Plasmon Branches Research Articles

Tailored plasmon polariton landscape in graphene/boron nitride patterned heterostructures

Surface plasmon polaritons (SPPs), which are electromagnetic modes representing collective oscillations of charge density coupled with photons, have been extensively studied in graphene. This has provided a solid foundation for understanding SPPs in 2D materials. However, the emergence of wafer-transfer techniques has led to the creation of various quasi-2D van der Waals heterostructures, highlighting certain gaps in our understanding of their optical properties in relation to SPPs. To address this, we analyzed electromagnetic modes in graphene/hexagonal-boron-nitride/graphene heterostructures on a dielectric Al2O3 substrate using the full ab initio RPA optical conductivity tensor. Our theoretical model was validated through comparison with recent experiments measuring evanescent in-phase Dirac and out-of-phase acoustic SPP branches. Furthermore, we investigate how the number of plasmon branches and their dispersion are sensitive to variables such as layer count and charge doping. Notably, we demonstrate that patterning of the topmost graphene into nanoribbons provides efficient Umklapp scattering of the bottommost Dirac plasmon polariton (DP) into the radiative region, resulting in the conversion of the DP into a robust infrared-active plasmon. Additionally, we show that the optical activity of the DP and its hybridization with inherent plasmon resonances in graphene nanoribbons are highly sensitive to the doping of both the topmost and bottommost graphene layers. By elucidating these optical characteristics, we aspire to catalyze further advancements and create new opportunities for innovative applications in photonics and optoelectronic integration.

Plasmon dispersion in bilayer cuprate superconductors

The essential building blocks of cuprate superconductors are two-dimensional CuO2 sheets interspersed with charge reservoir layers. In bilayer cuprates, two closely spaced CuO2 sheets are separated by a larger distance from the subsequent pair in the next unit cell. In contrast to single-layer cuprates, prior theoretical work on bilayer systems has predicted two distinct acoustic plasmon bands for a given out-of-plane momentum transfer. Here we report random phase approximation (RPA) calculations for bilayer systems which corroborate the existence of two distinct plasmon bands. We find that the intensity of the lower-energy band is negligibly small in most parts of the Brillouin zone, whereas the higher-energy band carries significant spectral weight. We also present resonant inelastic x-ray scattering (RIXS) experiments at the O K-edge on the bilayer cuprate Y0.85Ca0.15Ba2Cu3O7 (Ca-YBCO), which show only one dispersive plasmon branch, in agreement with the RPA calculations. In addition, the RPA results indicate that the dispersion of the higher-energy plasmon band in Ca-YBCO is not strictly acoustic but exhibits a substantial energy gap of approximately 250 meV at the two-dimensional Brillouin zone center. Published by the American Physical Society 2024

Plasmonic detection of the parity anomaly in a two-dimensional Chern insulator

In this paper, we present an analytic study on the surface plasmon polaritons in a two-dimensional parity anomaly Chern insulator. The two-dimensional conductivities derived from the BHZ model are antisymmetric, based on which two surface plasmon modes each contains two branches of dispersions have been found. In the absence of parity anomaly, the half-integer-valued Hall conductivities with positive and negative Dirac mass terms differ by a sign; two branches of each surface plasmon mode are exactly degenerate. However, the parity anomaly can lift such degeneracy and lead to significant modifications of these dispersion curves or even the occurrence of an extra branch of surface plasmons under particular condition. Our investigations have revealed the effects of the interplay of parity anomaly and topology on the dispersion relations of the surface plasmon polaritons, which may pave a possible way for the detection of the parity anomaly in a two-dimensional Chern insulator via plasmonic responses.

Time-resolved plasmon-assisted generation of optical-vortex pulses

The microscopic mechanism of the light-matter interactions that induce orbital angular momentum (OAM) in electromagnetic fields is not thoroughly understood. In this work, we employ Archimedean spiral vortex generators in time-resolved numerical simulations using the Octopus code to observe the behind-the-scenes of OAM generation. We send a perfect circularly-polarized plane-wave light onto plasmonic optical vortex generators and observe the resulting twisted light formation with complete spatio-temporal information. In agreement with previous works, we find that emission from the plasmonic spiral branches shapes the vortex-like structure and governs the OAM generation in the outgoing electromagnetic field. To characterize the generated beam further, we emulate the emission from vortex generators with current emitters preserving the spiral geometry. We subject a point-particle system to the generated field and record the orbital angular momentum transfer between the electromagnetic field and the point particle. Finally, we probe the OAM density locally by studying the induced classical trajectory of point particles, which provides further insight into the spatio-temporal features of the induced OAM.

