Acoustic Plasmons Research Articles

Broadband focusing of acoustic plasmons in graphene with an applied current

Non-reciprocal plasmons in current-driven, isotropic, and homogenous graphene with proximal metallic gates is theoretically explored. Nearby metallic gates screen the Coulomb interactions, leading to linearly dispersive acoustic plasmons residing close to its particle-hole continuum counterpart. We show that the applied bias leads to spectral broadband focused plasmons whose resonance linewidth is dependent on the angular direction relative to the current flow due to Landau damping. We predict that forward focused non-reciprocal plasmons are possible with accessible experimental parameters and setup.

Prominence of Terahertz Acoustic Surface Plasmon Excitation in Gas–Surface Interaction with Metals

The current understanding of the dynamics of gas-surface interactions is that all of the energy lost in the collision is transferred to vibrations of the target. Electronic excitations were shown to play a marginal role except for cases in which the impinging particles have energies of several electronvolts. Here we show that this picture does not hold for metal surfaces supporting acoustic surface plasmons. Such loss, dressed with a vibronic structure, is shown to make up a prominent energy transfer route down to the terahertz region for Ne atoms scattering off Cu(111) and is expected to dominate for most metals. This mechanism determines, e.g., the drag force acting on telecommunication satellites, which are typically gold-plated to reduce overheating by sunshine. The electronic excitations can be unambiguously discerned from the vibrational ones under mild hyperthermal impact conditions.

Acoustic graphene plasmon resonator based on gold nanowire arrays.

An acoustic graphene plasmon (AGP) resonator based on gold nanowire arrays structure is proposed in this paper. The resonator is designed by continuous graphene layer/gold nanowire arrays/optical resonant cavity. The infrared light excites the AGP in the graphene/gold arrays structure and propagates along the graphene surface. The coupling efficiency can be improved by the optical resonator cavity. The finite-difference time-domain method is used to simulate and optimize the property of the resonator. The results show that the resonator has a stronger optical limiting effect and higher coupling efficiency. The AGPs are a prospective platform that enhances light-matter interactions, reduces spread loss, and exhibits a double resonance absorption phenomenon in the studied mid-infrared wavelength range. The research results provide a basis for the design of optoelectronic devices and more.

Acoustic spoof surface plasmon polaritons for filtering, isolation and sensing

We study a waveguide structure supporting acoustic spoof plasmon polaritons (aSSPPs) perturbed by a defect, whose specifically tailored geometry enables controllable transmission characterized by a uniform phase distribution and very steep narrowband response. The structure is analyzed using transmission-line theory and numerical simulations, providing evidence for its use in advanced filters, isolators and sensor technology. In order to demonstrate the applicability of the aSSPP waveguide with defect, two selective narrowband filter designs are discussed and explored. Furthermore, we propose an acoustic isolator that exploits steady fluid flow to break reciprocity and provide large isolation in a narrowband region. We also propose a sensor for liquid analytes, in which the grooves of the aSSPP waveguide serve as microfluidic channels, while the sensing principle is based on the spectral shift of the transmission peak for different mixtures of water and glycerol. The sensor shows a good sensitivity and fast response, with a potential for further development for applications in water quality monitoring.

Tunable plasmon modes in doped AA-stacked bilayer graphene

We study plasmon modes in doped AA-stacked bilayer graphene (BLG) within the nearest-neighbor tight-binding and the random phase approximation. We obtain closed analytical expressions for the polarizability function which are used to obtain the low-energy dispersion relations of and the numerical results for both acoustic and optical plasmon modes. Our result reveal the potential of AA-stacked BLG to be used as a tunable plasmonic device. In particular we find that the long-wavelength acoustic plasmon disperse as $\omega_{+}\approx\sqrt{max(|\mu|,t_{1})q}$ with a phase space which shrinks and vanishes as the chemical potential approaches the interlayer hopping energy, preventing the existence of long-lived acoustic plasmon. Furthermore, we show that AA-stacked BLG support coherent optical plasmon only when the condition $(1+\frac{g_{\sigma}g_{v}e^{2}t_{1}d}{\kappa v_{F}^{2}}\frac{|\mu|}{t_{1}})^{1/2}<\frac{|\mu|}{t_{1}}$ is satisfied, specially indicating Landau damping of the optical plasmon in undoped AA-staked BLG even at long-wavelength limit. We also find that the optical plasmon mode disperses as $\omega_{-}\approx \Delta+Cq^{2}$ with constants that can be tuned by tuning the chemical potential.

