Plasmon Coupling Research Articles

Propagation Characteristics of Electrostatic Surface Plasma Waves at the Spin‐Polarized Quantum Plasma–Vacuum Interface

ABSTRACTThis investigation explores the characteristics of electrostatic surface plasma waves within the framework of a spin‐polarized quantum plasma. Utilizing the spin‐polarized quantum hydrodynamic model and incorporating essential elements like Fermi pressure and Bohm potential, we derive the dispersion relation governing surface plasma waves at a plasma–vacuum interface. Through Fourier decomposition of the hydrodynamic model, we establish the dispersion relation that outlines the behavior of surface plasmons under conditions of small amplitude. Quantum effects, encompassing degenerate pressure, and Bohm potential are considered with specific attention given to the spin polarization effect, treating spin up, and spin down electrons as distinct species. The resulting dispersion relation demonstrates that, regardless of the degree of spin matching, Bohm potential significantly alters the phase speed in the limit of a large wave vector. Increasing spin mismatch in the quantum plasma leads to a decrease in the phase speed of the surface mode for a fixed value of the plasmonic coupling parameter . Our findings bear relevance to graphene‐based plasmonic systems, aligning with some of the observations reported in Gao et al. (2013) and Guo et al. (2019).

Plasmon scattering at a junction in double-layer two-dimensional electron gas plasmonic waveguide

Abstract The scattering of plasmons at a junction within a double-layer two-dimensional electron gas plasmonic waveguide is studied via a full electromagnetic method. The dispersion relation is derived by utilizing the transfer matrix method and can be extended to the situation of an arbitrary number of layers. By numerically solving the dispersion equations, both the acoustic and optical plasmon modes are identified in this double layer system, and the unstable plasmon modes arising from plasmon coupling in different layers are discussed elaborately. Subsequently, the total fields are expanded with eigenmodes and matched at the interface to analyze the scattering characteristics at the junction. The results indicate that the total power of the plasmon mode is amplified when the electron fluid flows from a high concentration region to a low concentration region, and the amplification is more evident at a higher drift velocity. Additionally, we address the scattering of unstable plasmons caused by the two-stream instability and find that the transmitted plasmons are excited intensively at the incidence of the growing plasmon, leading to the plasmon amplification. The detailed examination of plasmon scattering at junction is the prerequisite for studying more complex structures of terahertz plasmonic devices and comprehending the corresponding amplification mechanism.

Enantioselective Molecular Detection by Surface Enhanced Raman Scattering at Chiral Gold Helicoids on Grating Surfaces.

Distinct advantages of surface enhanced Raman scattering (SERS) in molecular detection can benefit the enantioselective discrimination of specific molecular configurations. However, many of the recent methods still lack versatility and require customized anchors to chemically interact with the studied analyte. In this work, we propose the utilization of helicoid-shaped chiral gold nanoparticles arranged in an ordered array on a gold grating surface for enantioselective SERS recognition. This arrangement ensured a homogeneous distribution of chiral plasmonic hot spots and facilitated the enhancement of the SERS response of targeted analytes through plasmon coupling between gold helicoid multimers (formed in the grating valleys) and adjacent regions of the gold grating. Naproxen enantiomers (R(+) and S(-)) were employed as model compounds, revealing a clear dependence of their SERS response on the chirality of the gold helicoids. Additionally, propranolol and penicillamine enantiomers were used to validate the universality of the proposed approach. Finally, numerical simulations were conducted to elucidate the roles of intensified local electric field and optical helicity density on the SERS signal intensity and on the chirality of the nanoparticles and enantiomers. Unlike previously reported methods, our approach relies on the excitation of a chiral plasmonic near-field and its interaction with the chiral environment of analyte molecules, obviating the need for the enantioselective entrapment of targeted molecules. Moreover, our method is not limited to specific analyte classes and can be applied to a broad range of chiral molecules.

