OPTICAL DEPHASING OF ORGANIC-DYE MOLECULES DOPED IN CROSS-LINKED POLYVINYL-ALCOHOL DERIVATIVES - INCOHERENT PHOTON-ECHO AND HOLE-BURNING STUDIES

Optical dephasing of the zero-phonon line (ZPL) of organic dye molecul es doped in polyvinyl alcohol (PVA) and several cross-linked PVA deriv atives was studied by using the incoherent photon echoes and the photo physical persistent hole-burning. It was found in the incoherent photo n echo measurements that optical dephasing time of the ZPL increases w ith increasing the length of a cross-linker introduced to the PVA back bone. The difference in the temperature dependence of the dephasing ti me was also observed between a dye doped in PVA with and without the c ross-link. When a longer cross-linker was introduced, the phonon sideb and component in the incoherent photon echo signal became dominant in contrast to: that in PVA with a relatively short cross-linker, while t he dephasing time of the ZPL was still longer than that in PVA without the cross-link. Hole-burning studies showed that the phonon sideband spectrum of a dye doped in the cross-linked PVAs is essentially simila r to that in PVA. However, the depth of the zero-phonon hole burnt wit h the same burning power was found to depend on the PVA hosts with the cross-linkers. These our findings suggest that the introduced cross-l ink does not alter the electron-phonon coupling modes or pseudo-local mode, but it drastically reduces the effect of the two-level tunneling systems in PVA on the optical dephasing of a doped dye. As a result, the dephasing time of the dye in the cross-linked PVAs becomes longer than that in PVA without the cross-link. The decrease of the optical d ephasing by introducing the cross-link can be interpreted by assuming a void space in PVA, which is created near to the cross-linker. In add ition, absorption and fluorescence spectra in these systems were measu red, which imply the occurrence of weak dye-dye interaction when the d ye is doped in PVA with a much longer cross-linker.