Green function approach to nonlinear optical spectroscopy of molecular nanostructures.

摘要

The linear and nonlinear optical response of excitons in confined molecular systems are studied using Green function techniques. The momentum-dependent exciton radiative decay rate in molecular multilayer structures (superlattices) is studied and shown to have a superradiant behavior. On-resonance cooperative nonlinear response is found in small molecular lattices, and it saturates in large lattices. This suggests that there is a coherence size within which molecules have cooperative nonlinear response. The steady wave linear and nonlinear reflection spectrum of molecular multilayers, and the time resolved four wave mixing of a monolayer are calculated. The linear reflection signal can be used to probe the momentum-dependent exciton radiative decay rate. The nonlinear reflection signal probes two-exciton states. The two-exciton decay rate is found to be larger than the sum of the two single exciton decay rates. This nonadditivity is a result of hard core exclusion. The discrete nature of the linear reflection spectrum of molecular multilayers shows a clear evidence of exciton confinement. The size enhancement in the nonlinear optical response induced by Coulomb interactions and radiative coupling between different layers are demonstrated.;We further study the effects of disorder on the linear and nonlinear optical response of molecular excitonic systems. Using the coherent potential approximation, we calculate the energy-dependent exciton radiative decay rate and effective coherence size. Numerical calculations for two-dimensional molecular lattices with topological disorder show that the disorder reduces the radiative coherence size, i.e. diminishes the cooperativity among molecules. We also calculate degenerate four wave mixing in a three-dimensional molecular lattice with topological disorder. A disorder-induced new resonance is found, whose width is proportional to the energy dependent exciton diffusion constant. Numerical results for the diffusion coefficient are presented, and its dependence on exciton energies and molecular densities is explored.