Near-field Plasmon Research Articles

Direct Three-Dimensional Observation of the Plasmonic Near-Fields of a Nanoparticle with Circular Dichroism.

Characterizing the spatial distribution of the electromagnetic fields of a plasmonic nanoparticle is crucial for exploiting its strong light-matter interaction for optoelectronic and catalytic applications. However, observing the near-fields in three dimensions with a high spatial resolution is still challenging. To realize efficient three-dimensional (3D) nanoscale mapping of the plasmonic fields of nanoparticles with complex shapes, this work established autoencoder-embedded electron energy loss spectroscopy (EELS) tomography. A 432-symmetric chiral gold nanoparticle, a nanoparticle with a high optical dissymmetry factor, was analyzed to relate its geometrical features to its exotic optical properties. Our deep-learning-based feature extraction method discriminated plasmons with different energies in the EEL spectra of the nanoparticle in which signals from multiple plasmons were intermixed; this component was key for acceptable 3D visualization of each plasmonic field separately using EELS tomography. With this methodology, the electric field of the plasmon that induces far-field circular dichroism was observed in 3D. The field linked to this chiroptical property was strong along the swirling edges of the particle, as predicted by a numerical calculation. This study provides insight into the correlation between structural and optical chiralities through direct 3D observation of the plasmonic fields. Furthermore, the strategy of implementing an autoencoder for EELS tomography can be generalized to achieve competent 3D analysis of other features, including the optical properties of the dielectrics and chemical states.

Harnessing visible light for sustainable biodiesel production with Ni/Si/MgO photocatalyst

Sustainable energy sources frequently demonstrate greater reliability and resilience in comparison to conventional energy sources. Biodiesel, with its markedly reduced carbon footprint when compared to petroleum-based diesel fuel, owes this advantage to its production from renewable resources. Heterojunction photocatalysts have gained significant interest due to their immense promise in tackling environmental challenges. In this study, a highly efficient photocatalyst, Ni/Si/MgO, for biodiesel production under visible light irradiation was synthesized using a solid-phase reaction method with silica as the silicon source, along with Ni and MgO. The surface functionality of Ni/Si/MgO was crucial for achieving high efficiency of photocatalytic systems, as evident from XPS. The transesterification reaction on the Ni/Si/MgO photocatalyst proceeds by the formation of SiH and SiOH bonds over the catalyst. The photocatalytic activities of Ni/Si/MgO photocatalysts were higher than those of the Si/MgO nanoparticle when exposed to light. Achieving an optimal yield of 98 %, the biodiesel production was carried out under the following reaction conditions: A catalyst dosage of 2 % by weight was utilized, along with a methanol-to-oil molar ratio of 12:1, and the entire procedure was executed within a duration of 3.5 h. Plasmonic near-fields are speculated to be responsible for the increased transesterification activity along the Ni/Si/MgO interface. In order to carry out the transesterification reaction, electron-hole pairs are generated along the Ni/Si/MgO interface, where plasmonic near-fields are highly concentrated. This study contributes a significant perspective on mechanisms governing the process of efficient plasmonic photocatalysis responsive to visible light. These findings hold the potential to offer valuable guidance in the formulation and design of next-generation, high-performance photocatalysts.

Plasmonic absorption enhancement of MAPI-based perovskite solar cell with nanoparticles array

In this work, we propose a MAPI-based p-i-n perovskite solar cell with architecture ITO/PEDOT:PSS/MAPI/Rhodamine/TiO2/Ag. Although MAPI-based solar cells have prominent performance, there are some challenges for these devices. Poor optical absorption at near-infrared wavelengths is the most important challenge for MAPI-based solar cells. The effect of three kinds of periodic array nanoparticles on the absorption spectra of the perovskite layer has been investigated theoretically. The spherical bare Au nanoparticle, Au@SiO2, and Au@TiO2 core–shell nanorods have been used. Moreover, Rhodamine as a surface passivator to decrease the charge carrier recombination is used between perovskite and Electron Transport Layer (ETL). Our simulation is based on three dimensions Finite-Difference Time-Domain (FDTD) method and we use Lumerical software. The effect of these nanoparticles on electric field intensity, light absorption, short-circuit current density, and generation rate are investigated for the visible and near-infrared bands. We found that Au@TiO2 can improve light absorption more than Au and Au@SiO2 at the near-infrared band and this increment is due to the growth of plasmonic near-field of nanoparticles and light scattering enhancement from these nanoparticles.

