Low-energy Plasmon Research Articles

Interplay between structure and electronic properties of layered transition-metal dichalcogenides: Comparing the loss function of1Tand2Hpolymorphs

Transition-metal dichalcogenides (TMD) share the same global layered structure, but distinct polymorphs are characterized by different local coordinations of the transition-metal atoms. Here we compared the $1T$ and $2H$ families of metallic TMD, both in the bulk and in the two-dimensional forms. By means of first-principles time-dependent density functional calculations of the loss function, we established the direct connection between the low-energy plasmon properties and the crystal-structure symmetry. The different atomic environments affect the $d\ensuremath{-}d$ electron-hole excitations, which are prominent at low energies, resulting in distinct in-plane plasmon dispersions in the two families. Conversely, the different periodicity of the plasmon reappearance along the $c$ axis perpendicular to the layers can be used to distinguish the various crystal structures of TMD.

Multimodal plasmonics in fused colloidal networks.

Harnessing the optical properties of noble metals down to the nanometer-scale is a key step towards fast and low-dissipative information processing. At the 10-nm length scale, metal crystallinity and patterning as well as probing of surface plasmon (SP) properties must be controlled with a challenging high level of precision. Here, we demonstrate that ultimate lateral confinement and delocalization of SP modes are simultaneously achieved in extended self-assembled networks comprising linear chains of partially fused gold nanoparticles. The spectral and spatial distributions of the SP modes associated with the colloidal superstructures are evidenced by performing monochromated electron energy loss spectroscopy with a nanometer-sized electron probe. We prepare the metallic bead strings by electron beam-induced interparticle fusion of nanoparticle networks. The fused superstructures retain the native morphology and crystallinity but develop very low energy SP modes that are capable of supporting long range and spectrally tunable propagation in nanoscale waveguides.

Examination of ion-induced Auger electron spectra of Ti, Cr and Co in a mass-selecting Focused Ion Beam with a gold–silicon liquid metal source

Ion induced Auger electron spectroscopy is a technique where Auger electrons are produced as a result of energetic ion impact. In this paper, the ion–induced electron spectra of three of the transition metals; Ti, Cr and Co by Si++ and Au+ ions accelerated to 30 and 60 keV are studied. Aside from the low energy plasmon peaks, sharp M23M45M45 Auger transitions with high signal to noise ratio attributed to the metal targets and, in the samples bombarded by Si++ ions, L23MM transition from Si incident ion are also detected. Broad tails next to the main metal Auger peaks are attributed to interatomic transitions between the incident ion and target ions.

Electron energy loss spectroscopy of gold nanoparticles on graphene

Plasmon excitation decay by absorption, scattering, and hot electron transfer has been distinguished from effects induced by incident photons for gold nanoparticles on graphene monolayer using electron energy loss spectroscopy (EELS). Gold nano-ellipses were evaporated onto lithographed graphene, which was transferred onto a silicon nitride transmission electron microscopy grid. Plasmon decay from lithographed nanoparticles measured with EELS was compared in the absence and presence of the graphene monolayer. Measured decay values compared favorably with estimated radiative and non-radiative contributions to decay in the absence of graphene. Graphene significantly enhanced low-energy plasmon decay, increasing mode width 38%, but did not affect higher energy plasmon or dark mode decay. This decay beyond expected radiative and non-radiative mechanisms was attributed to hot electron transfer, and had quantum efficiency of 20%, consistent with previous reports.

Plasmons in spatially separated double-layer graphene nanoribbons

Motivated by innovative progresses in designing multi-layer graphene nanostructured materials in the laboratory, we theoretically investigate the Dirac plasmon modes of a spatially separated double-layer graphene nanoribbon system, made up of a vertically offset armchair and metallic graphene nanoribbon pair. We find striking features of the collective excitations in this novel Coulomb correlated system, where both nanoribbons are supposed to be either intrinsic (undoped/ungated) or extrinsic (doped/gated). In the former, it is shown the low-energy acoustical and the high-energy optical plasmon modes are tunable only by the inter-ribbon charge separation. In the later, the aforementioned plasmon branches are modified by the added doping factor. As a result, our model could be useful to examine the existence of a linear Landau-undamped low-energy acoustical plasmon mode tuned via the inter-ribbon charge separation as well as doping. This study might also be utilized for devising novel quantum optical waveguides based on the Coulomb coupled graphene nanoribbons.

