Plasmon Spectrum Research Articles

Polarizability, plasmons and damping in pseudospin-1 gapped materials with a flat band

The subject of our present investigation is the collective electronic properties of various types of pseudospin-1 Dirac-cone materials with a flat band and finite bandgaps in their low-energy spectra. Specifically, we have calculated the dynamical polarization, plasmon dispersions, as well as their decay rates due to Landau damping and presented the closed-form analytical expressions for the wave function overlaps for both the gapped dice lattice and the Lieb lattice. The gapped dice lattice is a special case of the more generalα-T3model such that its band structure is symmetric and the flat band remains dispersionless. On the other hand, the Lieb lattice has a flat band which appears at the lowest point of its conduction band. Our results for these two materials exhibit unique features in the plasmon spectra and their damping regions, which have never been previously reported. For example, the particle-hole modes of a Lieb lattice appear as finite-size regions, while the plasmon modes exist only in areas with small wave numbers but an extended range of frequencies.

Bi-path color tunable plasmonic micro-nano hybrid structures for encrypted printing.

Colored information is crucial for humans to perceive the world. Plasmonic spectra modulation can serve as an effective means to create different colors. Although several solutions for plasmonic color-printing have been proposed, further information encryption has not received any attention. Herein, we exhibit a fine color modulation strategy to construct noble-metal-based micro-nano hybrid structures in the bi-path of photo-thermal deformation and liquid-phase-chemical reaction. Ag/Ta2O5 bi-layer films are ablated at the center of the machined lines of nanosecond pulsed laser, while silver nanoparticles are formed in other regions by thermal radiation of the infrared laser, which can be further dissolved and shape-modulated in KCl solution under different periods. The variation of size and spacing of nano-Ag particles results in a precise shift of plasmonic spectra in visible region. Colored information can be hidden by adjusting the scan number and the energy density during laser processing, and will emerge after the subsequent chemical dissolution reactions. The bi-path color adjustment strategy is easy to operate and can play a role in key information protection and color image switching.

Electrodynamic approach for calculating the absorption spectra of plasmons in a rectangle with a two-dimensional electron gas excited by an incident electromagnetic wave

The purpose of this research is to develop an electrodynamic method for calculating the plasmon spectrum in a three-dimensional structure with a two-dimensional electron gas excited by an incident electromagnetic wave. Methods. The developed method is based on solving integral equations formed with respect to induced currents in the conducting parts of a three-dimensional structure. Results. The convergence of the method and the calculation time were studied. The conditions for the convergence of calculations of higher plasmon resonances in a rectangular structure with a two-dimensional electron gas are determined. The normal incidence of an arbitrarily polarized electromagnetic wave on a rectangle with a two-dimensional gas is studied. The spectra of the absorption, extinction, forward and back scattering cross sections of the incident wave are calculated. Conclusion. It is found that in a rectangular structure containing a two-dimensional electron gas, the spectrum of plasmon resonances is modified in comparison with established by two-dimensional models of problem formulation, in which the structure is assumed to be infinite and homogeneous in one of the directions. It has been established that the incident wave most effectively excites fundamental plasmon modes. Plasmonic modes exhibit strong charge accumulation at the edges of the rectangle, which significantly affects the resonant excitation frequencies of plasmonic modes.

Selective Damping of Plasmons in Coupled Two-Dimensional Systems by Coulomb Drag

The Coulomb drag is a many-body effect observed in proximized low-dimensional systems. It appears as emergence of voltage in one of them upon passage of bias current in another. The magnitude of drag voltage can be strongly affected by exchange of plasmonic excitations between the layers; however, the reverse effect of Coulomb drag on properties of plasmons has not been studied. Here, we study the plasmon spectra and damping in parallel two-dimensional systems in the presence of Coulomb drag. We find that Coulomb drag leads to selective damping of one of the two fundamental plasma modes of a coupled bilayer. For identical electron doping of both layers, the drag suppresses the acoustic plasma mode; while for symmetric electron-hole doping of the coupled pair, the drag suppresses the optical plasma mode. The selective damping can be observed both for propagating modes in extended bilayers and for localized plasmons in bilayers confined by source and drain contacts. The discussed effect may provide access to the strength of Coulomb interaction in 2d electron systems from various optical and microwave scattering experiments.

