Collective Plasmon Excitations Research Articles

Dynamical response function in sodium studied by inelastic x-ray scattering spectroscopy

We present the dynamic structure factor $S(Q,\ensuremath{\omega})$ of Na valence electrons in the range of momentum transfer $0.5{k}_{F}<Q<2.4{k}_{F}$ and energy transfer 3 eV $<\ensuremath{\omega}<$ 30 eV determined by inelastic x-ray scattering spectroscopy. In this range, we observe how the collective plasmon excitations decay into the single-particle excitation continuum. We compare the results to calculations using time-dependent density-functional theory with different approximations. The failure of both random-phase approximation and time-dependent local-density approximation (TDLDA) is shown to become important at ${k}_{F}<Q<2.4{k}_{F}$, while TDLDA with an additional inclusion of quasiparticle lifetime effects reproduces the experimental spectra well. The experimental valence-electron response reaches the single-particle spectrum surprisingly early, at $Q\ensuremath{\approx}1.5{k}_{F}$. This is manifested both in the spectral shape and the peak dispersion. The experimental spectra are nearly free of any fine structure, confirming that the peak-shoulder structure observed in many other materials is due to band-structure effects, which turn out to be negligible in Na.

Plasmon Spectra at Wurtzite Aluminum Gallium Nitride / Silicon Carbide Heterojunctions

AbstractWe present theoretical calculations for electron plasmon spectra at wurtzite Aluminum Gallium Nitride/ Silicon Carbide heterojunctions. Spontaneous and piezoelectric polarizations in wurtzite semiconductors give rise to polarization discontinuities at interfaces and to bound interface sheet charges. These charges are of the order of 1013 electrons per cm2 and give rise to two dimensional electron or hole gases near heterojunctions. Electron-electron interactions in the two-dimensional electron gases give rise to collective plasmon excitations. We calculate the dielectric function in these electron gases under the well-known and widely studied random phase approximation. Our calculations are relevant to the determination of the plasmon spectra at wurtzite Aluminum Gallium Nitride / Silicon Carbide heterojunctions and are of potential interest for determining the limits of mobility in two-dimensional electron gases. They are also of interest for terahertz electronics applications.

Dual wavelength sensing based on interacting gold nanodisk trimers

Fabrication and surface plasmon properties of gold nanostructures consisting of periodicarrays of disk trimers are reported. Using electron beam lithography, disk diameters assmall as 96 nm and gaps between disks as narrow as 10 nm have been achieved with anunprecedented degree of control and reproducibility. The disk trimers exhibit intense visibleand infrared surface plasmon resonances which are studied as a function of the diskdiameter and of the pitch between trimers. Based on simulations of the optical extinctionspectra and of the electric near-field intensity maps, the resonances are assigned to asingle trimer response and to collective surface plasmon excitations involvingelectromagnetic interaction between the trimers. The sensing properties of thedisk trimers are investigated using various coating media. The reported resultsdemonstrate the possible use of gold disk trimers for dual wavelength chemical sensing.

Theoretical analysis of the momentum-dependent loss function of bulk Ag

We analyze the momentum-dependent loss function of bulk silver, calculated via the random-phase approximation and based on the band structure from semi-local density functional theory calculations. In the energy range of 0 – 55 eV the spectrum reveals the existence of seven well-pronounced peaks, in accordance with experiment. The two lowest of these are collective plasmon excitations, while the higher ones originate from inter-band transitions. Due to the different nature of those peaks, they display a different dependence on the momentum transfer q. While the plasmon peaks show an approximately quadratic q-dependence, the peaks caused by inter-band transitions are characterized by a much less pronounced dispersion.

