T. Lopez-Rios

Use of surface plasmon excitation for determination of the thickness and optical constants of very thin surface layers

Abstract We present a method using attenuated total reflection (ATR) with excitation of surface plasma waves at different wavelengths and angles of incidence, which allows an accurate determination of the thickness and optical constants of absorbing surface layers from optical measurements only. This method is applied to the case of very thin Au surface layers on (111)Ag, and the results are discussed.

Contribution of longitudinal polarization waves to the optical properties of Ag surface layers

Abstract Diffential reflectivity measurements of thin Ag films deposited on Al and Au are reported. The silver plasmon excitation peak is broadened and shifts to higher energies with decreasing thickness. This is interpreted by taking into account longitudinal polarization waves and a shorter effective relaxation time of the free electrons. Good quantitative agreement is obtained.

Ellipsometry of metallic films and surfaces with nonlocal effects

Abstract We indicate the modifications of the classical ellipsometric parameters ψ 0 and Δ 0 , described via the complex function ϱ 0 = tan ψ 0 exp(iΔ 0 ), when nonlocal effects occur at a metallic surface. Various possible situations are considered: (a) two semi-infinite media e a and e b , separated by a sharp boundary, the medium e b displaying nonlocal effects; (b) a diffuse surface region with nonlocal anisotropic dielectric function; (c) a very thin homogeneous and isotropic layer propagating both longitudinal and transverse electromagnetic waves situated between two semi-infinite layers characterized by local dielectric functions. For all these situations, we show that it is, in principle, possible to determine e b from multiple angle of incidence ellipsometry, even in the absence of any information about the properties of the surface or of the surface region. Unfortunately, the technique requires very accurate measurements of δϱ / δθ a .

Optical evidence for longitudinal waves in very thin Ag layers

Abstract We report on differential measurements performed in situ on very thin Ag layers deposited in ultra-high vacuum on Au, with p-polarized light both in transmission and in reflection. The peaks located in the vicinity of the effective plasma frequency are shifted to higher energies with decreasing film thickness. It is claimed that this effect can be explained only if one assumes the presence of plasma waves in the Ag layers.

Roughness induced at Si(111) surfaces by high temperature heating

Abstract The roughness of Si(111) surfaces induced by heating at temperatures between 900 and 1000°C is studied by differential reflectivity measurements. For the lowest temperatures, the stepped-roughness of the surface is smoothed out by annealing the surface, leading to an increase of reflectivity for light energies below 3 eV. For the highest temperatures, a larger scale roughness (of the order of several tens of nm) is formed at the surface. This roughness increases with temperature and induces essentially a scattering of light leading to a decrease of reflectivity in the UV range.

Modification by pd adsorbates of the surface enhanced Raman scattering at Ag surfaces

The influence of very thin ( < 10 A) Pd deposits on the surface enhanced Raman scattering (SERS) of pyridine and ethylene adsorbed on cold films and island films of Ag is investigated as a function of the Pd thickness and the pyridine and ethylene coverage. The quenching of SERS by the Pd deposits was found to be larger as the pyridine coverage was smaller. Some similarities between SERS and the optical absorption of adsorbates are pointed out.

Abnormal optical absorption of quenched Ag films due to surface roughness

The optical properties of rough Ag films quenched on substrates cooled to 150 K are studied by differential reflectivity during annealing to room temperature. An abnormal optical absorption at about 2.5 eV is assigned to surface roughness.

Changes of the SERS background induced by metal monolayers

Abstract The Stokes and antiStokes SERS background of Ag island films covered by very thin Pd deposits is investigated in the 80–1600 cm−1 region. It is found that a Pd monolayer reduces the background intensity by a factor of five.

The reflectance spectroscopy of silver surface layers on gold and aluminium substrates

Abstract The results of reflectivity and differential reflectivity measurements at non-normal incidence for p-polarized light of thin films of silver on different substrates are presented. It was found that the peak associated with the silver plasma frequency broadens and shifts towards higher energies for thinner layers. A model of a thin layer with bulk silver optical constants cannot be used to explain this behaviour. Agreement with experiment is obtained if we take into account not only the shorter relaxation time of the free electrons but also the non-local effects associated with the longitudinal wave (polarization wave) near the plasma frequency (in our configuration these effects are particularly important).

Enhanced optical absorption by silver overcoated with rough layers of pyridine

Abstract The reflectivity of smooth Ag surfaces coated by pyridine layers of average thickness from a few to several hundred angstroms was measured. A pronounced absorption at 3.2 eV attributed to surface plasmon excitation by the roughness of the pyridine layer is observed. A simple model consisting of ellipsoidal particles of pyridine with the associated images account qualitatively for the observed absorption.