Optical dephasing in glasses: Theoretical comparison of the incoherent photon echo, accumulated grating echo, and two-pulse photon echo experiments

Abstract

We present a detailed theoretical analysis of three nonlinear optical dephasing experiments, the incoherent photon echo, the accumulated grating echo, and the two-pulse photon echo. It has been believed previously that these three experiments provide the same information about optical dephasing, and therefore about a system's dynamics. In systems such as chromophores in glasses, proteins, liquids, or complex crystals, in which spectral diffusion (slow time scale energy fluctuations) as well as homogeneousdephasing (fast time scale energy fluctuations) occur, it is proven that these techniques are not equivalent. While the two-pulse photon echo measures the homogeneous dephasing, the other two techniques are influenced by spectral diffusion. In general, the incoherent echo and the accumulated grating echo will measure dephasing rates which are faster than the two-pulse photon echo. The differences among the methods are calculated using a standard two-level system model of glasses. It is found that the differences depend on factors such as the pulse duration in the incoherent echo and the triplet life time in the accumulated grating echo. We also demonstrate that a combination of techiques can be used to map out the broad distribution of relaxation rates which occur in glasses and many other complex systems.