Showing papers on "Plasmon published in 1989"

Composite structures for the enhancement of nonlinear-optical susceptibility

Abstract: Calculations of the nonlinear-optical behavior are developed for model composites of nanospheres with a metallic core and nonlinear shell or with a nonlinear core and metallic shell suspended in a nonlinear medium. Optical phase conjugation is shown to be enhanced from each nonlinear region because the optical field can be concentrated in both the interior and the exterior neighborhoods of the particle and magnified at the surface-mediated plasmon resonance. For the model composite with a metallic core, a limited range of resonance tunability can be achieved by adjustment of shell thickness; the frequency range is dependent on the dielectric dispersion of the metal. For the composite with a metallic shell instead of a metallic core, this restriction is reduced so that tunability from ultraviolet to infrared can be attained. Enhancement of the phase-conjugate signal is calculated for the electrostrictive mechanism dominant in the microsecond time scale and for the electronic mechanism dominant in the picosecond time scale. Calculations based on the dielectric functions for gold and for aluminum indicate that phase-conjugate reflectivity enhancements of 108 can be achieved. The imaginary components of the composite dielectric functions are shown to limit the magnitude of the field enhancement at the surface-plasmon resonance and determine the absorption and figure of merit of the composite.

Enhanced Photon Emission in Scanning Tunnelling Microscopy.

Abstract: We have investigated the inverse photoelectric effect stimulated by electron tunnelling in the scanning tunnelling microscope (STM). Light emission spectra are reported for polycrystalline silver films. In accordance with similar observations for other tunnelling devices, the high-energy threshold of the spectral distributions has been found to equal the voltage between tip and sample. Characteristic maxima of the photon emission at ~1.9 eV and ~2.4 eV, independent of tunnel voltage, are attributed to resonant excitation and radiative decay of localized surface plasmon polariton modes at the silver surface. This represents the first observation of plasmon excitations with the STM and provides unambiguous evidence of local inelastic processes occurring on silver surfaces. We propose that our novel results may have particular relevance to the interpretation of optical phenomena on the atomic scale.

Valence-electron excitations in the alkali metals

Abstract: To study the dynamical response of the valence-electron gas in nearly-free-electron metals, we carried out an electron-energy-loss investigation on Na, K, Rb, and Cs. On polycrystalline samples we measured the volume plasmon dispersion, which showed strong deviations not only from the random-phase approximation, but deviations as well from the predictions of the current theories for Fermi liquids, including short-range exchange and correlation. In addition, anomalous dispersion of the plasmon half-width was found. Both results may be traced back to exchange and correlation effects not properly treated by the current Fermi-liquid theories. Furthermore, we observed the excitation of intraband transitions in Na and Rb, the dispersion of which yields the effective band mass for unoccupied states. The results for Na are at variance with recently published enhancement of the band mass for the occupied states. To elucidate the influence of band structure, measurements were also performed on a single-crystalline Na film and we observed the so-called zone-boundary collective state in Na.

Surface-plasmon dispersion in simple metals.

Abstract: Angle-resolved reflection inelastic electron scattering has been used to measure the dispersion of the surface plasmon for thick films of Na and K. The measured dispersion is negative at small momentum parallel to the surface as predicted by quantum-mechanical calculations of the dynamic response of the electrons at the surface of an interacting electron gas. A simple physical picture is presented to explain the origin of the negative surface-plasmon dispersion in simple metals.

Surface-plasmon energy and dispersion on Ag single crystals

Abstract: We report here the measurements of the energy and dispersion of the surface plasmons on Ag single crystals in UHV using angle-resolved electron-energy-loss spectroscopy. The energy of small-momentum surface plasmons depends upon crystal face and, for Ag(110), upon crystal orientation. The differences in the energies of the surface plasmons, which cannot be explained by a jellium model, are discussed in terms of the contributions to the dielectric response from surface states and truncated bulk band states.

Quasiparticle excitation spectrum for nearly-free-electron metals

Abstract: The quasiparticle excitation spectrum is calculated for the nearly-free-electron metals Li, Na, and Al by evaluation of the electron self-energy operator within the GW approximation and a generalized plasmon-pole model. The calculated quasiparticle energies for Na and Al are in excellent agreement with angle-resolved photoemission experiments. For Na in particular, the occupied-band width is significantly narrower than the free-electron value, as found in experiment. Inclusion of exchange and correlation effects in the dielectric matrix is shown to decrease the bandwidth relative to the random-phase-approximation result by a significant amount. Local-field effects, reflected in the off-diagonal elements of the dielectric matrix, are found to have little effect on the quasiparticle band structure of these simple metals.

Magnetoplasma effects in a quasi-three-dimensional electron gas

Abstract: Far-infrared magnetotransmission measurements on a quasi-three-dimensional modulation-doped semiconductor structure are reported in the frequency region of the bulk plasmon. When a magnetic field is applied tilted with respect to the film plane, two branches appear in the excitation spectrum resulting from the coupling between cyclotron resonance and the plasma oscillations perpendicular to the film plane. The experiment demonstrates the connection between the depolarization-shifted intersubband transitions and the bulk plasma resonance.

