 40 nm gaps of the SiNW oligomers, thus reaching the hot spot region. At 785 nm excitation, the AuNPs@SiNW dimer arrays provide the highest Raman signal, in agreement with electromagnetic simulations showing a high electric field enhancement at the Au/Si interface within the dimer gap region.

Unraveling the Dominant Size Effect in Polydisperse Solutions and Maximal Electric Field Enhancement of Gold Nanoparticles

Unraveling the Dominant Size Effect in Polydisperse Solutions and Maximal Electric Field Enhancement of Gold Nanoparticles

A fully-printed plasmonic nanoparticle-incorporated ZnO-based UV photodetector with very high responsivity and fast response

We report the development of a fully printed plasmonic Ag nanoparticle-enhanced ZnO-nanoparticle-based photodetector (PD) for the efficient detection of ultraviolet (UV) light. The contact electrodes with a gap of 200 µm are printed on a SiO2/Si substrate, and a micropattern of Ag nanoparticles (Ag NPs) is printed within the electrode gap to generate the plasmonic effect. The ZnO nanoparticle thin film is printed onto the array of Ag NPs to fabricate the plasmon-enhanced UV PD. The printed devices exhibit impressive performance with a peak responsivity of 48.8 A W−1, external quantum efficiency of 1.7 × 104%, and detectivity of 1.3 × 1013 Jones at 5 V bias. Moreover, the device shows an ultrafast photoresponse with a rise time of 24.3 µs and a fall time of 33.1 µs. Finite element method-based simulations confirm a significant field enhancement within the ZnO matrix upon incorporation of plasmonic Ag nanoparticles, explaining the increased photoresponse. The performance of the printed plasmon-enhanced UV-PD here offers a promising, simple, and inexpensive approach for the fabrication of future optoelectronic devices.

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.

Magnéli Ti4O7 nanowire induces locally enhanced electric field and electrocatalytic activity for rapid and efficient water disinfection via synergistic electroporation and electrochlorination

Waterborne pathogens in drinking water remains a prominent cause of morbidity and mortality in developing rural areas. Electrochlorination offers a promising alternative for decentralized water disinfection, but its efficiency was strongly restrained by the low Cl- concentrations in natural waters. Herein, we proposed a Magnéli Ti4O7 nanowire-assisted electrochemical approach via forming locally enhanced electric field over nanowire tips for reinforcing electroporation, and via forming locally enhanced charge density for promoting Cl- enrichment and subsequent electrochlorination on anodic nanowire tips. Ti4O7 nanowires induced the enhancement of electric field by 4 orders of magnitude and Cl- concentration by ∼3 orders of magnitude at nanowire tips, and promoted ∼3 times more active chlorine generation at a voltage of 3.0 V compared with non-nanostructured one. Synergistic electroporation and electrochlorination on nanowires enabled rapid and efficient disinfection with hydraulic retention time of ∼36 s and energy consumption below ∼5 J/L/log for inactivation of various gram-positive/negative bacteria and phage MS2. Analyses of cell morphology and membrane integrity revealed that electroporation-induced cell pores provided diffusion channels of reactive chlorine species for oxidative damage of cell structures, resulting in ∼1.5–4.5 times lower energy consumption than the thin-film, nanoparticle, and nanorod ones. The excellent operation stability for tap water with significant bacterial inactivation and active chlorine generation demonstrated its great application potential for decentralized water disinfection.

Optical axis-driven field enhancement in a hyperbolic medium.

Field enhancement of applied electric field is the foundation for the variety of applied domains at a nanoscale level. Traditionally, efforts to achieve field enhancement have required the use of complicated metamaterial-based structures with a transition behavior. Here, the electromagnetic field solution of the TM-polarized wave that interacts with an optical-axis-driven hyperbolic medium with a transition behavior is established. Detailed calculations reveal that such field enhancement can be achieved over a broad range of incident angles (i.e., near critical angle). Definitely, such flexibility of the incident angle for achieving the field enhancement enriches the understanding and provides novel prospective toward its practical realization.

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.

Modeling generation of harmonics in the water window region in hollow core waveguides by mid-infrared femtosecond pulses

We numerically investigate generation of harmonics in the water window region (down to 2.8 nm) by 2 μm femtosecond pulses propagating in hollow core waveguides filled with high pressure He. Numerical calculations are based on a three dimensional macroscopic model, which solves the pulse propagation by a split-step method, uses the strong field approximation to evaluate the single atom response, and integrates it coherently to obtain the harmonic field. Two configurations for the waveguides are considered: the standard one with a constant diameter of 70 μm and a conical one with a decreasing diameter from 70 to 50 μm. We demonstrate that harmonic field enhancement can be obtained in spectral domains of great practical interest, from 2.8 to 20 nm, and identify quasi-phase matching induced by multimode beating as the mechanism responsible for this enhancement.

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.

Multi-band enhanced nonreciprocal thermal radiation based on Weyl semimetals.

Previous studies manifested that the majority of structures that exhibit nonreciprocal thermal radiation in the mid-infrared are capable of achieving either single-band strong nonreciprocity or multi-band weak nonreciprocity at a large incidence angle. However, few structures can realize multi-band strong nonreciprocity at a small incidence angle. To address such scientific issues, we propose a tunable nonreciprocal thermal emitter based on gallium arsenide (GaAs)/graphene/Weyl semimetal (WSM). This device is capable of achieving strong nonreciprocity at 7.3 μm, 10 μm and 13.6 μm wavelengths at an incidence angle of 25.5°. It is shown that the field enhancement of the GaAs/graphene composite layer can improve the nonreciprocal response of the WSM layer. In addition, by changing the Fermi energy level of graphene and the axial vector b of the Weyl semimetal, tunable nonreciprocal thermal radiation can be realized. What's more, we find that the structure breaks Kirchhoff's law without lithography and an external magnetic field, which reveals the advantages of applying our research in the field of thermal radiation.