Plasmon-magnon interactions in two-dimensional honeycomb magnets

Two-dimensional honeycomb ferromagnets offer the unprecedented opportunity to study interactions between collective modes that in standard bulk ferromagnets do not cross paths. Indeed, they harbor an optical spin-wave branch, i.e. a spin wave which disperses weakly near the Brillouin zone center. When doped with free carriers, they also host the typical gapless plasmonic mode of 2D itinerant electron/hole systems. When the plasmon branch meets the optical spin-wave branch, energy and momentum matching occurs, paving the way for interactions between the charge and spin sector. In this Letter we present a microscopic theory of such plasmon-magnon interactions, which is based on a double random phase approximation. We comment on the possibility to unveil this physics in recently isolated 2D honeycomb magnets such as ${\rm Cr}_2{\rm Ge}_2{\rm Te}_6$.

Plasmons in a two-dimensional nonsymmorphic nodal-line semimetal

Recent experiments have established a type of nonsymmorphic symmetry-protected nodal lines in the family of two-dimensional (2D) composition-tunable materials ${\mathrm{NbSi}}_{x}{\mathrm{Te}}_{2}$. Here, we theoretically study the plasmonic properties of such nonsymmorphic nodal-line semimetals. We show that the nonsymmorphic character endows the plasmons with extremely strong anisotropy. There exist both intraband and interband plasmon branches. The intraband branch is gapless and has a ${q}^{1/2}$ dispersion. It is most dispersive and is independent of carrier density in direction normal to the nodal line, whereas along the nodal line, its dispersion is largely suppressed and its frequency scales linearly with carrier density. The interband branches are gapped and their long-wavelength limits are connected with Van Hove singularities of the band structure. We find that the single-particle excitations are strongly suppressed in such systems, which decreases the Landau damping of plasmons. These characters are further verified by first-principles calculations on 2D ${\mathrm{NbSi}}_{x}{\mathrm{Te}}_{2}$. Interesting features in static screening of charged impurity are also discussed. Our result reveals characteristic plasmons in a class of nonsymmorphic topological semimetals and offers guidance for its experimental detection and possible applications.

Possibility for the anisotropic acoustic plasmons in LaH10 and their role in enhancement of the critical temperature of superconducting transition

We discuss a possible role of the collective acoustic electronic excitations in the mechanism of the nearly room-temperature superconductivity in superhydrides at megabar pressures. While the dominant role of the phonon mechanism of superconductivity in such compounds is evident due to the measured isotope effect with deuterium (on LaD10), the small values of the effective Coulomb constant chosen in theoretical calculations of the critical temperature deserve further justification. We believe that the additional suppression of the Coulomb repulsion in superhydrides may be due to the appearance of additional acoustic plasmonic branches in their collective spectra. In LaH10 the conditions for such mechanism arise due to the hybridization of La 4f and H 1s states near the Fermi level in the vicinity of the L point of the Brillouin zone. We propose an analytical model approximation for the resulting conducting band and show that in a certain range of directions in quasimomentum space an acoustic branch should appear in the spectrum of the collective electronic excitations in LaH10. The contribution of these excitations to the suppression of the effective Coulomb constant is analyzed.

Plasmon modes in double-layer biased bilayer graphene

We investigate zero-temperature plasmon modes in a double-layer bilayer graphene structure under a perpendicular electrostatic bias. The numerical results demonstrate that there exist two collective modes which are undamped in the long wavelength limit. The finite potential bias decreases remarkably the plasmon energy in a wide range of wave-vector and makes plasmon branches become Landau damped at a higher wave-vector as compared to unbiased case. We find that the dependence of plasmon dispersions on the system parameters such as the inter-layer separation and carrier density is similar in two cases with and without electrostatic bias.

