Application of plasmons to the determination of surface profile and contact stress distribution

Free electrons in the surface of metals can be excited by evanescent waves producing surface plasmons. The surface plasmons are electromagnetic waves that by decaying can generate photons. In this paper is introduced a novel application of plasmons: the contouring of a metal surface and the determination of contact stresses using the transformation of evanescent waves into propagating waves via the plasmon generation. A laser beam illuminates a glass-air-metal interface. The angle of illumination of the laser beam is greater than the limit angle that produces total reflection. The light reflected from the glass interface is reflected again by a mirror that returns back the reflected beam producing double evanescent illumination. This illumination produces interference fringes that contain surface depth information as well as in plane strain information. Two examples of application are presented. The surface roughness mapping of a copper plate is made and the obtained values are in statistical agreement with the data obtained with a mechanical profilometer. The contact strains of a copper cylinder with glass are also measured. The large plastic strains measured in a region of 35x35 m of the contact area are of the same order of magnitude of the strains measured in the uniaxial stress-strain curve of copper at high loads. These large strains are in agreement with the K.L. Johnson’s model of contact strains of asperities.