Undamped plasmon modes and enhanced superconductivity in metal diborides

The anisotropic geometric and electronic structures of metal diborides (MB2) leads to many unusual optical and electronic properties. Here, we present a first-principles study on the low-energy collective electronic excitations and the superconducting performance of the MB2 (M = Be, Mg, Al, and Ca). We demonstrate the undamped cosine-like plasmon modes and the sine-like acoustic plasmons in these metal diborides. Interestingly, the energy of the acoustic plasmons shows a positive correlation with the E 2g phonon modes which are negatively correlated with the superconducting transition temperature (T c) of the MB2 materials. Moreover, hole doping can lower the energy of the acoustic plasmons and enhance the T c of CaB2 up to 48.2 K. Our work offers a theoretical guidance to regulate the plasmonic properties, as well as a new predictive factor for superconductivity.

Superlattice on the surface of a nanotube

The results of theoretical studies of the thermodynamic, kinetic, and high-frequency properties of the electron gas on the surface of a nanotube in a magnetic field in the presence of a longitudinal superlattice are presented. Nano-dimensions of the motion area lead to energy quantization. Its multiply connected structure in the presence of a magnetic field leads to effects that are derived from the Aharonov–Bohm effect. It is shown that the curvature of a nanotube, even in the absence of a magnetic field, causes new macroscopic oscillation effects such as de Haas–van Alphen oscillations, which are associated with the quantization of the transverse electron motion energy and with the root peculiarities of the density of electron states on the nanotube surface. Thermodynamic potentials and heat capacity of the electron gas on the tube are calculated in the gas approximation. The Kubo formula for the conductivity tensor of the electron gas on the nanotube surface is obtained. The Landau damping regions of electromagnetic waves on a tube are determined and the beats are theoretically predicted on the graph of the dependence of conductivity on tube parameters. In the hydrodynamic approximation, the plasma waves on the surface of a semiconductor nanotube with a superlattice are considered. It is shown that optical and acoustic plasmons can propagate along a tube with one kind of carrier. Electron spin waves on the surface of a semiconductor nanotube with a superlattice in a magnetic field are studied. The spectra and areas of collisionless damping of these waves are found. We have shown that the spin wave damping is absent in these areas if the tubes with a degenerate electron gas have small radius.

Quantum surface-response of metals revealed by acoustic graphene plasmons

A quantitative understanding of the electromagnetic response of materials is essential for the precise engineering of maximal, versatile, and controllable light–matter interactions. Material surfaces, in particular, are prominent platforms for enhancing electromagnetic interactions and for tailoring chemical processes. However, at the deep nanoscale, the electromagnetic response of electron systems is significantly impacted by quantum surface-response at material interfaces, which is challenging to probe using standard optical techniques. Here, we show how ultraconfined acoustic graphene plasmons in graphene–dielectric–metal structures can be used to probe the quantum surface-response functions of nearby metals, here encoded through the so-called Feibelman d-parameters. Based on our theoretical formalism, we introduce a concrete proposal for experimentally inferring the low-frequency quantum response of metals from quantum shifts of the acoustic graphene plasmons dispersion, and demonstrate that the high field confinement of acoustic graphene plasmons can resolve intrinsically quantum mechanical electronic length-scales with subnanometer resolution. Our findings reveal a promising scheme to probe the quantum response of metals, and further suggest the utilization of acoustic graphene plasmons as plasmon rulers with ångström-scale accuracy.

