We report experimental and theoretical investigations of electroabsorption in a polydiacetylene, and determine the complete mechanism of third-harmonic generation (THG) and two-photon absorption (TPA) in linear-chain π-conjugated polymers. The experimental electroabsorption is studied by transmission, rather than reflectance techniques. In addition to the Stark shift of the exciton, a significant feature is observed in the difference spectrum at a higher energy, where the linear absorption is negligible. The origin of this high-energy feature has been controversial. We report several extensive theoretical calculations within the extended Hubbard model, and are able to establish a universality that exists within one-dimensional Coulomb correlated models. We show that the high-energy oscillatory feature in the electroabsorption spectrum originates from the conduction-band threshold, which is separated from the exciton in polydiacetylenes. We also demonstrate that even-parity two-photon states that occur below the one-photon exciton are not observed in electroabsorption due to a cancellation effect. However, a dominant two-photon state that is predicted to occur in between the lowest optical exciton and the conduction-band threshold should be observable. The cancellation, which is only partial for the dominant two-photon state, can, however, reduce the intensity of the resonance due to the state. We show that the conduction-band threshold state, which is an odd-parity one-photon state, also plays an important role in other nonlinear optical processes such as third-harmonic generation and two-photon absorption.

Third-order optical nonlinearity in linear-correlated chains is dominated by four essential states: the ground state, the lowest optical exciton and the conduction-band threshold states, and the two-photon state that lies in between the two excited odd-parity states. The two most important predictions of our theoretical work are (a) third-harmonic-generation experiments on ideal isolated strands should find two, not merely one, three-photon resonances, originating from the exciton and the conduction-band threshold states, and (b) only one dominant two-photon resonance in the infinite-chain limit should be observable in THG and TPA. Extensive comparisons between theoretical predictions and experiments are made to prove the validity of the theory. Conjugated polymers other than the polydiacetylenes are discussed briefly.

• Received 8 September 1992

DOI:https://doi.org/10.1103/PhysRevB.48.1433