NONLINEAR OPTICAL-PROPERTIES OF POLY(PARA-PHENYLENEBENZOBISOXAZOLE)

The third-order nonlinear optical susceptibility chi(3)(-3-omega;omega,omega,omega) of the conjugated rigid-rod polymer poly(p-phenylenebenzo[1,2-d:5,4-d']bisoxazole) (PBO) has been investigated by third-harmonic generation spectroscopy and theoretical modeling in the wavelength range 0.9-2.4-mu-m (1.4-0.5 eV). The chi(3) spectrum of PBO exhibits a strong three-photon resonance with a three-photon resonance enhanced chi(3)(-3-omega;omega,omega,omega) value of 7.0 X 10(-11) esu at 1.2-mu-m. The nonresonant-chi(3) value at 2.4-mu-m was 8.1 X 10(-12) esu, which is large and comparable to some of the best conjugated polymers. The chi(3) dispersion data were found to be well described by a theoretical two-level essential states model. These results for the oxygen-containing heterocyclic rigid-rod polymer (PBO) were compared and contrasted with those previously reported for the structurally analogous sulfur-containing poly(p-phenylenebenzobisthiazole) (PBZT). The results indicate that the magnitude of chi(3) is essentially identical in PBO and PBZT, suggesting the absence of an effect of the heteroatom on the nonlinear optical response of this class of heterocyclic rigid-rod polymers in contrast to prior oligomer model compound studies which had shown a factor of 3 enhancement by the sulfur heteroatom.