Enhancement of plasmonic coupling on Si metallized with intense femtosecond laser pulses

Using a pump–probe technique, the reflectivity of a silicon grating surface irradiated with intense femtosecond (fs) laser pulses was measured as a function of the incidence angle and the delay time between pulses. After irradiating the surface with an intense s-polarized, 400 nm, 300 fs laser pulse, the reflectivity measured with a weak p-polarized, 800 nm, 100 fs laser pulse exhibited an abrupt decrease for an incidence angle of ~ 24°. The depth of the dip was greatest for a delay time of 0.6–10 ps, for which the reflectivity around the dip was highest. The surface was also found to be ablated most strongly for the conditions causing the deepest dip for a delay time of 5–10 ps. Surface plasmon polaritons (SPPs) on silicon metallized by the intense pulse are resonantly excited by the subsequent pulse, and the strong coherent coupling between the subsequent pulse and SPPs excited on the molten Si surface produced by high-density free electrons induces strong surface ablation due to the intense plasmonic near-field. The results clearly show that fs pulses can be used to significantly modulate the nature of nonmetallic materials and could possibly serve as a basic tool for the excitation of SPPs on nonmetallic materials using ultrafast laser–matter interactions.

Boosting biodiesel production over silicon heterojunction with visible light irradiation

Renewable energy sources offer greater reliability and resilience compared to traditional sources. Biodiesel, derived from renewable resources that absorb carbon dioxide during growth and production, boasts a significantly lower carbon footprint than petroleum-based diesel fuel. Heterojunction photocatalysts have emerged as a promising solution for environmental challenges. This study focused on efficient biodiesel production using visible light-irradiated Si/MgO heterojunctions. The XPS analysis confirmed the crucial role of surface functionality in achieving high photocatalytic efficiency. Transesterification occurs through SiH and SiOH bond formation on the catalyst. Finite-difference time-domain (FDTD) predicts the structure–activity relationship, showing stronger plasmonic nearfields in Si/MgO due to distinct dielectric constants. The Si/MgO photocatalyst exhibited superior photocatalytic activity under visible light, consistent with FDTD results. Biodiesel production was attained to 96% yield using 2 wt% catalysts, a 12:1 M ratio of methanol to Jatropha curcas oil, and a 3.5 hrs reaction time. Therefore, the work provided valuable insights into the mechanism of efficient plasmonic photocatalysis, paving the way for future advancements in novel high-performance photocatalysts.

Ultra-Broadband Solar Absorber and High-Efficiency Thermal Emitter from UV to Mid-Infrared Spectrum.

Solar energy is currently a very popular energy source because it is both clean and renewable. As a result, one of the main areas of research now is the investigation of solar absorbers with broad spectrum and high absorption efficiency. In this study, we create an absorber by superimposing three periodic Ti-Al2O3-Ti discs on a W-Ti-Al2O3 composite film structure. We evaluated the incident angle, structural components, and electromagnetic field distribution using the finite difference in time domain (FDTD) method in order to investigate the physical process by which the model achieves broadband absorption. We find that distinct wavelengths of tuned or resonant absorption may be produced by the Ti disk array and Al2O3 through near-field coupling, cavity-mode coupling, and plasmon resonance, all of which can effectively widen the absorption bandwidth. The findings indicate that the solar absorber's average absorption efficiency can range from 95.8% to 96% over the entire band range of 200 to 3100 nm, with the absorption bandwidth of 2811 nm (244-3055 nm) having the highest absorption rate. Additionally, the absorber only contains tungsten (W), titanium (Ti), and alumina (Al2O3), three materials with high melting points, which offers a strong assurance for the absorber's thermal stability. It also has a very high thermal radiation intensity, reaching a high radiation efficiency of 94.4% at 1000 K, and a weighted average absorption efficiency of 98.3% at AM1.5. Additionally, the incidence angle insensitivity of our suggested solar absorber is good (0-60°) and polarization independence is good (0-90°). These benefits enable a wide range of solar thermal photovoltaic applications for our absorber and offer numerous design options for the ideal absorber.

