Nonradiative Surface Plasmons Research Articles

Surface-Enhanced Fluorescence on SiO2-Coated Silver Island Films

Fluorescence is one of the molecular spectroscopic properties that is enhanced by placing the molecule on the rough surface of a coinage metal. The surface-enhanced fluorescence (SEF) can be directly observed in steady-state fluorescence experiments. The observations are the results of a delicate balance between the enhanced emission and the quenching due to energy transfer to nonradiative surface plasmons. In the present report, SiO2-coated silver films were fabricated at varying dielectric thickness. The surface of the films was analyzed with the use of atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). AFM confirms the surface roughness and XPS analysis indicates that the SiO2 coverage was successful. SEF and SERS (surface-enhanced Raman scattering) were observed on active 6, 10, and 14 nm silver films coated with SiO2. Similar results were obtained with a 6 nm silver film coated with 6 nm SiO. The SEF work was carried out on fluorescent molecules with different quantum yield, and the typical enhancement factor obtained for the fluorescent signal was approximately 10. Both the SiO2 and SiO overlayers provide stable surfaces with well-defined hydrophilic properties. Such stable constructions have applicability towards the advancement of SERS and SEF as routine analytical techniques in bio- and chemical sensors.

Surface electronic fluctuations in metals and Raman light scattering.

The effect of a surface on electronic fluctuations in anisotropic metals is studied theoretically. The Coulomb interacton of the electron-hole excitations is taken into account self-consistently. A system consisting of the Boltzmann equation for electronic fluctuations and the Maxwell equations for the interaction field is solved with appropriate boundary conditions. The cross section of inelastic light scattering is calculated. The scattered light observed out of the metal is a result of the collective electron interaction with radiation inside the metal. The scatteirng cross section consists of the bulk contribution of electron-hole pairs and plasmons, which differs from the scattering by an infinite metal only in the effect of the penetration of radiation in the skin layer. The surface contribution to the cross section is attributed to the electromagnetic excitations of the nonradiative into-vacuum surface plasmons (the sharp peak) and the radiative surface plasmons (the continuum with a peak).

Determination of the spatial extension of the surface-plasmon evanescent field of a silver film with a photon scanning tunneling microscope.

A photon scanning tunneling microscope is employed to probe the surface-plasmon field in the evanes- cent region of a silver film for p (parallel to the plane of incidence) and s (perpendicular to the plane of incidence) polarizations of the light beam at several angles of incidence near the critical angle. The in- teraction between the field and the probe is measured and compared to theoretical calculations involving a single four-media model. A systematic analysis of images obtained for several positions of the optical fiber above the film is presented and it is shown that, for tip-to-sample distances smaller than half the wavelength of the incoming light, the collected intensity curves are identical in any area of the sample. In this paper, we study the evanescent behavior of the surface plasmons (SP), previously described by Ritchie, ' excited in metallic films in the Kretschmann configuration. The extension, above the surface, of the nonradiative surface plasmons is detected locally by the optical probe of the photon scanning tunneling micro- scope (PSTM). This microscope described by Reddick, Warmack, and Ferrell at the Oak Ridge National Labo- ratory and since developed by several groups ' is the op- tical analogue of the well-known scanning tunneling mi- croscope (STM). In our configuration a transparent sample is placed on the base of a hemicylindrical prism (index of refraction n, ) illuminated by a laser beam in to- tal internal reliection (TIR) as represented in Fig. 1. To- tal internal reAection occurs at the prism-air interface and the evanescent field in the medium of lesser index of refraction (n3=1 for air) is converted into progressive waves by a sharpened optical fiber brought sufficiently close to the sample (typically a few tens of nanometers). The probe is scanned over the sample surface and varia- tions of the intensity detected by the fiber are measured and treated by a suitable detector-computer system. A theoretical study of the field collected by the optical fiber is given in Sec. II. A comparison between these cal- culations and the experiment is developed in Sec. III. In Sec. IV we discuss the PSTM-field isointensity lines in the medium above the sample as a function of the sample probe distance. We give our conclusions in Sec. V.

