Role of the conduction band in electroabsorption, two-photon absorption, and third-harmonic generation in polydiacetylenes

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Abstract
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.