Two-dimensional optical spectroscopy of molecular aggregates

Abstract

An overview of the theoretical background for the novel spectroscopic tool – two-dimensional (2D) optical spectroscopy – is presented. Principles of nonlinear polarization induction, signal generation, and detection are described. Concepts of heterodyned four-wave mixing experimental technique and 2D spectra construction are detailed and the scheme of third-order polarization calculation is consistently introduced. The system response function theory is formulated for a general multi-level quantum system considering the system-field interaction perturbatively. Equations of motion for the system density operator relevant to the third-order response are presented. Basic quantum systems of a two-level atom, two-level molecule, and a three-level system are considered and analytic expressions of the third-order signal are derived at certain limits. Molecular complexes are described using the Frenkel exciton approach. 2D spectra of the excitonically-coupled dimers of two-level and three-level chromophores are presented. Possibilities of extraction of separate spectral elements as well as performing quantum control by the two-colour 2D spectroscopy for the dimer of excitonically-coupled two-level systems are demonstrated. Effects of motional narrowing of one-dimensional J-aggregates of pseudoisocyanine and construction of the J-band as well as highly-efficient excitonic energy transfer in photosynthetic Fenna–Matthews–Olson complex are illustrated by simulated time-resolved 2D spectra. Keywords: optical two-dimensional spectroscopy, four-wave mixing, molecular excitons