Electro-absorption in a polydiacetylene

Abstract

The electro-absorption in a DCH-polydiacetylene has been investigated both experimentally and theoretically. In addition to the red Stark shift of the exciton, a significant feature in the difference spectrum is observed at a higher energy. Within an interacting electron model (extended Peierls-Hubbard model), the roles played by the dipole-allowed one-photon states and the dipole-forbidden two-photon states are studied in detail. It is found that the high-energy signals in the difference spectrum are due to the strong field-induced mixing of the one-photon state at the edge of a continuum structure, or ‘conduction band’ in the language of band theory, with its neighboring two-photon states. Contributions to the electro-absorption from the one- and two-photon states cancel deep within the band. Three-photon resonance due to the band edge is expected in third-harmonic generation (THG) and has been seen in both a polydiacetylene and in polysilanes.