A note on the angular distribution of the characteristic surface loss intensity of electrons with non-normal incidence to the foils

By using a quantized theory of non-radiative surface plasmon in a semi-infinite electron gas, where retardation is taken into account, the differential surface loss intensity of electrons in a metal foil is calculated for the case of non-normal incidence of electron to the metal surface. The result shows that the differential surface loss intensity neither has a zero at θ = 0 for the case of normal incidence nor has a zero at a forward direction ( θ ≠ 0, φ = 0) for the case of non-normal incidence, whose existences were predicted by electrostatic theories, but diverges as 1/θ at θ = 0 for both cases. It is also shown that, when the angle of incidence is greater than a certain critical value, there is a region of direction to which incident electrons can be scattered by exciting any one of surface plasmons with three different wave vectors.

Coupled surface plasmons excited by photons in a free-standing thin silver film.

We report the first observation of coupled modes of nonradiative surface plasmons excited by photons in a free-standing thin metal film. Excitation of these modes was achieved by introducing shallow periodic corrugations on the surfaces of a 44-nm-thick silver film, and the plasmon resonance absorption of photons was detected by a photoacoustic method.

Photoacoustic study of phase correlation of surface plasmons in a diffraction grating

.Nonradiative surface plasmons may be excited in a metal-coated diffraction grating by either p­ or s-polarized photons if the ruling is oriented neither perpendicular nor parallel to the plane of photon incidence. Phase correlation between these surface plasmons in a 520-line/mm Ag-coated diffraction grating was observed by measuring absorption of elliptically polarized 633-nm photons at the plasmon resonance angle. The absorption measurements were made by using a photoacoustic technique. The experimental results were analyzed by a first-order perturbation theory of photoab­ sorption in a rough surface. This allowed us to determine the phase difference between surface plasmons excited by p- and s-polarized photons. I. INTRODUCTION In contrast to conventional spectroscopic methods in which light intensities are measured outside the sample under study, photoacoustic methods 1 measure absorption of photons by probing heat generation occurring inside the sample as a result of nonradiative dissipation of excited states created by incident photons. This makes possible new experimental studies which have been difficult to per­ form by conventional experimental methods. In recent years we have used 2 - 6 this photoacoustic technique to study excitation and relaxation of surface plasmons 7 in metals by using the attenuated-total reflection (ATR) and grating methods. Most recently, we have investigated 6 plasmon resonance absorption in a Ag-coated diffraction grating as a function of the ruling orientation relative to the plane of incidence. Such a study has been hitherto dif­ ficult to perform by conventional spectroscopic methods because photons are diffracted out of the plane of in­ cidence. Surface plasmons may be excited in a diffraction grating by both p- and s-polarized photons if the ruling is neither perpendicular nor parallel to the plane of in­ cidence. These surface plasmons are excited at the same angle of photon incidence for both p and s photons and propagate in the same direction along the grating surface and are thus coherent to each other. Therefore, if they are excited simultaneously by photons having both p and s components in a certain phase relation, the phase correla­ tion of the plasmons may be observed directly in the reso­ nance absorptance values. In this paper we report experi­ mental results of this phase correlation for surface plasmons in a Ag-coated shallow diffraction grating. The plasmon resonance absorption was measured by the same photoacoustic method as in our previous studies 2 - 6 as a function of the relative phase between p and s components of elliptically polarized incident photons. The experimen­ tal results were analyzed numerically with the use of a first-order perturbation theory 8 of photoabsorption in a rough surface. This allowed us to determhle the phase difference between surface plasmons excited by p- and s­ polarized photons.

Phase conjugation of surface plasmon waves by the third order nonlinearity of a free electron gas

Phase conjugation of non-radiative surface plasmon waves by degenerate four-wave mixing utilizing the third order non-linear current of a free electron gas is considered. The third order nonlinear susceptibility of a free electron gas is calculated from hydrodynamic equations where the magnetic part of the Lorentz force is treated as a perturbation. Equations describing the growth of the phase conjugate wave to an input signal are given together with their solutions. A numerical estimate for the case of surface plasmos at the ruby laser frequency on a silver surface indicates that the detection of the phase conjugate wave is possible.