Temperature effects on plasmon modes in double-bilayer graphene structures

This paper aims to investigate collective excitations in a double-layer structure consisting of two parallel bilayer graphene sheets within random-phase-approximation. We determine the collective excitations in the system from the zeroes of the temperature-dependent dynamical dielectric function. Numerical results demonstrate that both optical in-phase and acoustic out-of-phase plasmon modes are found in the system, similar to other double-layer structures. Calculations prove that plasmon frequencies increase with the increase in temperature while the imbalance in doping density between two bilayer graphene sheets as well as the increase in carrier density decrease these frequencies. Similar to the case of zero temperature, increasing separation increases (decreases) acoustic (optical) plasmon frequency. Our computations illustrate that temperature affects remarkably and can reduce the effects of other factors on plasmon properties in double bilayer graphene systems.

Topological Graphene Plasmons in a Plasmonic Realization of the Su–Schrieffer–Heeger Model

Graphene hybrids, made of thin insulators, graphene, and metals can support propagating acoustic plasmons (AGPs). The metal screening modifies the dispersion relation of usual graphene plasmons leading to slowly propagating plasmons, with record confinement of electromagnetic radiation. Here, we show that a graphene monolayer, covered by a thin dielectric material and an array of metallic nanorods can be used as a robust platform to emulate the Su-Schrieffer-Heeger model. We calculate the Zak's phase of the different plasmonic bands to characterise their topology. The system shows bulk-edge correspondence: strongly localized interface states are generated in the domain walls separating arrays in different topological phases. We find signatures of the nontrivial phase which can directly be probed by far-field mid-IR radiation, hence allowing a direct experimental confirmation of graphene topological plasmons. The robust field enhancement, highly localized nature of the interface states, and their gate-tuned frequencies expand the capabilities of AGP-based devices.

Optical response of noble metal nanostructures: quantum surface effects in crystallographic facets

Noble metal nanostructures are ubiquitous elements in nano-optics, supporting plasmon modes that can focus light down to length scales commensurate with nonlocal effects arising from quantum confinement and spatial dispersion in the underlying electron gas. Quantum and nonlocal effects can be more prominent in crystalline noble metals, due to their lower intrinsic loss (when compared with their polycrystalline counterparts), and because particular crystal facets give rise to distinct electronic surface states whose signatures can be imprinted in the optical response of a structure. Here, we employ an atomistic method to describe nonclassical effects impacting the optical response of crystalline noble metal surfaces and demonstrate that these effects can be well captured using a set of surface-response functions known as Feibelman d -parameters determined from such quantum-mechanical models. In particular, we characterize the d -parameters associated with the (111) and (100) crystal facets of gold, silver, and copper, emphasizing the importance of quantum surface effects associated with electron wave function spill-out/spill-in and with the surface-projected band gap emerging from the atomic-layer corrugation. Furthermore, we show that the extracted d -parameters can be straightforwardly applied to describe the optical response of various nanoscale metal morphologies of interest, including metallic ultrathin films, graphene–metal heterostructures hosting ultraconfined acoustic graphene plasmons, and crystallographic faceted metallic nanoparticles supporting localized surface plasmons. We envision that the d -parameters presented here, along with the prescription to extract and apply them, could help circumvent computationally expensive first-principles atomistic calculations to describe quantum nonlocal effects in the optical response of mesoscopic crystalline metal surfaces, which are becoming widely available with increasing control over morphology down to atomic length scales for state-of-the-art experiments in nano-optics.

Effects of the Dirac cone tilts on coupled plasmon modes in electron systems with different dimensionalities

We study the effects of the Dirac cones tilts on coupled plasmon modes arising from the Coulomb electron–electron interaction for a system consisting of a two-dimensional (2D) system of Dirac electrons embedded in a three-dimensional (3D) Dirac semimetal. We particularly take into account the long-range Coulomb electron–electron interaction between electrons from two subsystems with the tilted Dirac cone anisotropic energy band dispersions and obtain the resulting coupled optical and acoustic plasmon modes from the dielectric functions of the subsystems for their various separations. The tilts of Dirac cones anisotropically increase the coupled plasmon mode energies and renormalize their spectral weights compared to the results for the untilted energy band dispersions. We demonstrate that the effects of tilts are more pronounced for the larger separations of the subsystems when their Coulomb coupling is weaker.

