Femtosecond incoherent photon echo from rhodamine B in propylene glycol. Inhomogeneous broadening and spectral diffusion at room temperature

Two-pulse degenerate four-wave mixing (photon-echo) experiments using “incoherent” light are reported for rhodamine B in propylene glycol and ethanol at 295 K. The signal detected in a particular direction is strongly asymmetric with respect to the time delay between the two pulses, implying that the electronic spectrum is inhomogeneously broadened on the time scale of the “waiting time” between interactions of the material system with the second and third electric fields. Similar experiments on nile blue yield a symmetric signal response characteristic of a homogeneously broadened system. The effective waiting times in experimental configurations employing parallel and perpendicularly polarized excitation pulses are discussed.