Temperature Cycling of Persistent Hole Spectra: A Method for Investigating the TLS Density in

Abstract

Persistent hole burning yields a spectral resolution of the order of the homogeneous linewidth, which, especially in the case of amorphous materials, greatly improved the utility of optical spectroscopy. However, the homogeneous linewidth itself can in general not be measured in a hole-burning experiment, since in virtually all disordered solids the hole width is affected by spectral diffusion [1,2]. The time [3] and temperature [4] dependence of spectral diffusion, on the other hand, can be investigated by fast hole-burning measurements and by thermal cycling, respectively. During a thermal cycle, spectral diffusion gives rise to an additional irreversible inhomogeneous broadening component of a hole spectrum because at a higher excursion temperature more tunneling states or TLS of the glassy matrix are in thermal equilibrium than at the burning temperature. This irreversible broadening yields information on the activation energies of the TLS [4,5].