] 2 7 Ju n 20 06 Strongly localized plasmon oscillations in a cluster of two metallic nanospheres and their influence on spontaneous emission of an atom

The plasmon oscillations in a cluster of two metallic nanospheres are studied theoretically. Particular attention is paid to the case of nearly touching spheres. Simple analytical expressions have been found for the spectra of plasmon oscillations of different symmetry in this case. A new type of the plasmon oscillations, which are strongly localized between the spheres, and which totally disappear at separation of the spheres, has been discovered. The found plasmon oscillations have a dramatic effect on optical properties of an atom localized between the spheres. Much attention has been paid, of late, to the experimental and theoretical study of optical properties of the metallic nanoparticles. This interest is mainly due to a considerable enhancement of local fields near the nanoparti-cles. An especially high increase occurs in the case of plasmon polariton resonances (" plasmons ") [1, 2] or phonon polariton resonances (" phonons ")[3]. On the basis of this effect one considers quite a number of possible applications. One of the most developed is the use of large local fields for enhancement of the Raman scattering cross-section [4]. Recent experiments have shown that such an increase may achieve 10-14 orders of magnitude, which may help to resolve separate molecules [5]-[7]. The local enhancement of the fields can also be used to increase the fluorescence intensity and to determine the structure of a single DNA strand without using the fluorescent labels [8, 9]. By using the nanoparticles of complex configuration one can provide enhancement of both the absorption and the emission of light by natural and artificial fluorophores [10]. Of particular interest and promise are the studies of optical properties of the clusters of two and more metallic nanoparticles, because by changing the cluster's geometry one can effectively control the spectra of the plasmon oscillations. This effect makes it possible to produce, for example, new types of biosensors [11]-[13]. A whole series of experimental [14]-[17] and theoretical [18]-[27] studies have been devoted to the two-particle clusters.