Roles of band states and two-photon transitions in the electroabsorption of a polydiacetylene

We report results of experimental and theoretical investigations of electroabsorption (EA) in a DCH polydiacetylene. In addition to the Stark shift of the exciton, a high-energy feature is seen in EA, whose origin has been the subject of controversy. Based on exact theoretical calculation of the EA within an extended Peierls-Hubbard model, we ascribe the high-energy signal to transitions to a continuum threshold. We briefly discuss the contributions of these band threshold states to the third-order optical nonlinearity, {chi}{sup (3)}.