Dramatic Electrochemical Tuning of the Plasmonic Spectrum of a Metal Nanoparticle on a Mirror

Enhancement of electric fields in the nanometric gap between plasmonic nanostructures is widely exploited for sensing, enhanced spectroscopies and photochemistry. Plasmonic properties of nanostructures with ultrathin gaps, such as nanoparticles (NPs) positioned on a flat metal surface (a mirror), have attracted much attention in recent years [1]. We studied the effect of electrochemical control on the plasmonic properties of single Au and Ag NPs on a mirror. We found that the coupled plasmonic resonance could be reversibly tuned over a range of tenths of nanometers, more than an order of magnitude higher than expected based just on capacitive charging of the NPs.Our experimental system consisted of single Au or Ag NPs (50 nm – 100 nm) adsorbed on a self-assembled monolayer (SAM) of either polyelectrolytes, cysteamine (CEA) or ethanedithiol (EDT) deposited on an atomically flat 100nm thick Au electrode prepared by template stripping. The NPs were either bare (prepared by H2 reduction) or citrate-stabilized and the system was immersed in a basic solution.Coupling to the mirror electrode resulted in the appearance of several bands in the plasmonic spectrum of individual NPs, as measured by dark-field microspectroscopy (Fig. 1a), and the lowest-energy band could be attributed to the coupled plasmon. Variation of the electrode potential generated a giant, though reversi shift of the spectrum. A plot of the peak position of the coupled plasmon band displayed an S-shaped, phase-transition like behavior (Fig. 1b).This remarkable result could not be observed with individual NPs deposited on non-metallic surfaces, and can therefore be attributed to a dramatic modulation of the coupling between the particles and the mirror with electrode potential.Figure 1. Scattering spectra (a) and variation of the spectral position of the coupled plasmon peak (b) of bare AuNP (ca. 60 nm) on CEA/Au electrode. Inset in (a): Scheme of NP on the mirror.

Correction to plasmon spectrum due to dark photons in a graphene periodic structure

Correction to plasmon spectrum due to dark photons in a graphene periodic structure

Relationship Between Stress Modulated Metallicity and Plasmon in Graphene Nanoribbons.

Nanoscale quantum plasmon is an important technology that restricts the application of optics, electricity, and graphene photoelectric devices. Establishing a structure-effect relationship between the structure of graphene nanoribbons (GNRs) under stress regulation and the properties of plasmons is a key scientific issue for promoting the application of plasmons in micro-nano photoelectric devices. In this study, zigzag graphene nanoribbon (Z-GNR) and armchair graphene nanoribbon (A-GNR) models of specific widths were constructed, and density functional theory (DFT) was used to study their lattice structure, energy band, absorption spectrum, and plasmon effects under different stresses. The results showed that the Z-GNR band gap decreased with increasing stress, and the A-GNR band gap changed periodically with increasing stress. The plasmon effects of the A-GNRs and Z-GNRs appeared in the visible region, whereas the absorption spectrum showed a redshift trend, indicating the range of the plasmon spectrum also underwent significant changes. This study provides a theoretical basis for the application of graphene nanoribbons in the field of optoelectronics under strain-engineering conditions.

Modeling Plasmonics and Electronics in Semiconducting Graphene Nanostrips

In recent decades, both academia and industry have shown noteworthy interest in investigating the semiconducting properties of graphene. Nevertheless, the lack of a suitable bandgap in graphene has restricted its practical applications in the current semiconductor industry. To overcome this limitation, graphene micro/nano-strips have been actively explored. The focus of the present study centers on modeling the electronic and plasmonic characteristics of graphene strips with varying widths: 2.7, 100, 135 nm, and 4 m. This analysis is conducted at ultralow energies (0.3 eV, or ~73 THz). We employ conventional density functional computations to estimate the Fermi velocity of graphene, refining the results via the GW approximation. Utilizing the accurate Fermi velocity, we employ a semi-analytical model to explore the ground state and plasmon properties (frequency and dispersion) of these graphene strips. Notably, this approach effectively replicates the density of states observed in narrow experimental graphene nano-strips (2.7 nm) grown on Ge(001) and, similarly, reproduces the plasmon spectrum found in synthesized graphene microstrips (4 μm) on Si/SiO2. Interestingly, our study also offers insights into the potential application of this approach in comprehending the plasmon frequency and plasmon dispersion of graphene nano-strips (~135 nm) acquired through liquid-phase exfoliation. The outcomes of this investigation present compelling evidence that the properties of graphene-based strips can be customized to fulfill specific requirements and applications. These findings hold significant promise for advancing graphene-based technologies, bridging the gap between fundamental research and tangible applications. Doi: 10.28991/ESJ-2023-07-05-01 Full Text: PDF

