Spectral diffusion of optical transitions in doped polymer glasses below 1 K

We have performed optical hole-burning experiments in the temperature range between 25 and 500 mK. Experimental results for the dye molecule free-base phthalocyanine embedded in the organic polymers polystyrene and polymethlmethacrylate are presented. The hole widths in these systems show a linear temperature dependence over the whole temperature range. Extrapolating to T{→}0 K we obtain lifetime-limited optical linewidths. Assuming a line-broadening mechanism through two-level systems we can interpret our data with a constant density of states, an assumption which has often been questioned.