Surface-plasmon-induced desorption by the attenuated-total-reflection method.

Abstract: Surface plasmons excited by a Nd:YAG (YAG, yttrium aluminum garnet) laser in an attenuated-total-reflection (ATR) geometry were used to desorb Al atoms from smooth Al films The ejected Al atoms were ionized by a pulse-excimer laser and measured by time-of-flight mass spectroscopy The desorption rate exhibits a sharp maximum at the surface-plasmon resonance angle The experiment demonstrates that surface-plasmon-induced desorption can be accomplished on smooth surfaces Since ATR excitation allows the use of a wide range of wavelengths, this method will be useful in the study of electronic desorption processes

Coupled plasmon and phonon excitations in the space-charge layer on GaAs(110) surfaces

Abstract: High-resolution electron-energy-loss spectroscopy (HREELS) is used to study the surface plasmon and phonon in the space-charge layer on doped GaAs(110) cleaved surfaces. Three losses are observed, with energies of about 27, 36, and 42 meV. The peak at 36 meV is attributed to an unscreened phonon in the depletion layer. The other two losses are due to the coupled modes of the plasmon and phonon propagating near the interface of the depletion layer and the bulk. The surface Fermi-level pinning is induced either by residual-gas interaction with the surface or by deliberate hydrogen adsorption; this in turn modifies the depletion-layer thickness. This modification results in a quite pronounced change of the relative intensities and noticeable energy shifts of these losses. A model energy-loss spectrum calculated in the framework of local-response theory is used to deduce the properties of the space-charge layer. Our model is the first to use a self-consistent free-carrier profile together with the Lindhard description of the local dielectric response.

Spatial light modulation using surface plasmon resonance

Abstract: A new type of spatial light modulator has been demonstrated, operating by the surface plasmon resonance effect. Devices have been constructed using nematic liquid crystal as the active material, and the operation of these devices is 50 times faster than the bulk switching of the liquid‐crystal cells. We have obtained resolution of 10–20 μm and contrast ratios better than 100:1.

Quantum theory of plasmons in lateral multiwire superlattices in the presence of subband quantization: Intersubband plasmons.

Abstract: We present a quantum theory of plasmons in lateral multiwire superlattices. The theory applies to lateral multiwire superlattices both with and without electron tunneling between wires. Special attention is paid to the strong-confinement case, where quantization effects are the most important and electron tunneling is weak. For a parabolic--confining-potential model, we find that the existence of an approximate sum rule results in an intersubband plasmon with energy shifted well beyond the subband-energy separation, and there are other intersubband plasmons with energies close to the subband-energy separation. Only the plasmon with the large energy shift creates a large electric dipole moment and thus couples strongly to the experimental probe. The coupling of intersubband plasmons to intrasubband plasmons is discussed.

Reexamination of the surface-plasma-wave technique for determining the dielectric constant and thickness of metal films

Abstract: The effect of surface plasmon generation on the reflectivity of thin metal films in attenuated-total-reflection prism configurations has been described as a means of determining the film dielectric function and thickness. Although this technique is capable of yielding accurate values for the film parameters with relatively simple equipment, there is an ambiguity in fitting to the measured reflectivity data. Previous research had shown that the ambiguity could be removed by further reflectivity measurements with different conditions; however, we find that this is not necessarily true. We describe the conditions under which a unique determination of the film parameters is ensured and demonstrate that even with ambiguous results the true solution can usually be determined by a supplementary optical measurement.

Optical-fibre surface-plasmon-wave polarisers with enhanced performance

Abstract: The effect of a thin metal film with positive dielectric permittivity on the performance of an optical-fibre surface-plasmon-wave polariser is presented. A dramatic improvement has been achieved with polarisation extinction ratios in excess of 45 dB over an extended range of superstrate refractive indices. >

On the laser‐wavelength dependence of plasmon surface polariton field‐enhanced Raman spectroscopy

Abstract: We analyze plasmon surface polariton field‐enhanced Raman spectroscopic data for the CH stretching vibrational range (2800–3000 cm−1) of cadmium arachidate multilayer assemblies, taken as a function of the exciting laser wavelength λL. In addition to the well‐known λ−4 dependence of ordinary (photon) Raman scattering we find a variation of the intensity ratio of the methylene antisymmetric to symmetric stretching band (νa=2885 cm−1 and νs=2850 cm−1, respectively) which can be attributed quantitatively to a specific surface plasmon field property: a longitudinal field component parallel to the propagation direction adds to the dominant perpendicular field with increasing relative strength for decreasing wavelength.

Electron-hole plasma dynamics in CdTe in the picosecond regime.

Abstract: La temperature et la densite du plasma ainsi que la renormalisation de la bande interdite sont tracees durant la relaxation qui suit l'excitation. Les spectres de luminescence transitoire sont analyses, tout d'abord dans un modele de particule libre le plus courant, puis en incluant l'elargissement des collisions des porteurs de charge calcule dans l'approximation du pole de plasmon

Theoretical approach for quantitative analysis of electron spectroscopies: E.P.E.S. and E.E.L.S.