MMP simulation of coupled gold nanoparticles as nanoantennae: Efficient tuning of localized surface plasmon resonances

We study localized surface plasmon excitations on a set of two coupled two-dimensional (2D) gold nanoparticles as a nanoantenna by using the multiple multipoles (MMP) method. We investigate the plasmon spectrum as a function of the nanoparticle's size, the background index of refraction, and the separating gap between the nanoparticles in order to provide proper means to tune the underlying plasmon resonance efficiently. Accurate multiple multipoles (MMP) computations are conducted with precise control over error measures, particularly for configurations of coupled particles with varying inter-particle distances and illumination conditions. Our findings demonstrate that by selecting these parameters carefully, nanoantennae can amplify emitter radiation across a wide range of spectra from ultraviolet to near-infrared. Numerical calculations have revealed that the MMP method is accurate, easy to handle, and efficient even on a low-cost, state-of-the-art PC.

Identification of dark axisymmetric plasma modes in partially gated two-dimensional electron gas disk

We investigate the dark axisymmetric plasmon spectrum for partially gated two-dimensional gas of the disk shape. The extension of the central gate spot provides a change in plasmon dispersion from the root to a linear dependence on the wave vector. Intriguingly, the interaction between neighboring modes occurs because of stepwise change in carrier screening across the circumference of the gate. This behavior is unexpected when changing from a fully screened to unscreened two-dimensional gas by increasing the dielectric width under the gate [A.L.Fetter, Phys.Rev.B 33, 5221 (1986)]. Our results provide the accurate identification of axisymmetric plasmon modes recently observed in experiment and, in addition, pave the way for feedback plasmon resonator study.

Dipolar Ligands Tune Plasmonic Properties of Tin-Doped Indium Oxide Nanocrystals.

Surface functionalization with dipolar molecules is known to tune the electronic band alignment in semiconductor films and colloidal quantum dots. Yet, the influence of surface modification on plasmonic nanocrystals and their properties remains little explored. Here, we functionalize tin-doped indium oxide nanocrystals (ITO NCs) via ligand exchange with a series of cinnamic acids with different electron-withdrawing and -donating dipolar characters. Consistent with previous reports on semiconductors, we find that withdrawing (donating) ligands increase (decrease) the work function caused by an electrostatic potential shift across the molecular layer. Quantitative analyses of the plasmonic extinction spectra reveal that varying the ligand molecular dipole affects the near-surface depletion layer, with an anticorrelated trend between the electron concentration and electronic volume fraction, factors that are positively correlated in as-synthesized NCs. Electronic structure engineering through surface modification provides access to distinctive combinations of plasmonic properties that could enable optoelectronic applications, sensing, and hot electron-driven processes.

Site-selective growth and plasmonic spectral properties of L-shaped Janus Au-Ag gold nanodumbbells for surface-enhanced Raman scattering

Ligand-mediated interface control has been broadly applied as a powerful tool in constructing asymmetric multicomponent nanoparticles (AMNP), and induces the anisotropic growth with fine-tuning morphology, composition, plasmonic property and functionality. As a new kind of AMNP, the synthesis of Janus Au-Ag nanoparticles with tunable negative surface curvature is still a challenge. Here, we demonstrate that the synergistic surface energy effects between gold nanodumbbells (Au NDs) with a negative surface curvature and 4-mercaptobenzoic acid (4-MBA) can direct the site-selective growth of anisotropic silver domains on gold nanodumbbells (Au NDs@Ag NPs). By adjusting the 4-MBA concentration-dependent interfacial energy, the Au NDs@Ag NPs could be continuously tuned from dumbbell-like core–shell structures, to L-shaped Janus, and then rod-like core–shell structures with directional and asymmetric spatial distributions of resizable Ag domains by site-selective growth. Based on the calculation results of discrete dipole approximation (DDA) method, it has been found that the Au NDs@Ag L-shaped Janus NPs with Ag island domains created polarization orientation-dependent plasmonic extinction spectra and hot spots around the negatively curved waist and Ag domains. The L-shaped Janus Au NDs@Ag NPs exhibited significantly plasmonic spectrum properties with four apparent LSPR peaks that cover from visible to near-infrared range and higher surface-enhanced Raman scattering (SERS) activity compared with the original Au NDs. The best SERS enhancement factor of 1.41 × 107 was achieved. This synergistic surface energy effect-based method involving the asymmetric growth of silver coating on gold nanoparticles with negatively curved surface presents a new method to design and fabricate nanometer optical devices based on asymmetric multicomponent nanoparticles.