Plasmonic near-field spatiotemporal characterizations of an asymmetric copper bowtie nanostructure

Photon-induced near-field electron microscopy (PINEM), developed from ultrafast transmission electron microscopy, enables near-field imaging with nanometer spatial resolution and femtosecond temporal resolution. We report the plasmonic near-field distribution and lifetime analysis for an asymmetric copper bowtie nanostructure having unequal opening angles. The PINEM images show surface plasmon polaritons and local surface-plasmon resonance excitations with various polarizations. Combined with finite-element simulations, the polarization- and structure-dependent distribution and intensity variations of the near-field were analyzed. The lifetime difference of a plasmonic near-field excited by different polarizations is also discussed. The temporal and spatial characterization of the plasmonic near-field is important for the further studies of plasmonic near-field manipulation and designs of plasmonic devices having specific functions.

Mechanism for plasmon-generated solvated electrons

Solvated electrons are powerful reducing agents capable of driving some of the most energetically expensive reduction reactions. Their generation under mild and sustainable conditions remains challenging though. Using near-ultraviolet irradiation under low-intensity one-photon conditions coupled with electrochemical and optical detection, we show that the yield of solvated electrons in water is increased more than 10 times for nanoparticle-decorated electrodes compared to smooth silver electrodes. Based on the simulations of electric fields and hot carrier distributions, we determine that hot electrons generated by plasmons are injected into water to form solvated electrons. Both yield enhancement and hot carrier production spectrally follow the plasmonic near-field. The ability to enhance solvated electron yields in a controlled manner by tailoring nanoparticle plasmons opens up a promising strategy for exploiting solvated electrons in chemical reactions.

DNA Nanostructure-Guided Plasmon Coupling Architectures

DNA Nanostructure-Guided Plasmon Coupling Architectures

Plasmofluidic-Based Near-Field Optical Trapping of Dielectric Nano-Objects Using Gold Nanoislands Sensor Chips.

Near-field optical manipulation has been widely used for guiding and trapping nanoscale objects close to an optical-active interface. This near-field manipulation opens opportunities for next-generation biosensing with the capability of large-area trapping and in situ detection. In this article, we used the finite element method (FEM) to analyze the motion mechanism of nano-objects (50-500 nm) in the near-field optics, especially localized surface plasmon resonance (LSPR). The size-dependent optical forces and hydrodynamic forces of subwavelength nanoparticles (<500 nm) in different hydrodynamic velocity fields were calculated. When the strength of the local electric field was increased, LSPR with two-dimensional gold nanoislands (AuNIs) showed improved capability for manipulating nano-objects near the vicinity of the AuNI interface. Through the experiments of in situ interferometric testing 50-500 nm nano-objects with constant number concentration or volume fraction, it was confirmed that the local plasmonic near-field was able to trap the dielectric polystyrene beads smaller than 200 nm. The plasmofluidic system was further verified by testing biological nanovesicles such as exosomes (40-200 nm) and high- and low-density lipoproteins (10-200 nm). This concept of direct dielectric nano-objects manipulation enables large-scale parallel trapping and dynamic sensing of biological nanovesicles without the need of molecular binding tethers or labeling.

Fast fabrication and judgement of tip-enhanced Raman spectroscopy-active tips

The quality of the scanning tip is crucial for tip-enhanced Raman spectroscopy (TERS) experiments towards large signal enhancement and high spatial resolution. In this work, we report a controllable fabrication method to prepare TERS-active tips by modifying the tip apex at the atomic scale, and propose two important criteria to in-situ judge the tip's TERS activity for tip-enhanced Raman measurements. One criterion is based on the downshift of the first image potential state to monitor the coupling between the far-field incident laser and near-field plasmon; the other is based on the appearance of the low-wavenumber Raman peaks associated with an atomistic protrusion at the tip apex to judge the coupling efficiency of emissions from the near field to the far field. This work provides an effective method to quickly fabricate and judge TERS-active tips before real TERS experiments on target molecules and other materials, which is believed to be instrumental for the development of TERS and other tip-enhanced spectroscopic techniques.

Silver Nanopillar Arrayed Thin Films with Highly Surface-Enhanced Raman Scattering for Ultrasensitive Detection.

Surface-enhanced Raman scattering (SERS) technique based on surface plasmon resonance has been considerably investigated in recent years due to its superior sensitivity in the detection of organic or biological molecules at trace levels. However, most research usually focuses on artificial architectures as SERS substrates that always have a complex and expensive micro-nanofabrication process. The high cost of masks for SERS substrates becomes a key obstacle for the widespread commercialization of SERS technology. In this paper, a biomimetic SERS substrate composed of silver-coated nanopillar arrays on the top of a cicada wing was advanced to overcome these challenges as both substrates and masks. Benefiting from the high near-field plasmon resonance coupling at the limited space among neighboring nanopillars, a dramatically increased SERS signal can be achieved using rhodamine 6G (R6G) as a model molecule. Encouragingly, the analytical enhancement factor of the order of more than 108 has been conveniently realized with a reliable detection concentration of R6G of about 100 pM or less. This work provides a promising route for designing cost-effective and highly sensitive SERS substrates and the related mask fabrication using our previously proposed template transfer nanoimprint.