Extraordinary midinfrared transmission of rectangular coaxial nanoaperture arrays

We analyze the extraordinary transmission effect in the periodic and random nanoaperture arrays in optically thick metal films. Experimental studies are combined with simulations to show that extraordinary transmission effect at midinfrared wavelengths is enhanced when the decaying waveguide mode of a single nanoaperture couples through the collective surface plasmon excitations created through periodicity. Transmission characteristics of the individual nanoapertures are investigated by randomizing the apertures. By comparing rectangular to coaxial rectangular nanoapertures, both of which support TE01-like decaying waveguide modes, we study the effect of nanoaperture geometry on the efficiency of the transmission.

Photoabsorption in sodium clusters on the basis of time-dependent density-functional theory

The photoabsorption spectra of a continuous series of Na(n) clusters (n<or=14, n=20, n=40) have been calculated using a time-dependent density-functional scheme. Accordingly, we present these spectra and show that they are in very good agreement with other theoretically and experimentally obtained photoabsorption spectra. Furthermore, we discuss the influence of the cluster structure on the photoabsorption spectrum for some selected clusters and present for several cluster sizes photoabsorption spectra of different geometrical isomers. The spectra of clusters with five or more atoms are dominated by a few large peaks which can be interpreted as collective plasmon excitations.

Ensembles of plasmonic nanospheres at optical frequencies and a problem of negative index behavior

Arrays of metallic nanoparticles support individual and collective plasmonic excitations that contribute to unusual phenomena like surface enhanced Raman scattering, anomalous transparency, negative index, and subwavelength resolution in various metamaterials. We have examined the electromagnetic response of dual Kron's lattice and films containing up to three monolayers of metallic nanospheres. It appears that open cubic Kron's lattice exhibits `soft' electromagnetic response but no negative index behavior. The close-packed arrays behave similarly: there are plasmon resonances and very high transmission at certain wavelengths that are much larger than the separation between the particles, and a `soft' magnetic response, with small but positive effective index of refraction. It would be interesting to check those predictions experimentally.

Multiple scattering of charged particles in a free-electron gas

Fil: Archubi, Claudio Dario. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Instituto de Astronomia y Fisica del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomia y Fisica del Espacio; Argentina. Comision Nacional de Energia Atomica. Gerencia del Area de Energia Nuclear. Instituto Balseiro; Argentina. Comision Nacional de Energia Atomica. Gerencia del Area de Investigacion y Aplicaciones No Nucleares. Gerencia de Fisica (Centro Atomico Bariloche); Argentina

Surface plasmon in metallic nanoparticles: Renormalization effects due to electron-hole excitations

The electronic environment causes decoherence and dissipation of the collective surface plasmon excitation in metallic nanoparticles. We show that the coupling to the electronic environment influences the width and the position of the surface plasmon resonance. A redshift with respect to the classical Mie frequency appears in addition to the one caused by the spill-out of the electronic density outside the nanoparticle. We characterize the spill-out effect by means of a semiclassical expansion and obtain its dependence on temperature and the size of the nanoparticle. We demonstrate that both, the spill-out and the environment-induced shift are necessary to explain the experimentally observed frequencies and confirm our findings by time-dependent local density approximation calculations of the resonance frequency. The size and temperature dependence of the environmental influence results in a qualitative agreement with pump-probe spectroscopic measurements of the differential light transmission.

Unconventional metal-insulator transition in two dimensions

We show, by using a correlated Jastrow wave function and a mapping onto a classical model, that the two-dimensional Mott transition in a simple half-filled one-band model can be unconventional and very similar to the binding-unbinding Kosterlitz-Thouless transition of vortices and anti-vortices, here identified by empty and doubly occupied sites. Within this framework, electrons strongly interact with collective plasmon excitations that induce anomalous critical properties on both sides of the transition. In particular, the insulating phase is characterized by a singular power law behavior in the photoemission spectrum, that can be continuously connected to the fully-projected insulating state, relevant to strongly correlated low-energy models.