Flat and correlated plasmon bands in graphene/α−RuCl3 heterostructures

We develop a microscopic theory for plasmon excitations of graphene/${\alpha}$-RuCl${_3}$ heterostructures. Within a Kondo-Kitaev model with various interactions, a heavy Fermi liquid hosting flat bands emerges in which the itinerant electrons of graphene effectively hybridize with the fractionalized fermions of the Kitaev quantum spin liquid. We find novel correlated plasmon bands induced by the interplay of flat bands and interactions and argue that our theory is consistent with the available experimental data on graphene/${\alpha}$-RuCl${_3}$ heterostructures. We predict novel plasmon branches beyond the long-wavelength limit and discuss the implications for probing correlation phenomena in other flat band systems.

Collective excitations and quantum size effects on the surfaces of Pb(111) films: An experimental study* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11874404 and 11634016), the National Key Research and Development Program of China (Grant Nos. 2016YFA0302400, 2016YFA0202300, and 2017YFA0303600), and the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB33000000). Xuetao

Pb(111) film is a special system that exhibits strong quantum size effects in many electronic properties. The collective excitations, i.e., plasmons, in Pb(111) films are also expected to show signatures of the quantum size effect. Here, using high-resolution electron energy loss spectroscopy, we measured the plasmons on the surface of Pb(111) films with different film thicknesses and analyzed the plasmon dispersions. One surface plasmon branch exhibits prominent damping in the small momentum range, which can be attributed to the interaction between the top and bottom interfaces of the Pb(111) films. With the film thickness increasing, the critical momentum characterizing the damping in Pb(111) films decays not only much slower in Pb(111) films than in other metal films, and even in films with the thickness up to 40 monolayers the damping still exists. The slow decay of the surface plasmon damping, manifesting the strong quantum size effect in Pb(111) films, might be related to the strong nesting of the Fermi surface along the (111) direction.

Plasmons in two-dimensional topological insulators

We analyze collective excitations in models of two-dimensional topological insulators using the random phase approximation. In a two-dimensional extension of the Su-Schrieffer-Heeger model, edge plasmonic excitations with induced charge-density distributions localized at the boundaries of the system are found in the topologically non-trivial phase, dispersing similarly as one-dimensional bulk plasmons in the conventional Su-Schrieffer-Heeger chain. For two-dimensional bulk collective modes, we reveal regimes of enhanced inter-band wave function correlations, leading to characteristic hardening and softening of inter- and intra-band bulk plasmonic branches, respectively. In the two-dimensional Haldane Chern insulator model, chiral, uni-directional edge plasmons in nano-ribbon architectures are observed, which can be characterized by an effective Coulomb interaction cross section. Bulk collective excitations in the two-dimensional Haldane model are shown to be originated by single-particle band structure details in different topological phases.

Ultra-compact broadband 3-db metal–dielectric-metal plasmonic power splitter

Power splitters are considered essential components in optical interconnects. In this research work, two different designs of 1×2 3-dB small-footprint high-performance plasmonic power splitters with less than 1% loss in each branch are proposed and numerically investigated using the finite-difference-time-domain method. Based on the simulation results, we show that unequal splitting ratio can be achieved by breaking the symmetry of the two plasmonic branches either by changing the width or the position of one of the splitter branches with respect to the other one. Moreover, the proposed splitters’ designs have the advantage of improving the alignment tolerance of the two plasmonic branches with respect to the centre of the dielectric waveguide in addition to broadening the spectrum response of the splitter. Our proposed technique can be extended to 1×N power splitters in addition to being employed in various high density photonic integration and nano-plasmonic circuits such as directional couplers and Mach–Zehnder interferometers.

