Linear and nonlinear optical properties of an erbium two-photon dye salt

The optical properties of erbium tri-4-{[4-(2-{4-[5-(4-tert-butyl-phenyl)-[1,3,4]oxadiazol-2-yl]-phenyl}-vinyl)-phenyl] -phenyl-amino}-benzoate, viz., an erbium two-photon dye salt, were investigated in THF solutions. Using the corresponding two-photon dye monomer acid as a reference material, we observed that the photoluminescence of the erbium two-photon dye salt was quenched significantly while nonlinear absorption for infrared nanosecond excitation was enhanced compared to the monomeric form. On the other hand, femtosecond Z-scan measurements suggest the same two-photon absorption coefficient for the dye in both forms. A possible explanation is that the presence of erbium enhances the intersystem crossing rate and that the nonlinear absorption effect must be due to two-photon absorption and subsequent triplet-triplet excited-state absorption for nanosecond pulsed excitation. Our analysis of the nonlinear absorption (power limiting) data suggests that the intersystem crossing rate is 3 orders of magnitude larger for the erbium two-photon dye salt compared with that in the monomer form.