Super-resolution Imaging of Plasmonic Near-Fields: Overcoming Emitter Mislocalizations.

Plasmonic nano-objects have shown great potential in enhancing applications like biological/chemical sensing, light harvesting and energy transfer, and optical/quantum computing. Therefore, an extensive effort has been vested in optimizing plasmonic systems and exploiting their field enhancement properties. Super-resolution imaging with quantum dots (QDs) is a promising method to probe plasmonic near-fields but is hindered by the distortion of the QD radiation pattern. Here, we investigate the interaction between QDs and “L-shaped” gold nanoantennas and demonstrate both theoretically and experimentally that this strong interaction can induce polarization-dependent modifications to the apparent QD emission intensity, polarization, and localization. Based on FDTD simulations and polarization-modulated single-molecule microscopy, we show that the displacement of the emitter’s localization is due to the position-dependent interference between the emitter and the induced dipole, and can be up to 100 nm. Our results help pave a pathway for higher precision plasmonic near-field mapping and its underlying applications.

Carbon Nanotubes for the Optical Far-Field Readout of Processes That Are Mediated by Plasmonic Near-Fields

As science progresses at the nanoscopic level, it becomes more and more important to comprehend the interactions taking place at the nanoscale, where optical near-fields play a key role. Their phenomenology differs significantly from the propagative light we experience at the macroscopic level. This is particularly important in applications such as surface-enhanced spectroscopies for single-molecule detection, where often the optimization of the plasmonic structures and surfaces relies on far-field characterizations. The processes dominating in the far-field picture, though, are not the same dominating in the near-field. To highlight this, we resort to very simple metallic systems: Isolated gold nanorods in solution. We show how single-walled nanotubes can be exploited to read out processes occurring at the near-field level around metallic nanoparticles and make the information accessible in the far-field region. This is implemented by monitoring the spectral profile of the enhancement of the photoluminescence and Raman signal of the nanotubes for several excitation wavelengths. Through this excitation-resolved study, we show that the far-field optical read-out detects the transversal and longitudinal dipolar plasmonic oscillations of gold nanorods, whereas the near-field read-out through the nanotubes reveals other mechanisms to dominate. The spectral position of the maximum enhancement of the optical near-field mediated signals are located elsewhere than the far-field bands. This dichotomy between near-field and far-field response should be taken into account when optimizing plasmonic nanostructures for applications such as surface-enhanced spectroscopies.

Material Nanoprocessing with Femtosecond-Laser-Induced Plasmonic Near-Fields

Material Nanoprocessing with Femtosecond-Laser-Induced Plasmonic Near-Fields

The fine spectral tuning of plasmon mode and quantum emitter coupling and itscomparison with the dark-hot resonances

Localization of incident fields into very small volumes (hot-spots) allows strong light-matter interactions at the hot spots. This makes path interference effects, Fano resonances, visible. Fano resonances can appear due to the coupling of a bright plasmon (i) to a longer lifetime dark plasmon mode or (ii) to a quantum object (QO) of a much longer lifetime. The second phenomenon provides a very important utility: the voltage tunability of the linear/nonlinear response. The level-spacing of the QO, such as defect-centers and quantum dots, are voltage-tunable which can make a sharp Fano resonance appear and disappear. Here, we compare the two phenomena by solving the equations of motions for the near-field plasmon amplitudes, derived from two different Hamiltonians. While the two plasmon amplitudes look similar to each other, except for the population inversion parameter `$y$', our results show that quantum emitter enables fine spectral tuning of the plasmon amplitude, thus, providing better enhancement.

Nanoscale Visualization of a Photoinduced Plasmonic Near-Field in a Single Nanowire by Free Electrons.