Resonant electron–plasmon interactions in drifting electron gas

In this paper, we investigate the resonant electron–plasmon interactions in a drifting electron gas of arbitrary degeneracy. The kinetic-corrected quantum hydrodynamic model is transformed into the effective Schrödinger–Poisson model, and the driven coupled pseudoforce system is obtained via separation of variables from the appropriately linearized system. It is noted that in the low phase–speed kinetic regime, the characteristic particle-like plasmon branch is significantly affected by the correction factor, which is a function of electron number density and temperature. It is shown that the electron current density of drifting electron gas sharply peaks at two distinct drift wavenumbers for a given value of electron density, temperature, plasmon energy, and damping parameter. The Fano-resonance of current density profile confirms the electron–plasmon resonant interaction in the presence of underlying interference effect. The electron drift current density shows fundamentally different resonance effects for plasmon energies with a wavenumber below and above a critical wavenumber. Moreover, an extension to the multistream model is presented, and the total current density of drifting electron gas in the presence of resonant electron–plasmon interactions is obtained. We further investigate the kinetic correction effect on matter-wave energy dispersion of the electron gas. It is also found that the increase in the electron number density leads to an increase in effective mass and consequently a decrease in electron mobility, whereas the increase in electron temperature has the converse effect. The kinetic correction is noted to significantly lower the quasiparticle conduction band minimum. The current model may be further elaborated to investigate the electron beam–plasma interactions.

Field-modulated photoninduced plasmons of armchair graphene nanoribbons

Band structures of armchair graphene nanoribbons (AGNRs) under crossed external fields are calculated within the tight-binding model. Magnetic field reduces energy-gap and flattens energy dispersions of band-edge states. Electric field strongly distorts energy dispersions leading to a reduction of energy-gap, a shift in band-edge states and many local maxima and minima. The complex dielectric function is further calculated in long-wavelength limit in which many peak-dip structures reveal characteristics of transparency and absorption. Furthermore, plasmon spectra are determined by frequency-position, sharpness, and number of peak-dip structures those are related to field strength and ribbon’s geometry. The field-induced extra transition channels could make a change in the frequency, peak-height, and number of branch of plasmons that can make AGNRs into a tunable terahertz nano-material. The photoninduced plasmons are further reflected in the edge-structures of reflectance spectra, unveiling the transition between transparency and absorption. Reflectance spectra strongly depend on external fields and ribbon’s geometry that will be verified by experimental measurements.

Plasmon modes in N-layer graphene structures at zero temperature

We investigate the plasmon frequency and the broadening function corresponding to Landau damping in a N-MLG structure consisting of N, up to 5, MLG sheets at zero temperature. Our results present that plasmon dispersions in the system include an in-phase optical mode and $$N - 1$$ out-of-phase acoustic ones. The optical (acoustic) plasmon frequency is higher (lower) than that of MLG at the same parameters. The broadening function of plasmon dispersions is quite similar to that in case of taking into account temperature effects. In addition, the increase in separation between layers increases (decreases) acoustic (optical) plasmon frequency, making plasmon curves become identical at smaller wave vector. Besides, the imbalance in carrier density in graphene sheets affects significantly on plasmon frequencies and plasmon pattern because it separates some plasmon branches far away from the others. However, the number of plasmon branches separated from the others depends mainly on the number of layers containing different carrier density but is independent of the order of these layers in the system.

Many-body effects and optical properties of single and double layer α- lattices

An extensive analytical and numerical investigation has been carried out to examine the role played by many-body effects on various α- materials under an off-resonance optical dressing field. Additionally, we explore its dependence on the hopping parameter α as well as the electron–light coupling strength λ0. The obtained dressed states due to mutual interaction between Dirac electrons and incident light are shown to demonstrate rather different electronic and optical properties in comparison with those in the absence of incident light. Specifically, various collective transport and optical properties of these electron dressed states are discussed in detail and compared for both single- and double layer α- lattices. All of these novel properties are due to the presence of a middle flat band and the interband transitions between it and an upper conduction band. Also, coupled plasmon dispersions for interacting double layer α- lattices are calculated, revealing a lower acoustic-like plasmon branch with tunable group velocity determined by both the layer separation and Fermi energy of each layer. Finally, a many-body theory is presented within the random-phase approximation for calculating the optical absorbance of doped multi-layered α- lattices in a linearly-polarized light field. We anticipate that the discoveries reported here could impact the design of the next-generation nano-optical and nano-plasmonic devices.