Enhanced Sensitivity and Homogeneity of SERS Signals on Plasmonic Substrate When Coupled to Paper Spray Ionization–Mass Spectrometry

This work reports on the development of an analyte sampling strategy on a plasmonic substrate to amplify the detection capability of a dual analytical system, paper spray ionization–mass spectrometry (PSI-MS) and surface-enhanced Raman spectroscopy (SERS). While simply applying only an analyte solution to the plasmonic paper results in a limited degree of SERS enhancement, the introduction of plasmonic gold nanoparticles (AuNPs) greatly improves the SERS signals without sacrificing PSI-MS sensitivity. It is initially revealed that the concentration of AuNPs and the type of analytes highly influence the SERS signals and their variations due to the “coffee ring effect” flow mechanism induced during sampling and the degree of the interfacial interactions (e.g., van der Waals, electrostatic, covalent) between the plasmonic substrate and analyte. Subsequent PSI treatment at high voltage conditions further impacts the overall SERS responses, where the signal sensitivity and homogeneity significantly increase throughout the entire substrate, suggesting the ready migration of adsorbed analytes regardless of their interfacial attractive forces. The PSI-induced notable SERS enhancements are presumably associated with creating unique conditions for local aggregation of the AuNPs to induce effective plasmonic couplings and hot spots (i.e., electromagnetic effect) and for repositioning analytes in close proximity to a plasmonic surface to increase polarizability (i.e., chemical effect). The optimized sampling and PSI conditions are also applicable to multi-analyte analysis by SERS and MS, with greatly enhanced detection capability and signal uniformity.

A nanophotonic approach to signal-to-noise ratio of quantum dot infrared photodetectors with surface plasmonic excitation

A nanophotonic approach to the signal-to-noise ratio (SNR) of an InAs-based quantum dot infrared photodetector (QDIP) with surface plasmonic excitation is reported. A 100 nm-thick Au film, perforated with a 3.1 μm-period, 2-dimensional square array of holes, referred to as a metal photonic crystal (MPC), is employed as a plasmonic coupler for it. Under the irradiance on the MPC integrated atop the QDIP, the fundamental surface plasma wave (SPW) is excited along their interface at ∼10.3 μm in wavelength. It carries the near-field that interacts with the quantum dots (QDs) under the interface. Depending on the presence of the coupler, the QDIP generates two different current–voltage (I-V) characteristics; one from the QDs normally working without MPC and the other from those coupled to the SPW near-field with it. The two I-V’s unfold the signal and noise current of each case with photoconductive gain. In their relation, generation-recombination and shot noises which are correlated with the detector signal current are directly affected by the plasmonic coupling but other sources including thermal noise are not relevant to it. Relying on these differences, the I-V analysis allows the SNR of each case and shows ∼20× enhancement by the plasmonic coupling. It reveals that most of the noise current is attributed to thermal fluctuations inherent to the quantum confinement of the QDIP. The SNR lower than the quantum efficiency in plasmonic enhancement is addressed.

Noble metal-free plasmonic Cu/WO3-x@ZNC bi-functional composite for ciprofloxacin photocatalytic degradation and photothermal water evaporation via visible-infrared exposure

Herein, coupling of noble metal-free plasmonic copper nanoparticles with tungsten suboxide and supporting on zeolite nanoclay (Cu/WO3-x@ZNC) composite will be introduced for bi-functional photocatalytic ciprofloxacin (CIP) degradation and water photothermal evaporation under visible/infrared (Vis/IR) exposure. Reduced band-gap of WO3-x via oxygen vacancies creation and localized surface plasmon resonance (LSPR) formation by Cu nanoparticles contributed significantly the extension and intensification of composite's photo-absorption range. Furthermore, small mesoporous structure of ZNC enhanced CIP adsorption and charge carriers separation where the reported photocatalytic efficiencies were 88.3 and 81.7% upon IR and Vis light exposure respectively. It was evidenced that plasmonic hot electrons (e−.s) and hydroxyl radicals (OH•-) performed the basic functions of the photocatalytic process. At the other side, oxygen vacancies existence, plasmonic effect, and confining thermal characteristics of WO3-x, Cu, and ZNC correspondingly induced water photothermal evaporation with efficiencies up to 97.5 and 72.8% under IR and Vis illumination respectively. This work introduces synthesis of a novel bi-functional photocatalytic-photothermal composite by metal sub-oxide and non-noble metal plasmonic coupling and supporting on naturally-derived carrier for water restoration under broad spectral exposure.