Toward 10 meV Electron Energy-Loss Spectroscopy Resolution for Plasmonics

Energy resolution is one of the most important parameters in electron energy-loss spectroscopy. This is especially true for measurement of surface plasmon resonances, where high-energy resolution is crucial for resolving individual resonance peaks, in particular close to the zero-loss peak. In this work, we improve the energy resolution of electron energy-loss spectra of surface plasmon resonances, acquired with a monochromated beam in a scanning transmission electron microscope, by the use of the Richardson-Lucy deconvolution algorithm. We test the performance of the algorithm in a simulated spectrum and then apply it to experimental energy-loss spectra of a lithographically patterned silver nanorod. By reduction of the point spread function of the spectrum, we are able to identify low-energy surface plasmon peaks in spectra, more localized features, and higher contrast in surface plasmon energy-filtered maps. Thanks to the combination of a monochromated beam and the Richardson-Lucy algorithm, we improve the effective resolution down to 30 meV, and evidence of success up to 10 meV resolution for losses below 1 eV. We also propose, implement, and test two methods to limit the number of iterations in the algorithm. The first method is based on noise measurement and analysis, while in the second we monitor the change of slope in the deconvolved spectrum.

Ab initioanalysis of plasmon dispersion in sodium under pressure

We present an ab initio study of the electronic response function of sodium in its five known metallic phases from 0 to 180 GPa at room temperature. The considered formalism is based on a interpolation scheme within time-dependent density functional theory that uses maximally localized Wannier functions, providing an accurate sampling of the reciprocal space. Besides showing an excellent agreement with inelastic x-ray scattering experiments [Phys. Rev. Lett. 107, 086402 (2011), Proc. Natl. Acad. Sci. USA 108, 20434 (2011)], our calculations reveal that the drastic decrease of the optical reflectivity measured in the high pressure phases oP8 and tI19 [Proc. Natl. Acad. Sci. USA 106, 6525 (2009)] is associated with a new low-energy plasmon arising from collective interband excitations. Additionally, our calculations predict the existence of an anisotropic interband plasmon in the stability pressure range of fcc Na (65 to 105 GPa).

Plasmon excitation in C60 fullerene dimers

Plasmon resonances in C60 fullerene dimers are investigated using time-dependent density functional theory. Owing to larger separation between molecules, there exist capacitive coupling plasmon modes in fullerene dimers. With the decrease of the gap distance, low-energy capacitive coupling plasmon modes show red shift. When the gap distance further decreases, because of the electrons tunneling across the dimer junction, plasmon resonance modes of C60 fullerene dimers are significantly modified, and the charge transfer plasmon modes occur. C60 fullerene dimer is different from metallic nanostructures dimmer. As the gap distance is again reduced, the charge transfer plasmon modes are not blue-shifted, but they are further red-shifted. In the range of the visible spectrum, C60 fullerene dimmers have strong absorption peaks.

Plasmons in spatially separated rolled-up electron-hole double-layer systems

Using the two-component random phase approximation, we report the collective mode spectrum of a quasi-one-dimensional spatially separated electron-hole double-layer system characterized by rolled-up type-II band aligned quantum wells. We find two intra-subband collective excitations, which can be classified into optic and acoustic plasmon branches, and several inter-subband plasmon modes. At the long wavelength limit and up to a given wave vector, our model predicts and admits an undamped acoustic branch, which always lies in the gap between the intra-subband electron and hole continua, and an undamped optic branch residing within the gap between the inter-subband electron and hole continua, for all values of the electron-hole charge separations. This theoretical investigation suggests that the low-energy and Landau-undamped plasmon modes might exist based on quasi-one-dimensional, two-component spatially separated electron-hole plasmas, and their possibility could be experimentally examined.

