Abstract

Polydiacetylenes (PDAs) have been widely studied because of the large optical nonlinearity associated with the delocalized π-electron system. They have a generic structure shown in Fig. 1. The conjugation between the main chain and the side branches R and R' is usually insignificant and hence the side branches have little direct effect on the electronic structure of the main chain, except for the possible steric effect observed, for instance, in the hydrogen-bonded crystals. The main chain, therefore, has the inversion symmetry and the optical second-harmonic generation (SHG) is not expected. However, in studying the anisotropy of the optical nonlinearity of highly oriented Langmuir-Blodgett (LB) films of heptacosa-10, 12-diynoic acid, we found a strong SHG in some cases. We ascribe the observed SHG to the defects in the LB films.

© 1988 Optical Society of America