Reversible Modulation of Plasmonic Coupling of Gold Nanoparticles Confined within Swellable Polymer Colloidal Spheres.

Dynamic optical modulation in response to stimuli provides exciting opportunities for designing novel sensing, actuating, and authentication devices. Here, we demonstrate that the reversible swelling and deswelling of crosslinked polymer colloidal spheres in response to pH and temperature changes can be utilized to drive the assembly and disassembly of the embedded gold nanoparticles (AuNPs), inducing their plasmonic coupling and decoupling and, correspondingly, color changes. The multi-responsive colloids are created by depositing a monolayer of AuNPs on the surface of resorcinol-formaldehyde (RF) nanospheres, then overcoating them with an additional RF layer, followed by a seeded growth process to enlarge the AuNPs and reduce their interparticle separation to induce significant plasmonic coupling. This configuration facilitates dynamic modulation of plasmonic coupling through the reversible swelling/deswelling of the polymer spheres in response to pH and temperature changes. The rapid and repeatable transitions between coupled and decoupled plasmonic states of AuNPs enable reversible color switching when the polymer spheres are in colloidal form or embedded in hydrogel substrates. Furthermore, leveraging the photothermal effect and stimuli-responsive plasmonic coupling of the embedded AuNPs enables the construction of hybrid hydrogel films featuring switchable anticounterfeiting patterns, showcasing the versatility and potential of this multi-stimuli-responsive plasmonic system.

Compact and broadband polarization beam splitter based on a bridged hybrid plasmonic waveguide coupler on a thin-film lithium niobate.

We propose a compact and broadband polarization beam splitter (PBS) based on a thin-film lithium niobate. The hybrid plasmonic waveguide (HPW) is used as a bridge in a three-waveguide coupler to achieve large birefringence. A degree of phase-matching is introduced for optimization. The proposed PBS exhibits a 125-nm bandwidth (1495-1620 nm) of polarization extinction ratio (PER) over 20 dB or an 85-nm bandwidth (1515-1600 nm) of PER over 25 dB, with a device length of only 81 μm. An ultra-low insertion loss of 0.04 dB is achieved around the wavelength of 1550 nm. In addition, the proposed device exhibits high robustness of the fabrication tolerance.

A new highly sensitive flexible SERS substrate: CVD graphene/copper foil decorated by Ag NPs

Graphene-mediated surface-enhanced Raman scattering (SERS) substrates are currently being investigated for their potential in ultrasensitive detection. Graphene (G) combined with plasmonic metal nanostructures is expected to provide an exceptional SERS response. Here, we prepared a flexible multilayer composite structure by directly decorating silver nanoparticles (Ag NPs) on CVD graphene/copper foil under ultrasonic conditions. Due to the multi-dimensional plasmonic coupling of precious metals, extra chemical enhancement and exceptional molecule harvesting capability of G interlayer, the as-prepared Ag/G/Cu multilayer composite structure manifested ultrahigh sensitivity with the detection limit of Rhodamine 6G (R6G) as low as 1.0 × 10-14 M and a high enhancement factor (EF) of 8.86 × 108. The high performance flexible SERS substrate, with its ingenious process and simple structure, demonstrates excellent potential for applications in environmental protection.

