6533b7d0fe1ef96bd125b8c0

RESEARCH PRODUCT

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

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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.