Probing correlated states with plasmons.

Understanding the nature of strongly correlated states in flat-band materials (such as moiré heterostructures) is at the forefront of both experimental and theoretical pursuits. While magnetotransport, scanning probe, and optical techniques are often very successful in investigating the properties of the underlying order, the exact nature of the ground state often remains unknown. Here, we propose to leverage strong light-matter coupling present in the flat-band systems to gain insight through dynamical dielectric response into the structure of the many-body ground state. We argue that because of the enlargement of the effective lattice of the system arising from correlations, conventional long-range plasmon becomes "folded" to yield a multiband plasmon spectrum. We detail several mechanisms through which the structure of the plasmon spectrum and that of the dynamical dielectric response is susceptible to the underlying order, revealing valued insights such as the interaction-driven band gaps, spin-structure, and the order periodicity.

Interference of excitons and surface plasmons in the optical absorption spectra of monolayer and bilayer graphene

By using spectroscopy ellipsometry, we investigated the optical absorption spectra of monolayer and bilayer graphene in the deep-ultraviolet region up to 6.42 eV as a function of temperature. The optical absorption spectra of monolayer and bilayer graphene can be fitted by two Breit-Wigner-Fano (BWF) spectral functions, in which each BWF is originated by the interference of discrete excitonic spectra and continuous surface plasmon spectra. Each BWF peak shows narrower linewidth and larger peak intensity with increasing temperature. The unusual temperature dependence of the BWF peaks is understood by a shorter lifetime of the surface plasmon at a higher temperature, which gives a smaller BWF asymmetric parameter. The optical absorption by the surface plasmon is usually observed only at the surface of the three-dimensional crystal or two-dimensional material. The contribution of the surface plasmon becomes relatively larger in the case of monolayer or bilayer graphene than that of single-crystal graphite because of the two-dimensional nature of electrons. The observed optical absorption spectra are confirmed by calculating the complex dielectric function using the first-principles calculations.

Selectively Exciting and Probing Radiative Plasmon Modes on Short Gold Nanorods by Scanning Tunneling Microscope-Induced Light Emission

We study the plasmon modes of gold nanorods (as short as ∼100 nm) on a nonmetallic conductive substrate using scanning tunneling microscope-induced light emission (STM-LE) with a nonplasmonic tungsten tip at room temperature in high vacuum (10–7 mbar). The far-field light is identified as the radiative decay of plasmon modes on the nanorods excited by inelastic electron tunneling. The spatial intensity distributions of the first three longitudinal multipolar modes on nanorods are spatially resolved on the order of 10–20 nm. These intensity distributions are related to the radiative electromagnetic local density of states and agree very well with numerical simulations. We discover that the presence of the tungsten tip with a high-dielectric constant influences the line shapes of the plasmon spectra and enhances the strength of the plasmon peaks.

Modeling 2D Arrangements of Graphene Nanoribbons

In the last two decades, interest in graphene has grown extensively due to its extraordinary properties and potential for various applications such as sensing and communication. However, graphene is intrinsically a semimetal with a zero bandgap, which considerably delays its use where a suitable bandgap is required. In this context, quasi-one-dimensional counterparts known as graphene nanoribbons (GNRs) have demonstrated sizeable bandgaps and versatile electronic properties, which make them promising candidates for photonic and plasmonic applications. While progress has recently been made toward the synthesis of GNRs, theoretical models to envisage their electronic and optical properties have been restricted to ab initio approaches, which are not feasible for wide systems because of the large number of atoms tangled. Here, we use a semi-analytical model based on Dirac cone approximation to show the adjustable electronic and plasmonic characteristics of wide and experimental GNRs, both freestanding and non-freestanding. This approach utilizes the group velocity of graphene, which is calculated using density functional computations (vF=0.829×106 m s−1), as the primary input. Importantly, our research reveals that at the terahertz level, the plasmon-momentum dispersion is highly responsive to changes by varying the ribbon width or charge carrier concentrations, the other involved parameters can be manipulated by setting values from experiments or more sophisticated predictions. In particular, this model can replicate the electronic properties of GNRs on Ge(001) and GNRs on Au(111). From the plasmonic side, the plasmon spectrum of graphene microribbon arrays of 4 μm wide on Si/SiO2 and GNR arrays on Si are found in good agreement with experiments. The potential use of GNRs in sensing molecules such as chlorpyrifos-methyl is also discussed. Chlorpyrifos-methyl is chosen as the test molecule because it is a commonly used insecticide in agriculture, but its high toxicity to organisms and humans makes it a concern. It has been established that the plasmon resonances of all the studied GNRs occur at the same frequency as chlorpyrifos-methyl, which is 0.95 THz. Our findings can serve as a useful guide for future experiments.