Engineering the apparent quantum yield and emission rate of fluorophore molecules by coupling fluorophore dipoles with plasmon modes of gold using low frequency electric fields

Abstract Localized surface plasmons produced by gold and silver nanostructures have been utilized to enhance the intensity of fluorophore molecules. The issue with using nanostructure plasmons for fluorescence enhancement is their short-range nature (5–50 nm from the nanostructures), which limits accessibility to a few molecules. In addition, fluorophore dipoles needed to be aligned with the plasmon electric fields to maximize the fluorescence enhancement. To address these issues, we used low-frequency electric fields (<5 MHz) and commercially available nanorod and nanosphere samples and studied their effectiveness in enhancing the fluorescence of fluorophore-labeled short single-stranded DNA molecules (22 bases). We demonstrated that DNA molecules and nanorod particles can effectively be manipulated around the charging frequency of DNA molecules (∼3 MHz). Nanorod particles enhanced the fluorescence emission rate by ∼50-fold. When the 3 MHz electric field was introduced, the emission rate increased to over 700-fold. We also found that the introduction of a 3 MHz electric field aided the enhancement of the intrinsic quantum yield fluorophore molecules, which resulted in over a 1000-fold fluorescence enhancement. This enhancement was due to the very high electric produced by polarized DNA dipoles at 3 MHz, which resulted in a torque on fluorophore dipoles and subsequently aligning the fluorophore dipole axis with the plasmon electric field. At a fundamental level, our results demonstrate the role of the low-frequency electric field in the fluorophore–plasmon coupling. These findings can directly be applied to many fluorescence detection systems, including the development of biosensors.

(Invited) Emergent Chiroptical Properties in Assembled Molecules and Materials: From Native Chirality to Global Chirality

The emergence of chiroptical properties in molecules and materials through their asymmetric organizations has fascinated humankind in general and the scientific community in particular. By adopting molecules, plasmonic systems, and semiconductor quantum dots as examples, the translation of ‘local’ chirality in these building blocks to the ‘global’ chirality when they self-assemble as nanoobjects will be explained. The presentation covers the following aspects. In assembled molecular systems, induced circular dichroism (ICD) originates through the off-resonance coupling of transition dipoles, resulting in monosignated CD signals. In contrast, exciton-coupled circular dichroism (EC-CD) originates through the on-resonance exciton coupling, displaying bisignated CD signals.1-3 Bisignation in the CD responses of assembled plasmonic systems results from the resonant plasmon coupling, termed surface plasmon-coupled circular dichroism (SP-CD).4,5 The presentation will draw parallels between the emergence of EC-CD and SP-CD based on the coupling of their transition dipoles.3 More importantly, our studies have concluded that the sign of the EC-CD/SP-CD depends not only on the handedness of the assembly but also on the sign of the interaction energy between the neighboring dipoles.6 The presentation will also discuss how chiral surface domains of various self-assembled amino acid-based templates transfer chiral information to bound achiral chromophores. The origin of CD and CPL of achiral molecules on these templates are explained based on exciton coupling.7 The presentation will also provide fundamental insight into the interaction of chiral molecules with silicon nanoparticles and the emergence of its CD and CPL.8 An open challenge is to develop a universal model which can explain the chiroptical properties of assembled systems based on the transition dipolar coupling and interaction energy: our findings in this direction will be presented. Thomas R, Kumar J, George J, et al. Phys. Chem. Lett., 2018, 9, 919-932.Kar S, Swathi K, Sissa C, et al., Phys. Chem. Lett., 2018, 9, 4584-4590.Nizar NSS, Sujith M, Swathi K, et al. Soc. Rev.,2021, 50 , 11208-11226.George J, Thomas KG, J. Am. Chem. Soc. 2010, 132, 2502-2503.George J, Kar S, Anupriya ES, et al. ACS Nano 13, 2019, 4392-4401.Swathi K, Sissa C, Painelli A, et al. Commun. , 2020 , 56, 8281-8284.Somasundaran SM, Kompella SVK, Mohan TMN, et al. ACS Nano 2023 17, 11054-11069.Sujith M, Vishnu EK, Sappati S, et al. J. Am. Chem. Soc. , 2022, 144, 5074-5086. The theoretical collaborations with Dr. R. S. Swathi, IISER Thiruvananthapuram (India), Dr. Cristina Sissa and Prof. Anna Painelli, University of Parma (Italy), and Prof. S. Balasubramanian, JNCASR, Bangalore are greatly acknowledged. KGT acknowledges the J. C. Bose National Fellowship and Nanomission project (DST/NM/TUE/EE-01/2019), the Department of Science and Technology, Government of India, for financial support.

