Annihilation Enhanced Four-Wave Mixing in J-Aggregates

Exciton-exciton annihilation processes are known as influencing the nonlinear response of molecular aggregates in a great deal. Recently, by using simplified model of a collection of two-level quantum systems, significant enhancement of the four wave mixing efficiency due to the annihilation processes was demonstrated1. The comparison with experimental findings2 of DFWM at the excitonic resonance in J-aggregates of pseudoisocyanine chloride (PIC), showed good qualitative agreement for all data, but the wavelength dependence of the nonlinearity power index p-∂(log10 S)/∂(log10 I) of the DFWM signal S on the pump intensity I. In the two-state (single-exciton) approximation p is reduced from the expected value 3 even far in the wings of the J-band due to the power-broadening of the spectrum.