Ultrafast optical Kerr effect measurements of third-order nonlinearities in cross-conjugated iso-polydiacetylene oligomers

摘要

We present a study on the ultrafast third-order nonlinearities of a novel series of iso-polydiacetylene oligomers (iso-PDAs). Unlike polydiacetylenes that contain a linearly-conjugated backbone, iso-PDAs have a backbone that is cross-conjugated. A new Kerr-gate technique, differential optical Kerr effect (DOKE) detection, is used to measure third-order nonlinear susceptibilities, X~(3), and second hyperpolarizabilities, #gamma#, of monomer, dimer, trimen, pentamer, and heptamer samples in tetrahydrofuran (THF) solutions. A linear increase in #gamma# as a function of the number of repeat units is observed for all samples except the monomer, suggesting that the fixed-length linearly conjugateed segments dominate the electronic polarizatbility. An added increase to the oligomer nonlinearities due to communication along the cross-conjugated path is not observfed. The largest nonlinearity was observed in the heptamer sample, displaying a second hyperpolarizability relative to the THF solvent of #gamma#_heptamer/#gamma#THF=181(+-)9. In addition, an interesting feature arising out of the signal decay tail is present in the samples but absent in our THF solvent reference.