Challenging the nature of low-energy plasmon excitations in CaC6 using electron energy-loss spectroscopy

The nature of low-energy plasmon excitations plays an important role in understanding the low-energy electronic properties and coupling mechanism of different superconducting compounds such as CaC. Recent ab initio studies predict a charge carrier intraband plasmon in keeping with a low-energy acoustic plasmon. Here, we have studied the low-energy electronic excitations of CaC using high-resolution electron energy-loss spectroscopy in transmission at low temperatures. The analysis of the core level excitations leads to the conclusion that hybridization between graphite and calcium states plays an essential role in this graphite-intercalated compound. Regarding the low-energy plasmon excitation, we observe the formation of an intraband (charge carrier) plasmon with a negative dispersion at about 3.5 eV in sound agreement with the theory. Finally, a weak excitation around 1.2 eV with an almost linear dispersion relation can be observed as predicted for an acoustic plasmon that may mediate the superconducting coupling in CaC. However its optical limit at 1 eV challenges the theoretical predictions and safely rules out an electronic superconducting coupling mechanism in CaC

Dynamic screening and plasmon spectrum in bilayer graphene

We have theoretically studied the collective response properties of the two-dimensional chiral electron gas in bilayer graphene within the random phase approximation. The cooperation of external controlling factors such as perpendicular electric bias, temperature, doping, and substrate background provides great freedom to manipulate the dynamic dielectric function and the low-energy plasmon dispersion of the system. Intriguing situations with potential application are systematically explored and discussed. Extra undamped plasmon modes might emerge under electric bias. They have almost zero group velocities and are easy to manipulate.

Plasmon nanooptics with individual single wall carbon nanotubes

We discuss how low-energy collective interband plasmon excitations can be used to tune the optical properties of individual single wall carbon nanotubes. Two specific examples are considered. We show that interband plasmons can efficiently mediate enhanced electromagnetic absorption by pristine semiconducting carbon nanotubes and bipartite entanglement in hybrid metallic carbon nanotube systems. We develop a theory for (non-linear) optical monitoring and control of the above phenomena. Our findings pave the way for the development of a new generation of tunable optoelectronic device applications with individual carbon nanotubes.

Monitoring bipartite entanglement in hybrid carbon nanotube systems via optical 2D photon-echo spectroscopy

We consider a pair of spatially separated extrinsic atomic type species coupled to a low-energy surface plasmon resonance of a metallic carbon nanotube. By explicit calculations of non-linear response signals, we show that the two-dimensional photon-echo spectroscopy is a very sensitive tool for probing and monitoring the bipartite inter-atomic coherences and entanglement in such a hybrid quantum system. The non-linear spectroscopy can also be used to control the atom-nanotube coupling strength. We formulate practical recommendations for the experimental observation of the bipartite entanglement of extrinsic atomic species in hybrid carbon nanotubes.

Low energy two-dimensional plasmon in epitaxial graphene on Ni (111)

Single layer graphene has been obtained on Ni (111) by dissociation of ethylene. Angle-resolved electron-energy-loss spectra show a low energy plasmon dispersing up to about 2eV, resulting from fluctuation of a charge density located around the Fermi energy, due to hybridization between Ni and graphene states. The dispersion is typical of a two-dimensional charge layer, and the calculated Fermi velocity is a factor of ~0.5 lower than in isolated graphene.

Optical properties of calcium under pressure from first-principles calculations

Here we present theoretical ab initio calculations based on time-dependent density-functional theory of the macroscopic dielectric function of calcium from 0 to 110 GPa in the fcc, bcc, and sc phases. All the dielectric functions are calculated using very fine reciprocal space meshes since both eigenvalues and matrix elements entering in the density-response functions are interpolated using Wannier functions. Our calculations correctly predict the energies of the low- and high-energy plasmons observed in the fcc phase at room pressure. Moreover, we predict that in the sc phase a very low-energy interband plasmon emerges at around 50 GPa that dramatically modifies the behavior of the reflectivity making it almost vanish at the plasmon energy. As a consequence, calcium becomes an example of how optical properties can become complex under pressure.