Linear-response study of plasmon excitation in metallic thin films: Layer-dependent hybridization and dispersion

We present a theoretical study of collective plasmon excitation in metal thin films using the jellium model. The excitation spectra are calculated in the linear response theory and the time-dependent local density approximation. The evolution from surface plasmons at large thickness to the hybridized thin film plasmons at smaller thickness is obtained as functions of atomic layers and electron momenta. The energies of the hybridized plasmons follow qualitatively the classical electrodynamical model at large to intermediate thickness. For ultrathin films with a few atomic layers, these plasmon resonances evolve into intraband and interband transitions at small momenta. The latter results from the quantized electron states normal to the films.

Emergence of collective plasmon excitation in a confined one-dimensional electron gas

We present a theoretical study of the electronic excitation in a confined one-dimensional electron gas, which is utilized to model atomic chains created in recent experiments. The length dependence of the excitation spectra is obtained from the linear response theory within the random phase approximation and time-dependent density functional theory. As the length of the chains increases, the dipole excitation spectra show a transition from electron-hole pair excitations to collective plasmon excitation. The trend of the length-dependent plasmon resonance is predicted, and the nature of the plasmon resonance is also elaborated.

The effect of salt concentration on the optical modes of charged cylindrical nanotubes

We have conducted calculations of the collective plasmon excitations for an electron gas confined to the surface of a charged single-walled cylindrical nanotube in salt solutions. Both positively and negatively charged nanotubes are investigated. At high salt concentration, the surface potential approaches zero, and the spectrum is close to that of a neutral nanotube. The highest-frequency branch of the plasmon excitation spectrum exhibits a redshift and a blueshift for negatively and positively charged nanotubes, respectively. Such a result can be attributed to the change of the number of eigenstates around the Fermi energy. As salt concentration is increased, the surface potential is screened out, and the spectral line shift diminishes. Our results also show that the negatively charged nanotube can be more sensitive to the ambient salt concentration than the positively charged one. The optimal sensor device occurs when the nanotube carries a linear charge density close to that of DNA. The theoretical predictions suggest that charged nanotubes have the potential as sensors to probe salt concentration.

Photon emission from silver particles induced by a high-energy electron beam

Photon emission spectra from submicron silver particles induced by an electron beam have been measured using a light detection system combined with a 200-kV transmission electron microscope. Multiple peaks appear in the spectra associated with collective plasmon excitations produced in the particles by the incident electrons. The wavelengths of these peaks are observed to shift towards larger values with increasing particle diameter, as predicted by Mie theory. Moreover, photon maps have been obtained in a scanning mode and they indicate that those peaks correspond to the multipole modes of electromagnetic oscillations in metallic spheres (Mie resonances). The spectral shape of the emission and the dependence on particle size, impact parameter, and electron energy are well explained from theoretical calculations of the photon emission probability derived from a fully retarded analytical treatment of the interaction of fast electrons with metallic spheres.

The role of layer-thickness deviation in dispersion and structure of plasmons in finite superlattices

We study the effects of layer thickness variations on the collective plasmon excitation modes of finite superlattices. Unlike other symmetry lowering mechanisms, thickness variation does not strongly localize the surface modes. We find that the reason for this insensitivity lies in the fact that the collective modes of a given finite structure must evolve continuously from the single-finite-superlattice at zero thickness deviation into modes of a pair of uncoupled finite structures at large thickness variation. We also show that this behavior is analogous to the evolution of molecular orbitals from atomic orbitals as the internuclear separation is reduced, in contrast to the analogy of the superlattice modes as a stack of coupled quantum wells. This emphasizes the difference between the electromagnetic symmetry of the finite superlattice and the structural symmetry.

Calculation of plasmonic dispersion in quantum wires: angular degeneracy and shape-confinement effects

We present a random-phase approximation calculation of the collective plasmon excitation spectrum of a one-dimensional electron gas confined in cylindrical and rectangular semiconductor quantum wires. The plasmon spectrum for the cylindrical case is calculated considering up to three occupied subbands. Taking into account the twofold angular degeneracy of the single-particle energy spectrum, we derive different expressions for the electron-electron interaction when opposite angular quantum numbers are involved. That occurrence leads, even in a two-subband system, to the presence of new undamped plasmon modes for q → 0 not exhibited in previous work. Comparing plasmon modes between cylindrical and rectangular quantum wires, we find an investigable shape-confinement effect for the intrasubband plasma excitation in the lowest subband.

