Nonequilibrium spectral diffusion due to laser heating in stimulated photon echo spectroscopy of low temperature glasses

A quantitative theory is developed, which accounts for heating artifacts in three-pulse photon echo (3PE) experiments. The heat diffusion equation is solved and the average value of the temperature in the focal volume of the laser is determined as a function of the 3PE waiting time. This temperature is used in the framework of nonequilibrium spectral diffusion theory to calculate the effective homogeneous linewidth of an ensemble of probe molecules embedded in an amorphous host. The theory fits recently observed plateaus and bumps without introducing a gap in the distribution function of flip rates of the two-level systems or any other major modification of the standard tunneling model.Comment: 10 pages, Revtex, 6 eps-figures, accepted for publication in Phys. Rev.

Chromophore orientation and the formation of surface reliefs in azobenzene side-chain polymers

The occurrence of surface relief gratings in azobenzene polymers under illumination with two superimposed laser beams is a puzzling effect which was discovered only few years ago. The origin of the large mass transport which occurs far below the glass transition temperature is not yet understood. We have performed experiments with cw holography. Due to low laser fluence, thermal effects can mostly be neglected. We have studied the relation between the light induced orientation of the chromophores in the bulk of the material and the observed surface relief gratings. We have performed a number of studies: Surface relief gratings were characterized under different polarization geometries of the writing beams, furthermore the grating period was varied. We could show that there is a competing behavior between the diffraction efficiencies by the volume grating and the surface relief, respectively. In order to analyze the orientational distribution function of the azobenzene chromophores, we have performed confocal Raman microspectroscopy in different regions of the surface relief grating. Our results are not in agreement with the current theories for the formation of surface reliefs