Plasmon confinement in atomically thin and flat metallic films

Abstract

We report on the direct measurement of dispersion relations of plasmons confined in atomically thin metal films and wires by electron energy loss spectroscopy in wide energy-momentum range. Ultrathin Ag films are prepared on single crystal Si surfaces by molecular beam epitaxy, and its crystallinity is checked by electron diffraction. For the case of multi-atomic-layer Ag films, two plasmon modes are observed at around 3.9 eV and 1.8 eV which are localized at the top and the bottom surfaces of the films, respectively. For the case of Ag monoatomic layer, a single mode is observed that steeply disperses in the mid-infrared range. Nonlocal and quantum effects are found to be essential in understanding its full plasmon dispersion curve up to the critical wave number of Landau damping. For the case of Au atom chains, an anisotropic sound-wave-like plasmon dispersion is found that clearly shows 1D plasmon confinement in each atom chain.

Plasmons in nanoscale and atomic-scale systems

“…As shown in figure 4, the dispersion curve resembles that of the 1D plasmon but its origin is rather different [16,17,46,47]. The linear dispersion is a consequence of the redshift from the 2D plasmon at larger wavenumbers owing to the increased dynamical screening from the metallic substrate [49].…”

“…Dashed line shows an early dispersion prediction obtained in a simplified 1D calculation [48]. The shape of the dispersion curve is similar to that of the 1D plasmon but its origin is different [16,17,46,47]. Right: imaginary part of induced charge-density distribution (red solid line) corresponding to the acoustic surface plasmon in direction perpendicular to the Be(0001) surface (ab initio calculation).…”

“…For example, lithographically fabricated nanorod and nanoparticle antennas as well as gap-tuned random nanoislands have been successfully adopted to optimize the enhancement of vibrational signals by tuning the structural parameters [12][13][14]. Even in atom-scale metallic objects, such as atomic sheets and atomic wires, plasmons can be confined and can propagate [15][16][17]; moreover, they can exhibit standing-wave antenna resonance [15]. Because this research field is developing very rapidly, recent works presented here are just a few examples from a vast amount of ongoing research.…”

Low-dimensional plasmons in atom-scale metallic objects

“…For example, the electromagnetic wave with "micrometer vacuum wavelength" can be confined to the "atom-scale tiny object" and the band dispersion of the excitation come to exhibit very different feature from the one supported in larger threedimensional (3D) media ( Fig. 1) [2,3]. In the present paper, we report on the dimensionality effect on the plasmons in some atom-scale metallic structures.…”

“…With a momentum resolution in the 1/1000Å -1 range in low-energy electron energy loss spectroscopy (EELS), we can determine the characteristic dispersion curves of plasmons in the mid-to near infrared spectral range. From an atomically flat monoatomic Ag layer, plasmon of a 2D free-electron gas was observed [3,4]. An example of plasmon confinement in atomic scale quantum wires is also briefly introduced [5-7].…”