Inhomogeneous broadening induced long-lived integrated two-color coherence photon echo signal

Recent observations of the long-lasting nonlinear signals in a variety of light-harvesting complexes have initiated an active debate on the origin of long-lived coherence in the biological systems. In this work we show that disorder of site energy can induce a long-lived electronic coherence between two chromophores in a strongly coupled dimer system, in addition to the ensemble dephasing effect. This phenomenon is physically explained as the correlated fluctuation of excitons with the equal delocalization on two sites, when the site-energy distributions overlap to give resonance. Using the integrated two-color coherence photon echo signal as an example, we show that the coherence in such a system exhibits a biexponential decay with a slow component with a lifetime of hundreds of femtoseconds and a rapid component with a lifetime of tens of femtoseconds. The current result provides a possible microscopic basis for the electronic coherence to be the origin of the long-lived coherence signals to be considered along with other recently proposed mechanisms.