Optical Response of Rough Metal Surfaces and Island Films
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OPTICAL RESPONSE OF ROUGH METAL SURFACES AND ISLAND FILMS
M. URBAKH School of Chemistry, Tel Aviv University, Ramat Aviv, Tel Aviv 69978, Israel
ABSTRACT This study treats the optical properties of different two-dimensional inhomogeneous systems in which localized and delocalized surface modes excited by incident light can exist. The optical response is described with the use of kinetic equations linking together different types of surface and bulk modes. It is important that both electromagnetic interactions between local modes and the dependence of the optical characteristics of centers on the local geometrical structure are taken into account. It is shown that the behavior of experimentally observable quantities (enhancement ratio of the electric field at the surface, surface absorption, etc.) as functions of the frequency and concentration of local centers depends essentially on the excitation mechanism of local modes.
INTRODUCTION Already since the beginning of the century [1] the optical properties of rough metal surfaces and island films have been attracting considerable attention of experimentalists and theorists. Recently the interest in these problems has greatly increased. Intensive development of investigations in this area was stimulated not only by practical needs but also by the discovery of new phenomena associated with the enhancement of electromagnetic field near the surface, such as surface enhanced Raman scattering (SERS), anomalous intensive absorption, etc. [2]. It should be emphasized that the above-mentioned phenomena were discovered on the so-called microrough surfaces for which the characteristic dimensions of inhomogeneities are no larger than some hundreds of Angstroms and thus is much less than the light wavelength, X . In what way do rough surfaces differ optically from ideally smooth ones? First, light reflection from rough surfaces gives rise not only to a specular reflected wave but also to diffusely reflected waves, leaving the surface at arbitrary angles. Second, the presence of roughness makes possible the generation of surface plasmons (SP) by the electromagnetic wave incident on the metal, and also scattering of SP, which results in their conversion into bulk excitations or other states of SP. Finally, one more feature of rough surfaces is important from the point of view of the enhancement effects. Namely, electromagnetic modes can be excited which are practically localized in the vicinity of inhomogeneities with a small radius of curvature. According to the results of optical experiments, microrough surfaces can be divided into two groups. To the first group belong the surfaces for which the characteristic dimension of inhomogeneities in the direction normal to the interface (height) is much less than the dimensions of inhomogeneities along the surface (slight roughness). The optical properties of slightly rough surfaces have been studied in detail both experimentally and theoretically [3,4]. To the second group belong the surfaces for which the dimension of inhomogeneitie
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