Nonperturbative theory of temperaturehyphen;dependent optical dephasing in crystals. I. Acoustic or optical phonons

摘要

We continue the discussion of the nonperturbative theory of zerohyphen;phonon impurity lines in crystals formulated by Osadrsquo;ko and more recently by us. In that paper, a diagrammatic analysis was undertaken which led to analytic results for the thermal width and shift of the zerohyphen;phonon line. Here we evaluate these results for two model phonon densities of states: the Debye model for acoustic phonons, and a sharply peaked density of states appropriate for optical phonons. For Debye phonons we find that at low temperatures the standard (perturbative) theory of optical dephasing is qualitatively correct, but at high temperatures it becomes very inaccurate. Moreover, if one of the excited state phonon frequencies tends to zero (a soft mode), then perturbation theory breaks down completely. For optical phonons our results give an Arrhenius temperature dependence for the linewidth and place an upper bound on the value of the preexponential. In paper II of this series we will discuss dephasing by pseudohyphen;local phonons, and in paper III we will compare our results to absorption, photon echo, and hole burning experiments.