EXCITONIC RESONANT EFFECT IN THE 3RD-ORDER NONLINEAR OPTICAL-PROPERTIES OF BLUE-FORM AND RED-FORM POLYDIACETYLENE FILMS

The third-order nonlinear optical susceptibility |χ(3)| has been evaluated for blue- and red-form polydiacetylene (PDA) oriented films by means of the Maker fringe method. For the red-form PDA, care was taken to minimize the conformation disorder effect. The observed |χ(3)| spectra for the blue- and red-form films show prominent peaks at about 0.65 (blue) and 2.35 eV (red) which are just three-photon resonant to the 1Bu excitons located at 1.95 (blue) and 2.35 eV (red) in the respective forms of PDA. The results are discussed by a three-level model taking account of the results of electro-absorption measurements on the same films. © 1990.