Light emission from surface-plasmon and waveguide modes excited by N atoms near a silver grating

We examine the coupling of excited N atoms, at distances $d$ away from a silver grating, to the usually nonradiative surface-plasmon and waveguide modes of a metal-dielectricvacuum system. For $dl{({\ensuremath{\epsilon}}_{D}\ensuremath{-}1)}^{\ensuremath{-}\frac{1}{2}}\frac{\ensuremath{\lambda}}{4}$, where ${\ensuremath{\epsilon}}_{D}$ is the dielectric constant of the dielectric layer and $\ensuremath{\lambda}$ is the wavelength of light at the frequency of the excitation, only the surface-plasmon mode exists. It is directly excited by the near field of the N atoms and radiates via the grating. For $d\ensuremath{\ge}{({\ensuremath{\epsilon}}_{D}\ensuremath{-}1)}^{\ensuremath{-}\frac{1}{2}}\frac{\ensuremath{\lambda}}{4}$, waveguide modes also exist. They also can decay radiatively via the grating. Conservation of momentum parallel to the surface to within an integer number of Bloch waves of the grating determines the angular distribution of the emitted radiation. The agreement between the experimentally measured and theoretically predicted angular distributions is excellent. The dielectric constant of condensed ${\mathrm{N}}_{2}$ is deduced to be ${\ensuremath{\epsilon}}_{D}=1.51\ifmmode\pm\else\textpm\fi{}0.02$ at $\ensuremath{\lambda}=523$ nm.

Radiative decay of surface plasmons excited by fast electrons on periodically modulated silver surfaces

The radiative decay of non-radiative surface plasmons excited by fast electrons on sinusoidally modulated Ag surfaces has been investigated systematically for different modulation heights. The position and half-width of the surface-plasmon dispersion of weakly modulated gratings are in agreement with theoretical values derived for smooth surfaces; for stronger modulated gratings, displacement and broadening of the surface-plasmon dispersion and multiple-scattering are found.

Light emission of nonradiative surface plasmons from sinusoidally modulated silver surfaces

Surface plasmons have been excited with the prism method at sinusoidally modulated silver surfaces. The scattered light intensity of the first diffraction order has been investigated for gratings with different modulation heights H. The scattered light intensity increases up to modulation heights H of ca. 70 Å with the square of H. We discuss the limits for applying first order scattering theories to surface plasmon scattering experiments.

Surface plasmons in liquid mercury: Propagation in a nonuniform transition layer

We have examined the propagation of nonradiative surface plasmons in the presence of an inhomogeneous transition zone in the surface region of a conductor. The model we treat in the present paper is the Epstein stratified conductivity profile that has been proposed by Bloch and Rice for liquid mercury. Assuming that photon-plasmon coupling occurs through frustrated total reflection, we evaluate the optical response of mercury to surface-plasmon excitation and find it to be sensitive to the existence of such a zone. Measurements of the dispersion relation should, therefore, provide valuable information regarding the range and degree of such electronic inhomogeneity; preliminary measurements are in good agreement with the theory (Sec. VI). In addition to inhomogeneity in the conductivity of nearly-free electrons in the liquid metal, we have also investigated the effect of the localized resonances near the surface, and we speculate under what conditions the surface-plasmon dispersion relation may exhibit "anomalous dispersion" due to plasmon-localized-state interaction. Finally, the existence of an upper plasmon branch induced by the inhomogeneity is treated. In general, the results of the present investigation suggest that optical surface-plasmon spectroscopy should be a useful technique for the study of nonuniform conducting surfaces.

Surface polaritons in a circularly cylindrical interface: Surface plasmons

A theoretical investigation of surface polaritons in the circularly cylindrical geometry is presented. The complete set of Maxwell's equations (retardation effects are not neglected) is solved with the simple dielectric function, $\ensuremath{\epsilon}(\ensuremath{\omega})=1\ensuremath{-}\frac{{\ensuremath{\omega}}_{p}^{2}}{{\ensuremath{\omega}}^{2}}$, where ${\ensuremath{\omega}}_{p}$ is the bulk plasma frequency. The resulting transcendental equation for the eigenfrequencies is solved via numerical methods for three representative values of the cylindricality constant, $\ensuremath{\alpha}=\frac{{\ensuremath{\omega}}_{p}a}{c}$, where $a$ is the cylinder radius and $c$ the velocity of light. In addition to the real nonradiative surface plasmons, various virtual radiative surface plasmons exist with properties depending rather strongly on $\ensuremath{\alpha}$. The results are compared with existing experimental data. Further experiments are proposed in order to reveal the most interesting features of the surface plasma modes.

