Optical dephasing of organic dye molecules doped in cross-linked polyvinyl alcohol derivatives: Incoherent photon echo and hole-burning studies

Abstract

Optical dephasing of the zero-phonon line (ZPL) of organic dye molecules doped in polyvinyl alcohol (PVA) and several cross-linked PVA derivatives was studied by using the incoherent photon echoes and the photophysical persistent hole-burning. It was found in the incoherent photon echo measurements that optical dephasing time of the ZPL increases with increasing the length of a cross-linker introduced to the PVA backbone. The difference in the temperature dependence of the dephasing time was also observed between a dye doped in PVA with and without the cross-link. When a longer cross-linker was introduced, the phonon sideband component in the incoherent photon echo signal became dominant in contrast to that in PVA with a relatively short cross-linker, while the 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 similar to that in PVA. However, the depth of the zero-phonon hole burnt with the same burning power was found to depend on the PVA hosts with the cross-linkers. These our findings suggest that the introduced cross-link 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 dephasing by introducing the cross-link can be interpreted by assuming a void space in PVA, which is created near to the cross-linker. In addition, absorption and fluorescence spectra in these systems were measured, which imply the occurrence of weak dye–dye interaction when the dye is doped in PVA with a much longer cross-linker.