Abstract: We propose a new contribution for the quantitative analysis of the electron spectroscopies E.P.E.S. and E.E.L.S. By a synthesis of different theoretical works, we take into account the travel of primary electrons and the different interaction processes which occur in the sample. Simple formulations are derived an give an easy interpretation of the experimental results obtained on In and InP(100) substrates. The paper is devoted to: E.P.E.S. for binary alloys, E.E.L.S. and probabilities of loss processes (volume plasmon, surface plasmon...)

IR surface plasmon (polariton) phase spectroscopy

Abstract: New experimental evidence is presented which shows that the reliability and the precision of the determination of the optical constants of the metallic and dielectric surfaces can be improved by using the surface electromagnetic wave absorption spectroscopy and SEW phase spectroscopy simultaneously.

Optical plasma resonance in semiconductors: Novel concepts for modulating far‐infrared light

Abstract: A novel concept for modulating far‐infrared light is presented based on the ‘‘photon‐plasmon, plasmon‐photon’’ interaction with a thin layer of a free‐electron gas in a n‐doped semiconductor. For oblique incidence of p‐polarized light at the plasma frequency of the electrons, high‐reflection properties arise due to plasma resonance. The influence of all the constituent parameters (angle of incidence, electron mobility and density, thickness of the layer, etc.) on the reflectivity is discussed. The plasma frequency, and hence the reflectivity, can be modulated by changing the effective mass or the density of the electrons. This leads to a small, integrable, and fast modulable mirror, which can be used as a switching device (electrically or optically driven) or even as a way of frequency modulation.

Excitation of surface plasmons by finite width beams

Abstract: The field properties of surface plasmons produced by realistically bounded beams incident in various attenuated total reflection (ATR) geometries are examined. Analytical results are first derived for the general case dealing with a beam field incident in a multilayered configuration. We show that, at the phase matching condition, the reflected field can be severely distorted in comparison with the incident beam shape. We also find that the power intensity inside the metal medium can be much smaller than that expected under the assumption of plane wave incidence. However, when the beamwidth is larger than the proportion range of the excited plasmon, the power intensity profiles and find that they exhibit distinguishing characteristics. In particular, for an incident Gaussian beam, the location of maximum power density on the metal surface shifts with respect to the center of the incident beam by a distance of the order of the plasmon propagation length. For a rectangular beam incident at the phase matching condition, on the other hand, the propagation range of the coupled surface plasmon can be found directly from the profile of the reflected field. We also show the overall process of beam wave coupling in the ATR geometry can be simulated by a spatial operating system having the response of either a differentiator (for the reflected field) or an integrator (for the transmitted field).

Superconductivity and acoustic plasmons in the two-dimensional electron gas.

Abstract: We calculate the superconducting transition temperature T/sub c/ for the two-dimensional, two-component electron gas. We use an effective interaction which approximates vertex corrections by means of local-field factors, which are obtained self-consistently; this interaction includes but is not limited to the ''acoustic plasmon'' (AP) mode. The gap equation which gives T/sub c/ includes the full frequency dependence of the interaction. We find two main results: (i) the T/sub c/'s obtained are low (mK); (ii) the AP itself essentially does not contribute to the pairing transition, which is rather due to low-frequency electron-hole pair excitations.

Plasmons and high-temperature superconductivity

Abstract: Long-wavelength charge fluctuation models are constructed for 1:2:3 superconductors. These include planar plasmons (..omega..approx. ..sqrt..q ) plus numerous acoustical plasmons (..omega..approx.q). We show that these modes do not cause pairing of electrons into a superconductor state. Our results rule out interlayer and intralayer pairing by long-wavelength charge fluctuations.

Collective and vibrational excitations on the n-doped GaAs(110) surface

Abstract: A detailed high-resolution electron-energy-loss-spectroscopy study of the surface collective and vibrational excitations of the clean n-doped GaAs(110) cleaved surface is presented. Measurements were taken exploiting a wide range of primary-beam energy and at two different dopant concentrations. Due to their coupling in the sample with higher doping, both the free-carrier plasmon and the Fuchs-Kliewer mode show dispersion. The phonon intensity behavior, when the primary-beam energy is changed, is opposite to that of the plasmon. The intensity behavior of the latter mode, in particular, suggests its spatial confinement beneath the surface.

Laser-field effects on the interaction of charged particles with a degenerate electron gas

Abstract: We formulate a general description of the inelastic interaction between charged particles and a degenerate plasma in the presence of a strong laser field. The excitations in the plasma are described according to the random-phase-approximation formalism, in terms of the dielectric function of the medium, and including the effects of the laser field on the dynamical response. The energy exchange and the scattering rate of the particle in the plasma are modified by multiphoton processes. The formalism describes the excitation of plasmons and electron-hole pairs, with simultaneous emission or absorption of photons. We calculate the contribution of these processes to the energy-loss rate and to the mean free path of the particle in the range of solid-state densities. New effects due to the laser field are expected for intensities similar to those used in laser fusion experiments.