Low temperature optical dephasing in polymethylmethacrylate: A photon echo study below 1 K

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Abstract
The first photon echo experiments on polymethylmethacrylate (PMMA) down to temperatures below 1 K are reported. A linear temperature dependence of the linewidth of the chromophore zinc-meso-tetraphenylporphine (ZnTPP) is obtained below 1 K. Based on this result a flat distribution of two-level system energies E on the time scale of our experiment is deduced. Above 1 K an activated process with an energy of 5 cm−1 is found, which explains the power law dependence usually measured by photon echo experiments in the temperature region 1.2–2.1 K. The fit parameters of the proposed process are shown to be in agreement with hole burning measurements and with specific heat data on PMMA.