Understanding plasmon dispersion in nearly free electron metals: Relevance of exact constraints for exchange-correlation kernels within time-dependent density functional theory

Small-wavevector excitations in Coulomb-interacting systems can be decomposed into the high-energy collective longitudinal plasmon and the low-energy single-electron excitations. At the critical wavevector and corresponding frequency where the plasmon branch merges with the single-electron excitation region, the collective energy of the plasmon dissipates into single electron-hole excitations. The jellium model provides a reasonable description of the electron-energy-loss spectrum (EELS) of metals close to the free-electron limit. The random phase approximation (RPA) is exact in the high-density limit but can capture the plasmonic dispersion reasonably even for densities with rs > 1. RPA and all beyond-RPA methods investigated here, result in a wrong infinite plasmon lifetime for a wavevector smaller than the critical one where the plasmon dispersion curve runs into particle-hole excitations. Exchange-correlation kernel corrections to RPA modify the plasmon dispersion curve. There is however a large difference in the construction and form of the kernels investigated earlier. Our current work introduces recent model exchange-only and exchange-correlation kernels and discusses the relevance of some exact constraints in the construction of the kernel. We show that, because the plasmon dispersion samples a range of wavevectors smaller than the range sampled by the correlation energy, different kernels can make a strong difference for the correlation energy and a weak difference for the plasmon dispersion. This work completes our understanding about the plasmon dispersion in realistic metals, such as Cs, where a negative plasmon dispersion has been observed. We find only positive plasmon dispersion in jellium at the density for Cs.

Plasmon modes in double-layer gapped graphene at zero temperature

Plasmon dispersions and damping rate of plasma oscillations in a double-layer gapped graphene made of two parallel mono layer gapped graphene sheets grown on dielectric separation are calculated within random-phase-approximation at zero temperature. By using long wavelength limit expansion, analytical expressions for optical and acoustic plasmon frequencies have been formed, and the formulae demonstrate that the considerable difference in analytical form for plasmon frequencies comes from the factor depending on the band gap, compared to gapless situation. Numerical results show that only large band gap decreases remarkably plasmon frequencies of two modes in the range of large wave vector. Acoustic plasmon branch becomes shorter than that in case of zero gap while optical one seems independent with small band gap. In addition, interlayer separation and carrier density affect on collective excitations and damping rate when taking into account the band gap quite similarly to those in case of zero gap.

Design-controlled synthesis of IrO2 sub-monolayers on Au nanoflowers: marrying plasmonic and electrocatalytic properties.

We develop herein plasmonic-catalytic Au-IrO2 nanostructures with a morphology optimized for efficient light harvesting and catalytic surface area; the nanoparticles have a nanoflower morphology, with closely spaced Au branches all partially covered by an ultrathin (1 nm) IrO2 shell. This nanoparticle architecture optimizes optical features due to the interactions of closely spaced plasmonic branches forming electromagnetic hot spots, and the ultra-thin IrO2 layer maximizes efficient use of this expensive catalyst. This concept was evaluated towards the enhancement of the electrocatalytic performances towards the oxygen evolution reaction (OER) as a model transformation. The OER can play a central role in meeting future energy demands but the performance of conventional electrocatalysts in this reaction is limited by the sluggish OER kinetics. We demonstrate an improvement of the OER performance for one of the most active OER catalysts, IrO2, by harvesting plasmonic effects from visible light illumination in multimetallic nanoparticles. We find that the OER activity for the Au-IrO2 nanoflowers can be improved under LSPR excitation, matching best properties reported in the literature. Our simulations and electrocatalytic data demonstrate that the enhancement in OER activities can be attributed to an electronic interaction between Au and IrO2 and to the activation of Ir-O bonds by LSPR excited hot holes, leading to a change in the reaction mechanism (rate-determinant step) under visible light illumination.

Plasmon modes in N-layer gapped graphene

We investigate plasmon dispersions and Landau damping in multi-layer gapped graphene structures consisting of N, up to 5, gapped graphene sheets grown on dielectric spacer within random-phase-approximation at zero temperature. First of all, the results show that one in-phase optical and N-1 out-of-phase acoustic plasmon modes are found in the system. In addition, the imbalance of carrier density and the separation between layers affect remarkably on plasmon characters, and plasmon frequencies increase pronouncedly with the increase in the number of layers in the structure. Especially, the increasing band gap leads to the significantly decreasing plasmon frequencies in large wave-vector region. Finally, the number of low carrier density graphene sheets is equal to the number of plasmon branches separated from the others whereas the order of these layers affects weakly on plasmon curves.