Role of band structure and local-field effects in the low-energy collective electronic excitation spectra of 2H-NbSe2

We present a study of the electron dynamics in the layered compound 2$H$-NbSe${}_{2}$. First-principles calculations are used to obtain the band structure employed in the evaluation of the loss function with inclusion of local-field (LF) effects. Two different symmetry directions [(100) and (010)] were explored in the hexagonal basal plane. In both cases, a low-energy charge-carrier plasmon (CCP) at $\ensuremath{\sim}$1 eV presenting a negative dispersion over a wide momentum transfer range is found, in agreement with recent experimental results [Wezel et al., Phys. Rev. Lett. 107, 176404 (2011)]. On the contrary, in the (001) perpendicular direction, the CCP has negative dispersion at small momenta only, presenting strong positive dispersion at larger momenta. Our calculations reveal that this behavior can be explained without invoking many-body effects, as long as band structure effects are properly included in the evaluation of the excitation spectra. In addition to this CCP mode, we find another one with an arclike oscillating dispersion along the perpendicular direction, as well as the appearance of a CCP replica at high momenta due to LF effects.

Collective mode in a well-ordered organic monolayer

We report on the observation of a low-energy surface plasmon mode in the case of a highly ordered organic monolayer adsorbed onto a metallic substrate. Careful investigation using high-resolution electron energy loss spectroscopy, tuning the kinematics, demonstrates that this mode presents a strong energy dependence with the transferred momentum. We treated this mode using a Drude ansatz and discussed its positive quadratic dispersion. We established the physical origin of this surface plasmon as due to a space-charge region arising from an electron-vibration coupling at the interface between the organic monolayer and the metallic substrate. The existence of a space-charge region at an organic-metal interface is of particular interest as it brings a new enlightenment in the understanding of the physical mechanisms involved at such interfaces, of which charge injection plays a decisive role for device performances.

Single-wall carbon nanotubes as coherent plasmon generators

The possibility of low-energy surface plasmon amplification by optically excited excitons in small-diameter single wall carbon nanotubes is theoretically demonstrated. The nonradiative exciton-plasmon energy transfer causes the buildup of the macroscopic population numbers of coherent localized surface plasmons associated with high-intensity coherent local fields formed at nanoscale throughout the nanotube surface. These strong local fields can be used in a variety of new optoelectronic applications of carbon nanotubes, including near-field nonlinear-optical probing and sensing, optical switching, enhanced electromagnetic absorption, and materials nanoscale modification.

Efficient terahertz emission from InAs nanowires

We observe intense pulses of far-infrared electromagnetic radiation emitted from arrays of InAs nanowires. The THz radiation power efficiency of these structures is about 15 times higher compared to a planar InAs substrate. This is explained by the preferential orientation of coherent plasma motion to the wire surface, which overcomes radiation trapping by total-internal reflection. We present evidence that this radiation originates from a low-energy acoustic surface plasmon mode of the nanowire. This is supported by independent measurements of electronic transport on individual nanowires, ultrafast THz spectroscopy and theoretical analysis. Our combined experiments and analysis further indicate that these plasmon modes are specific to high aspect ratio geometries.

Symmetry-dependent screening of surface plasmons in ultrathin supported films: The case of Al/Si(111)

A joint theoretical and experimental study of plasmon excitations for Al overlayers on Si(111) has been carried out. The presence of the substrate is found to drastically modify the hybridization and charge density response of the surface plasmons of the metal overlayers. The symmetric mode, which is polarized toward the Al/Si interface, is strongly damped in intensity and significantly redshifted in energy. However, the antisymmetric mode, which is polarized to the metal-vacuum interface, is essentially unaffected by the presence of the substrate. A low-energy acoustic plasmon mode is also found in a one monolayer Al film and is almost unaffected by the substrate. The calculated plasmon dispersions with substrate are in good agreement with experimental data measured by electron energy loss spectroscopy. Our results suggest that interaction and screening at the subnanometer scale are symmetry dependent, a conclusion that may have general implications in other thin films and related structures.