Electro-absorption measurements and optical nonlinearities of DCH polydiacetylene

摘要

Theoretical calculations have predicted the importance of two-photon absorptions in the third order optical nonlinearities of polydiacetylenes. We have used electro-absorption techniques to make these two-photon transitions slightly one-photon allowed and therefore observable in the one-photon absorption spectrum. Our findings, although unable to ascertain the existence of below-gap two-photon states, nevertheless provide an unexpected view of the conduction band in conjugated chain polymers. The observation of a field induced oscillatory change in the absorption around the vicinity of the band edge and comparison of this signal with theoretical calculations leads to the conclusion that the conduction band is composed of both one-photon and two-photon states. This oscillatory signal cannot be solely explained by one-photon or two-photon states but must include both types for this high energy signal to be reproduced in the theoretical calculations.