Acoustic plasmons in type-I Weyl semimetals

Massless chiral fermions emergent in inversion symmetry-breaking Weyl semimetals (WSMs) reside in the vicinity of multiple low symmetry nodes and thus acquire strongly anisotropic dispersion. We investigate the longitudinal electromagnetic modes of two-component degenerate Weyl plasma relevant to the realistic band structure of type-I WSM. We show that the actual spectrum of three dimensional collective density excitations in TaAs family of WSM is gaples due to emergence of acoustic plasmons corresponding to out-of-phase oscillations of the plasma components. These modes exist around the [001] crystallographic direction and are weakly damped, thanks to large difference in the Weyl velocities of the $W_1$ and $W_2$ quasiparticles when propagating along [001]. We show that acoustic plasmons can manifest themselves as slow beatings of electric potential superimposed on fast plasmonic oscillations upon charge relaxation. The revealed acoustic modes can stimulate purely electronic superconductivity, collisionless plasmon instabilities, and formation of Weyl soundarons.

Polyoxazolines with a Vicinally Double-Bioactivated Terminus for Biomacromolecular Affinity Assessment.

Interactions between proteins and carbohydrates with larger biomacromolecules, e.g., lectins, are usually examined using self-assembled monolayers on target gold surfaces as a simplified model measuring setup. However, most of those measuring setups are either limited to a single substrate or do not allow for control over ligand distance and spacing. Here, we develop a synthetic strategy, consisting of a cascade of a thioesterification, native chemical ligation (NCL) and thiol-ene reaction, in order to create three-component polymer conjugates with a defined double bioactivation at the chain end. The target architecture is the vicinal attachment of two biomolecule residues to the α telechelic end point of a polymer and a thioether group at the ω chain end for fixating the conjugate to a gold sensor chip surface. As proof-of-principle studies for affinity measurements, we demonstrate the interaction between covalently bound mannose and ConA in surface acoustic wave (SAW) and surface plasmon resonance (SPR) experiments.

Coulomb drag in metal monochalcogenides double-layer structures with Mexican-hat band dispersions

We theoretically study the Coulomb drag resistivity and plasmon modes behavior for a system composed of two parallel p-type doped GaS monolayers with Mexican-hat valence energy band using the Boltzmann transport theory formalism. We investigate the effect of temperature, T, carrier density, p, and layer separation, d, on the plasmon modes and drag resistivity within the energy-independent scattering time approximation. Our results show that the density dependence of plasmon modes can be approximated by p 0.5. Also, the calculations suggest a d 0.2 and a d 0.1 dependencies for the acoustic and optical plasmon energies, respectively. Interestingly, we obtain that the behavior of drag resistivity in the double-layer metal monochalcogenides swings between the behavior of a double-quantum well system with parabolic dispersion and that of a double-quantum wire structure with a large carrier density of states. In particular, the transresistivity value reduces exponentially with increasing the distance between layers. Furthermore, the drag resistivity changes as T 2/p 4 (T 2.8/p 4.5) at low (intermediate) temperatures. Finally, we compare the drag resistivity as a function of temperature for GaS with other Mexican-hat materials including GaSe and InSe and find that it adopts higher values when the metal monochalcogenide has smaller Mexican-hat height.

Acoustic Plasmons in Graphene Sandwiched between Two Metallic Slabs

We study the effect of two metallic slabs on the collective dynamics of electrons in graphene positioned between the two slabs. We show that if the slabs are perfect conductors, the plasmons of graphene display a linear dispersion relation. The velocity of these acoustic plasmons crucially depends on the distance between the two metal gates and the graphene sheet. In the case of generic slabs, the dispersion relation of graphene plasmons is much more complicated, but we find that acoustic plasmons can still be obtained under specific conditions.