Correction to: Characteristics of Photon–Plasmon Coupling in Uniaxial Chiral Filled Slab Waveguide Bounded by Graphene Layers

Correction to: Characteristics of Photon–Plasmon Coupling in Uniaxial Chiral Filled Slab Waveguide Bounded by Graphene Layers

Plasmon Mediated Single Photon Emission from a Nanocrystal Ensemble.

Quantum photonic devices require robust sources of single photons to perform basic computational and communication protocols. Thus, developing scalable, integrable, and efficient quantum light sources has become crucial for the realization of quantum photonic devices. Single quantum dots are promising sources of quantum light due to their tunable emission wavelength. Here, we show the emergence of quantum-emitter-like antibunched emission behavior when multiple quantum dots are located in the vicinity of plasmonic particles. To evaluate the robustness of this phenomenon, we consider both monometallic and bimetallic particles. We find that the photoluminescence intensity of the plasmon coupled quantum dots fits well to a single sublinear power law exponent that is distinct from the behavior of CQD aggregates. Significantly, we find that plasmon coupling results in reduced flickering, thus enabling the realization of a more stable and reliable single photon source. Possible roles of emergent excitonic interactions in the coupled system are discussed.

Photoresponse performance of Au (nanocluster and nanoparticle) TiO2: Photosynthesis, characterization and mechanism studies

In this study, gold (Au), nanoclusters (NC) and heterogeneous titanium dioxide (TiO2) were successfully synthesized by low-temperature photoelectron spectroscopy. The chemical structure of Au-TiO2 was characterized by XRD, EDX, XPS and HR-TEM. Optical properties were evaluated using PL and UV–Vis, and electrical properties were evaluated using CV, CP, UPS and EIS parameters. The results showed that Au nanocluster (NCs) were converted into Au nanoparticles (NPs) during high sunlight, and the transfer mechanism was examined. In addition, the produced nanoparticles can form a Schottky barrier with TiO2, which affects the band structure. Moreover, the simultaneous synthesis of nanoparticles and nanoclusters adds an extra dimension to the understanding of the photoelectronic behavior of structured photoelectrodes. Moreover, this research shows that good control of the photography process can result in more photos. This improvement is related to several factors, including the plasmonic field and coupling line bending.

Coherent coupling of localized surface plasmons and surface plasmons in borophene-based metamaterial

The strong coherent coupling in different electromagnetic modes can control the light-matter interaction more conveniently. Here, we theoretically researched the hybridization between the borophene surface plasmon (BSP) mode and the borophene localized surface plasmon (BLSP) mode in borophene grating structure. This coupling effect leads to the emergence of multiple hybrid modes. The absorption spectra of the system are investigated through finite difference time domain (FDTD) simulation and coupled oscillator model (COM). Results show that the coherent coupling of BSP and BLSP can be achieved by adjusting the carrier density of the borophene gratings. A Rabi splitting effect with frequency of 21.6 THz can be observed. Furthermore, we investigated the effects of geometric structural parameters, incident angle, and relaxation time on the correlated coupling spectra. Our work may deepen the understanding of light–matter interactions and provide a reference for borophene-based active photonic devices in the near-infrared region.

Unveiling Coupled Dark Plasmon Modes in CdO Structures

Unveiling Coupled Dark Plasmon Modes in CdO Structures

Emission Wavelength Control Based on Coupling of Surface Plasmon and Microcavity Mode in Organic Light-Emitting Diodes with Metal/Dielectric/Metal Anodes

Emission Wavelength Control Based on Coupling of Surface Plasmon and Microcavity Mode in Organic Light-Emitting Diodes with Metal/Dielectric/Metal Anodes

Welded Gold Nanoparticle Assemblies Defined Plasmonic Coupling.

