Engineering molecular aggregate spectra

We show that optical properties of linear molecular aggregates undergo drastic changes when aggregates are deposited on a metal surface. The dipole-dipole interactions of monomers with their images can result in strong restructuring of both the exciton band and the absorption spectrum, depending on the arrangement of the monomer transition dipoles with respect to the surface. 1 Introduction Molecular aggregates, in spite of more than 70 years of history after their discovery by Jelley [1] and Scheibe [2], still attract a great deal of attention due to their extraordinary optical and transport properties: narrow absorption band, superradiant emission, giant higher order susceptibilities and light harvesting [3,4]. Nowadays, the problem of controlling the optical properties of molecular aggregates is a challenging task. In an isolated aggregate, the energy of optical transition is determined by the transition energy of a monomer, which is fixed for a particular monomer, as well as by the transfer (dipolar) interactions, which are also fixed by the aggregate morphology. Therefore, there is almost no way to influence the optical absorption for a single aggregate. Being embedded into a host, an aggregate interact with it, which results in the so-called " solvent " shift of its absorption line. Difference in local solvent environment gives rise to an inhomogeneously broadened absorption band. Coupling of aggregates to vibrational degrees of freedom leads to homogeneous broadening of the aggregate absorption band. In different hosts, both the position of the aggregate absorption peak and its width vary to some extent. However , both effects are usually not very pronounced (see, e.g., Ref. [5]). As it has been pointed out in a number of papers, the optical response of J-aggregates changes substantially when J-aggregates are deposited on colloidal noble metal nanopar-ticles. Thus, super-radiant lasing at low threshold [6] and restructuring of the absorption spectra [7,8,9,10]) have