Observation of acoustic plasma waves with a velocity approaching the speed of light

We investigate the electrodynamics of a disk-shaped two-dimensional electron system in the proximity of a screening gate. The two-dimensional plasmons in the system under study exhibit the linear dispersion characteristic of acoustic plasma waves. We experimentally achieve the limit when the velocity of acoustic waves approaches the speed of light. We show that retardation effects lead to strong coupling between acoustic plasmons and the light, which is manifested in the renormalization of the electron effective mass. We develop a theory to substantiate the observed phenomena.

Anisotropic electronic and plasmonic properties of 2H-TiS2

Plasmon characteristics in a monolayer of 2H-TiS2 in the presence of strain are carefully analyzed by employing the density functional theory along with random phase approximation. The anisotropic properties of 2H-TiS2 are carefully analyzed under biaxial strains. An anisotropic semi-Dirac cone in the electronic band-structure is observed at a specific biaxial strain configuration. This unique characteristic can be explained by the hybridization of orbitals at the conduction band minimum and valence band maximum which can be effectively tuned by applying strain. The effect of anisotropic electronic band-structure on the properties of plasmons is investigated by utilizing the electron energy loss spectrum analysis. A dispersion along the Γ−K direction and a linear dispersion along the were found. The linear dispersion characterizes an extrinsic acoustic plasmon mode that can be tuned by the carrier concentration.

Real-space imaging of acoustic plasmons in large-area graphene grown by chemical vapor deposition

An acoustic plasmon mode in a graphene-dielectric-metal structure has recently been spotlighted as a superior platform for strong light-matter interaction. It originates from the coupling of graphene plasmon with its mirror image and exhibits the largest field confinement in the limit of a sub-nm-thick dielectric. Although recently detected in the far-field regime, optical near-fields of this mode are yet to be observed and characterized. Here, we demonstrate a direct optical probing of the plasmonic fields reflected by the edges of graphene via near-field scattering microscope, revealing a relatively small propagation loss of the mid-infrared acoustic plasmons in our devices that allows for their real-space mapping at ambient conditions even with unprotected, large-area graphene grown by chemical vapor deposition. We show an acoustic plasmon mode that is twice as confined and has 1.4 times higher figure of merit in terms of the normalized propagation length compared to the graphene surface plasmon under similar conditions. We also investigate the behavior of the acoustic graphene plasmons in a periodic array of gold nanoribbons. Our results highlight the promise of acoustic plasmons for graphene-based optoelectronics and sensing applications.

Evidence for a spin acoustic surface plasmon from inelastic atom scattering

Closed-shell atoms scattered from a metal surface exchange energy and momentum with surface phonons mostly via the interposed surface valence electrons, i.e., via the creation of virtual electron-hole pairs. The latter can then decay into surface phonons via electron-phonon interaction, as well as into acoustic surface plasmons (ASPs). While the first channel is the basis of the current inelastic atom scattering (IAS) surface-phonon spectroscopy, no attempt to observe ASPs with IAS has been made so far. In this study we provide evidence of ASP in Ni(111) with both Ne atom scattering and He atom scattering. While the former measurements confirm and extend so far unexplained data, the latter illustrate the coupling of ASP with phonons inside the surface-projected phonon continuum, leading to a substantial reduction of the ASP velocity and possibly to avoided crossing with the optical surface phonon branches. The analysis is substantiated by a self-consistent calculation of the surface response function to atom collisions and of the first-principle surface-phonon dynamics of Ni(111). It is shown that in Ni(111) ASP originate from the majority-spin Shockley surface state and are therefore collective oscillation of surface electrons with the same spin, i.e. it represents a new kind of collective quasiparticle: a Spin Acoustic Surface Plasmon (SASP).