Swift electrons can undergo inelastic interactions not only with electrons but also with near-fields, which may result in an energy loss or gain. Developments in photon-induced near-field electron microscopy (PINEM) enable direct imaging of the plasmon near-field distribution with nanometer resolution. Here, we report an analysis of the surface plasmonic near-field structure based on PINEM observations of silver nanowires. Single-photon order-selected electron images revealed the wavelike and banded structure of electric equipotential regions for a confined near-field integral associated with typical absorption of photon quanta (nℏω). Multimodal plasmon oscillations and second-harmonic generation were simultaneously observed, and the polarization dependence of plasmon wavelength and symmetry properties were analyzed. Based on advanced imaging techniques, our work has implications for future studies of the localized-field structures at interfaces and visualization of novel phenomena in nanostructures, nanosensors, and plasmonic devices.

Strong Magnetic Plasmon Resonance in a Simple Metasurface for High-Quality Sensing

We present a simple metasurface consisting of a periodic array of two closely spaced Ag nanowires standing on Ag substrate, to achieve a very strong magnetic plasmon (MP) resonance. We show that the strong MP resonance in the simple metasurface is essentially an antisymmetric resonance caused by the near-field plasmon hybridization in the pair of metallic nanowires. The magnetic field intensity and the spectral linewidth of the MP resonance can be effectively tuned by changing the height of nanowires and the spacing between the two closely spaced nanowires. More importantly, owing to the huge magnetic field enhancement, the ultra-narrow linewidth and the nearly-zero reflection at the strong MP resonance, the metasurface has very high sensitivity (S = 750 nm/RIU, S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">*</sub> = 45/RIU) and figure of merit (FOM = 94, FOM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">*</sub> = 750000). The results suggest that the proposed simple metasurface may find great potential applications in label-free biosensing.

Angle-independent plasmonic substrates for multi-mode vibrational strong coupling with molecular thin films

Vibrational strong coupling of molecules to optical cavities based on plasmonic resonances has been explored recently because plasmonic near-fields can provide strong coupling in sub-diffraction limited volumes. Such field localization maximizes coupling strength, which is crucial for modifying the vibrational response of molecules and, thereby, manipulating chemical reactions. Here, we demonstrate an angle-independent plasmonic nanodisk substrate that overcomes limitations of traditional Fabry-Pérot optical cavities because the design can strongly couple with all molecules on the surface of the substrate regardless of molecular orientation. We demonstrate that the plasmonic substrate provides strong coupling with the C=O vibrational stretch of deposited films of PMMA. We also show that the large linewidths of the plasmon resonance allow for simultaneous strong coupling to two, orthogonal water symmetric and asymmetric vibrational modes in a thin film of copper sulfate monohydrate deposited on the substrate surface. A three-coupled-oscillator model is developed to analyze the coupling strength of the plasmon resonance with these two water modes. With precise control over the nanodisk diameter, the plasmon resonance is tuned systematically through the modes, with the Rabi splitting from both modes varying as a function of the plasmon frequency and with strong coupling to both modes achieved simultaneously for a range of diameters. This work may aid further studies into manipulation of the ground-state chemical landscape of molecules by perturbing multiple vibrational modes simultaneously and increasing the coupling strength in sub-diffraction limited volumes.

2-D Study of Near-field Generated by Surface Plasmon Resonance of Short Axis of Gold Nano-rods on SiO2 and CeO2/SiO2 Substrates for Optimisation of Water Splitting Reaction

Distinguishing near-field intensity distribution and plasmon resonance peak wavelength of surface plasmon resonance of gold nano-rods (AuNRs) can provide information for the optimisation of localised surface plasmon and gap plasmon resonances of gold nano-rods. This work, shows the influence of how refractive indexes of the surrounding medium and adjacent surface, as well as, AuNRs pair inter-particle gap contributes to the generation of surface plasmon resonances. The simulation model presented consists of AuNRs pair situated on silicon dioxide (SiO2) and cerium dioxide/silicon dioxide (CeO2/SiO2) substrates with air and water as the surrounding media. The results show high near-field intensities at AuNRs/CeO2 interfaces with CeO2 refractive index (n = 2.38), and the near-field intensities contributed by the gap resonance is minimal between the AuNRs of inter-particle gap of 10 nm, however, the near-field intensities become significant near to the AuNRs/CeO2 interfaces because of reduced near-field interference. The simulation set-up provides the conditions for water splitting in thermochemical redox reaction of CeO2/CeO2-x resulting in the production of hydrogen. AuNRs pair with inter-particle gap of 5 nm situated on 10 nm thick CeO2 of CeO2/SiO2 substrate shows the most favorable conditions for water splitting.