Resonance optical response of small dielectric clusters

The optical response of small clusters made of two-level molecules is investigated with particular emphasis on the near-resonance region. It is seen that the dielectric-function concept is still useful for describing first-order properties such as particle polarizability and particle-absorption profile; however, near resonance the dielectric response is strongly affected by dephasing and by the energy-level shifts resulting from clustering of molecules in space. Finite-size effects on the dielectric response are also more pronounced near resonance. Second-order response (light scattering) cannot be described by the dielectric function if dephasing processes are important (as they usually are near resonance) and different response functions must be used for different observables. For a cluster of N molecules the light-scattering cross section is made of two contributions: a coherent part (proportional to N2) which may be described by the dielectric-function concept and an incoherent part (proportional to N). Implications for surface effects on the optical properties of molecules adsorbed on such clusters are considered.