Oriented Thin Films for Nonlinear Optics

Nonlinear optical dichroism technique is shown to be an useful tool for the 1D conjugated polymer chain orientation study. When done as a function of rotation angle it gives the order parameters. These parameters can be also obtained by coupling this technique with linear dichroism method. The technique is illustrated by third harmonic generation on epitaxied polydiacetylene thin films either on single-crystalline or on preoriented ultrathin films by rubbing. Whereas an almost perfect orientation is obtained in the case of epitaxy of p-DCH, poly [1,6-bis(N-carbazolyl)-2,4-hexadiyne], on KAP, potassium acid phthalate, single crystal, this is smaller for epitaxy on preoriented substrate. The results are correlated with thin film texture measurements with X-rays. A good overall agreement is seen except that X-ray data show better order than nonlinear optical dichroism. A possible origin of this difference is shortly discussed.