DISPERSION OF THE 3RD-ORDER NONLINEAR-OPTICAL PROPERTIES OF POLY(PARA-PHENYLENE BENZOBISTHIAZOLE) AND ITS MOLECULAR COMPOSITES WITH POLYAMIDES

With third-harmonic generation \chi(3)(-3-omega; omega, omega, omega)\ has been measured in the range 0.8-2.4-mu-m for thin films of poly(p-phenylene benzobisthiazole) (PBZT) and its molecular composites with nylon-66 and poly(trimethylhexamethylene terephthalamide). For pure PBZT, \chi(3)(-3-omega; omega, omega, omega)\ = (8.3 +/- 1.7) x 10(-11) esu near the peak of the three-photon resonance at 1.3-mu-m, and \chi(3)(-3-omega; omega, omega, omega)\ = (6.0 +/- 1.5) x 10(-12) esu off resonance at 2.4-mu-m. A new theoretical treatment that includes the effects of inhomogeneous broadening and vibronic transitions on chi(3) and permits accurate analytic fits of the dispersion data is presented. By preparation of molecular composites it has been discovered that these composites induce macroscopic ordering, or orientational anisotropy, of the PBZT, which can lead to an apparent enhancement of chi(3). The implications of the dispersion of the nonlinear properties of these materials are discussed. We have also measured the damage threshold of PBZT and found it to be approximately 50 GW/cm2 for 30-50-ps pulses at 1.9-mu-m.