Simple and Tailorable Synthesis of Silver Nanoplates in Gram Quantities.

Due to plasmonic and catalytic properties, silver nanoplates are of significant interest; therefore, their simple preparation in gram quantities is required. Preferably, the method is seedless, consists of few reagents, enables preparation of silver nanoplates with desired optical properties in high concentration, is scalable, and allows their long-term storage. The developed method is based on silver nitrate, sodium borohydride, polyvinylpyrrolidone, and H2O2 as the main reagents, while antifoam A204 is implemented to achieve better product quality on a larger scale. The effect of each component was evaluated and optimized. Solution volumes from 3 to 450 mL and concentrations of silver nanoplates from 0.88 to 4.8 g/L were tested. Their size was tailored from 25 nm to 8 μm simply by H2O2 addition, covering the entire visible plasmon spectra and beyond. They can be dried and spontaneously dispersed after at least one month of storage in the dark without any change in plasmonic properties. Their potential use in modern art was demonstrated by drying silver colloids on different surfaces in the presence of reagents or purified, resulting in a variety of colors but, more importantly, patterns of varying complexity, from simple multi-coffee-rings structures to dendritic forms and complex multilevel Sierpiński triangle fractals.

Recombination with plasmon emission in HgTe/CdHgTe multiple quantum well heterostructures

The work is devoted to the theoretical study of the process of recombination of nonequilibrium charge carriers with the emission of plasmons in structures with two and three quantum wells, and also in an infinite number quantum-well structure under conditions of inverse band population. The plasmon spectra in these structures have been calculated. The dependence of the average probability of electron recombination with plasmon emission on the concentration of nonequilibrium carriers for three temperatures has been found. It has been shown that an increase in the number of quantum wells leads to a slight increase in the average recombination probability.

Semiclassical theory for plasmons in spatially inhomogeneous media

Recent progress in experimental techniques has made the quantum regime in plasmonics accessible. Since plasmons correspond to collective electron excitations, the electron–electron interaction plays an essential role in their theoretical description. Within the Random Phase Approximation, this interaction is incorporated through a system of equations of motion, which has to be solved self-consistently. For homogeneous media, an analytical solution can be found using the Fourier transform, giving rise to Lindhard theory. When the medium is spatially inhomogeneous, this is no longer possible and one often uses numerical approaches, which are however limited to smaller systems. In this paper, we present a novel semi-analytical approach for bulk plasmons in inhomogeneous media based on the semiclassical (or WKB) approximation, which is applicable when the charge density varies smoothly. By solving the equations of motion self-consistently, we obtain the expressions of Lindhard theory with a spatially varying Fermi wavevector. The derivation involves passing from the operators to their symbols, which can be thought of as classical observables on phase space. In this way we obtain effective (Hamiltonian) equations of motion for plasmons. We then find the quantized energy levels and the plasmon spectrum using Einstein–Brilllouin–Keller quantization. Our results provide a theoretical basis to describe different setups in quantum plasmonics, such as nanoparticles, quantum dots and waveguides.

Application scope of multipole method for decomposition of plasmonic spectrum

The electromagnetic multipole method has been extensively adopted for a better understanding of the physical mechanism of localized surface plasmon. In this work, we quantitatively decomposed the total extinction spectra of typical silver nanoparticles into multipole components, which are shown to be quite different from qualitative assignment of resonance peaks reported previously. Taking the quasi-normal mode method as reference standard, we examined the scope of application of the electromagnetic multipole method in assigning plasmonic resonance peaks, and pointed out the necessity of employing eigenmode-based approaches when interpreting plasmonic spectra, which can hopefully lead to more physical insights into the underlying mechanisms of plasmonics phenomena.