Application of Plasmons to the Determination of Surface Profile and Contact Strain Distribution

Free electrons in the surface of metals can produce surface plasmons if they are excited by evanescent waves. The surface plasmons are electromagnetic waves that by decaying can generate photons. These photons produce propagating waves that feed energy to an oscillating cavity. From this effect a Fabry–Perot like interferometer is generated. In view of this basic knowledge a truly novel application on how these plasmons can be used experimentally is introduced in this article. Essentially, by using the transformation of evanescent waves into propagating waves via the plasmon generation it is possible to perform high accuracy contouring of any metal surface as well as the determination of its contact stresses. A laser beam illuminates a glass-air-metal interface and through a proper optical setup it will produce double evanescent illumination. This illumination generates interference fringes that contain surface depth information as well as in-plane strain information. In order to show the robustness of the proposed technique, three different applications are presented in the paper. First, the surface roughness mapping of a copper plate is determined and the obtained values are in statistical agreement with the data measured by a mechanical profilometer. Second, the surface profiles of nickel alloy surfaces used as calibrated standards for the determination of Ra values are obtained. The measured Ra values statistically agree with the values of the different standards. Finally, the contact strains of a copper cylinder on glass are also measured. The contact strains follow the trend predicted by the K.L. Johnson’s model of contact. Local regions (asperities) experiment large plastic deformations