QUANTUM ELECTRODYNAMICS OF MOLECULAR NANOSTRUCTURES

We present a microscopic account of the linear and nonlinear optical r esponse of an assembly of molecules with nonoverlapping charge distrib utions and arbitrary geometry. Our approach requires only the knowledg e of single-molecule wave functions. The microscopic polarization is d efined by a dipole distribution for each transition; we do not make th e dipole approximation and it is then unnecessary to introduce the Ewa ld summation technique. Equations of motion are derived which provide a quasiparticle (anharmonic oscillator) picture of the optical respons e. As an application, we calculate both the linear susceptibility chi( 1) and the light scattering signal off a crystal in d dimensions (d = 1, 2, and 3). We find that retardation does not affect chi(1), which c ontains a shift in the exciton frequency compared with the single mole cule, but no signature of spontaneous emission. However, the scattered field is retarded and shows cooperative spontaneous emission in reduc ed dimensionality d = 1 and 2. The present approach can be applied to ordered nanostructures as well as disordered systems such as liquids a nd addresses fully the effects of retardation, polaritons, and coopera tivity in linear as well as nonlinear optical processes.