Nanoparticle assemblies with interparticle ohmic contacts are crucial for nanodevice fabrication. Despite tremendous progress in DNA-programmable nanoparticle assemblies, seamlessly welding discrete components into welded continuous three-dimensional (3D) configurations remains challenging. Here, we introduce a single-stranded DNA-encoded strategy to customize welded metal nanostructures with tunable morphologies and plasmonic properties. We demonstrate the precise welding of gold nanoparticle assemblies into continuous metal nanostructures with interparticle ohmic contacts through chemical welding in solution. We find that the welded gold nanoparticle assemblies show a consistent morphology with welded efficiency over 90%, such as the rod-like, triangular, and tetrahedral metal nanostructures. Next, we show the versatility of this strategy by welding gold nanoparticle assemblies of varied sizes and shapes. Furthermore, the experiment and simulation show that the welded gold nanoparticle assemblies exhibit defined plasmonic coupling. This single-stranded DNA encoded welding system may provide a new route for accurately building functional plasmonic nanomaterials and devices.

Dependent scattering and plasmon coupling in concentrated suspensions of optically hard nanoparticles

This study establishes the accuracy and efficacy of the recently developed radiative transfer with reciprocal transactions (R2T2) method for quickly simulating radiation transfer through concentrated thick suspensions of optically hard nanoparticles featuring a large mismatch in refractive and/or absorption indices compared with their surrounding medium. Concentrated suspensions of optically hard nanoparticles exhibit strong light scattering and dependent scattering effects including both near-field interactions among particles and interferences of scattered waves in the far-field. Concentrated suspensions of metallic nanoparticles also exhibit plasmon coupling effect that leads to widening of absorption peak and red-shift in the peak surface plasmon resonance wavelength. However, predicting these complex interactions between EM waves and particles in thick and concentrated suspensions by explicitly solving Maxwell's equations is computationally intensive, if not impossible. Conventional solutions like Lorenz–Mie theory combined with independent scattering approximation do not account for dependent scattering and plasmon coupling. Furthermore, the dense medium radiative transfer theory is a far-field approximation that does not account for near-field effects, leading to significant errors in predictions, as illustrated in this study. By contrast, the R2T2 method's predictions showed excellent agreement with the solutions of Maxwell's equations obtained using the superposition T-matrix method for thin films containing optically hard particles. The method also rigorously accounted for multiple scattering as well as plasmon coupling in thick concentrated suspensions. These results could facilitate the design of plasmonic suspensions used in various energy and environmental applications.

Formation of Linear Plasmonic Heterotrimers Using Nanoparticle Docking to DNA Origami Cages.

The fabrication of complex assemblies with interesting collective properties from plasmonic nanoparticles (NPs) is often challenging. While DNA-directed self-assembly has emerged as one of the most promising approaches to forming such complex assemblies, the resulting structures tend to have large variability in gap sizes and shapes, as the DNA strands used to organize these particles are flexible, and the polydispersity of the NPs leads to variability in these critical structural features. Here, we use a new strategy termed docking to DNA origami cages (D-DOC) to organize spherical NPs into a linear heterotrimer with a precisely defined geometrical arrangement. Instead of binding NPs to the exterior of the DNA templates, D-DOC binds the NPs to either the interior or the opening of a 3D cage, which significantly reduces the variability of critical structural features by incorporating multiple diametrically arranged capture strands to tether NPs. Additionally, such a spatial arrangement of the capture strand can work synergistically with shape complementarity to achieve tighter confinement. To assemble NPs via D-DOC, we developed a multistep assembly process that first encapsulates an NP inside a cage and then binds two other NPs to the openings. Microscopic characterization shows low variability in the bond angles and gap sizes. Both UV-vis absorption and surface-enhanced Raman scattering (SERS) measurements showed strong plasmonic coupling that aligned with predictions by electrodynamic simulations, further confirming the precision of the assembly. These results suggest D-DOC could open new opportunities in biomolecular sensing, SERS and fluorescence spectroscopies, and energy harvesting through the self-assembly of NPs into more complex 3D assemblies.