Surface plasmon enhanced substrate mediated photochemistry on roughened silver

The wavelength dependent photochemical cross sections for three adsorbates (OCS, NO, SO2) on roughened silver have been measured, and contrasted with the behavior on Ag(111). Surface roughness leads to significant enhancements of the photochemical cross sections for all three adsorbates. The enhancement exhibits a maximum at 350±5 nm. Competing enhancement mechanisms are considered. Temperature programmed desorption measurements show that new adsorption sites are available on the surface, but that these are not uniquely associated with the enhanced cross section. The coincidence of the peak enhancement for both photodissociation of OCS and photodesorption of NO and SO2 suggests a substrate mediated mechanism. It is proposed that the enhancement arises from surface plasmon excitation on the roughened surface. This mechanism may contribute to an enhanced cross section in two ways. First the collective surface plasmon excitation can decay to single particle, hot electron, excitations. The hot electrons so generated may attach to the adsorbates, to cause the photochemistry observed. Secondly, the enhanced local electric field at the surface can generate electron–hole pair excitations, which may also attach to the adsorbate.

Novel finite- q plasmons in p-type GaAs/Al xGa 1− xAs quantum wells

We present a calculation of collective plasmon excitations in p-type GaAs/AlGaAs quantum wells, which is based on the random phase approximation and takes, within the 4×4 k· p model, band-structure effects exactly into account. We predict that the strong dependence of the subband wave functions on the in-plane wave vector, which does not exist in n-type quantum wells, leads to the existence of a novel, additional inter-subband plasmon at finite wave vectors and to a strong coupling of the intra- and the inter-subband plasmons, even in symmetric wells.

Coherence control of currents in semiconductors: a materials perspective

The phase of optical beams can induce and control novel effects in solids via quantum interference. Our focus has primarily been on generation and control of photocurrents in bulk semiconductors using harmonically related beams and GaAs as a prototypical material. Here we consider a general bulk semiconductor and investigate how its material properties influence photocurrent generation and evolution. The main factors to consider are the current injection efficacy, determined by the current injection tensor, as well as optical and carrier dephasing effects. From a simple Kane band model and k· p perturbation theory the magnitude of the injection tensor is seen to scale with the band gap, E g , as E g −2. Lack of phase-matching between beams due to material dispersion can reduce the peak current by up to an order of magnitude but its influence differs significantly among such common semiconductors as Ge, GaAs, GaP and ZnSe and there is no direct correlation with E g . In general, the rate of carrier dephasing can be expected to increase with increasing band gap. Finally, we consider a simple treatment of the dynamics of the injected currents and identify dissipative and collective (plasmon) excitation regimes. From the k· p model we also provide insight into the initial k-space carrier distributions for excitation from heavy or light hole bands.

Plasmon behavior of Zn from electron-energy-loss spectroscopy

The momentum-dependent dielectric function of Zn was measured using electron-energy-loss spectroscopy (EELS) in transmission. The loss function in the low-energy region is found to be a superposition of a sharp absorption feature due to the excitation of electrons from the $3d$ level at a binding energy of 9.4 eV and a broad not well pronounced feature at about 14 eV, which is attributed to the collective plasmon excitation of the free charge carriers. This latter feature becomes more pronounced with increasing momentum transfer due to a strong decrease of the core-level oscillator strength. The decrease of oscillator strength leads to an initially vanishing dispersion of the plasmon, and to a reduction of the plasmon line width with increasing momentum transfer. This behavior can qualitatively be understood in the framework of a Drude-Lorentz approach.