Polarization properties of surface plasma radiation excited by electrons

Some interesting features of the radiation associated with non-radiative surface plasmons, excited by electrons have not been explained previously. We give here an explanation of the origins of these features, based on recent work by Elson and Ritchie.

Surface plasmons at the surface of liquid mercury

We have evaluated the surface plasmon dispersion relation in the presence of the inhomogeneous surface zone which has been proposed for liquid mercury. Assuming that photon-plasmon coupling occurs through the mechanism of frustrated total reflection, we find the dispersion relation to be sensitive to the existence of such a zone, providing retardation effects are taken into account. The results, it is believed, define nonradiative surface plasmon absorption as a useful technique for the study of conducting surfaces.

Coherent surface bremsstrahlung in low-energy-electron diffraction and reflection-high-energy-electron diffraction

The classical theory of transition radiation generated when charged particles travel through a dielectric-vacuum interface is reconsidered in order to include the effects of the diffraction of the particle beam occurring at the surface. If the surface is atomically smooth, coherent Bragg reflection and transmission occur as well as specular reflection and transmission. Each Bragg beam is shown to provide a source of classical radiation which will be called coherent surface bremsstrahlung and whose theoretical energy spectrum and angular distribution are calculated. Ordinary transition radiation is recovered and corresponds to the particular case of the transmitted-particle trajectory undisturbed by the surface. The coherent surface bremsstrahlung is shown to be particularly intense when the primary beam strikes the surface at glancing incidence. For metal surface, the maximum radiative yield of the specular beam occurs at the wavelength of the nonradiative surface plasmon. For a rough surface, diffuse particle scattering is also produced by the surface roughness which will correspondingly generate incoherent surface bremsstrahlung. Both coherent- and incoherent-surface-radiation mechanisms may be important for the interpretation of recently observed anomalous radiation emitted in glancing-incidence experiments.

Optical and Photoelectric Properties of Mg in the Vacuum Ultraviolet

Optical and photoelectric measurements on Mg are presented for photon energies from 2 to 12 eV. A minimum in the reflectance at \ensuremath{\sim} 7 eV is interpreted in terms of absorption due to the excitation of nonradiative surface plasmons at the rough Mg surface. By fitting the data to a theoretical expression for nonradiative-surface-plasmon excitation, rms surface-roughness heights $\ensuremath{\delta}$ and autocorrelation lengths $\ensuremath{\sigma}$ have been obtained. Photoelectric-yield measurements show a strong maximum for photons of \ensuremath{\sim} 7 eV, very close to the energy at which the reflectance is a minimum. It is suggested that this peak in the photoelectric yield is due to the decay of nonradiative surface plasmons into one-electron excitations which are observed in the photoemission. In addition to the data below 12 eV, photoelectron energy distributions are presented for photon energies of 13.5 and 14.9 eV. Examination of all the photoelectron energy distributions fails to reveal structure resulting from photoelectrons losing energy by creating surface plasmons while escaping from the surface. If this process occurs in Mg it is overshadowed in our data by electrons that have suffered one or more inelastic electron-electron scattering events.

Polarization studies of radiation emitted by surface plasmons in silver

Measurements are reported on the polarization of radiation emitted by the roughness induced decay of nonradiative surface plasmons optically excited by light in thin Ag films. This radiation is of mixed polarization when the incident light is p-polarized. The s-polarized component of the radiation is observed to be considerably less intense than the p-polarized component.

Optical constants of lithium in the region of 2–3.8 eV determined by the excitation of surface plasma oscillations

Optical constants of lithium thin films, thickness about 500 Å, have been determined in the region of 2–3.8 eV by reflection versus angle of incidence measurements, involving the excitation of nonradiative surface plasmons in the film by a prism method.

Collective Electronic Motion in a Metallic Slab

The virtual modes are obtained for a free-electron-gas model of a metallic slab. These modes, excitations of the crystal which couple to radiative fields outside the slab, correspond to a mixture of collective electron motion and photons. It is shown that a knowledge of the virtual modes leads to a complete understanding of those aspects of the optical properties related to the long-wavelength collective behavior of the electrons. In particular, the existence of radiative surface modes is established. These modes markedly affect the low-frequency optical properties. Manifestations of the virtual modes in experimental situations other than optical investigations are discussed. The dispersion of the ordinary nonradiative surface plasmons at long wavelengths is also calculated using the same general technique. The result is in accord with a previous calculation.