Abstract

Conjugated heterocyclic rigid rod polymers are of special interest as nonlinear optical (NLO) materials because of their robust physical properties, which include excellent mechanical strength and thermal stability.¹ Recent advances¹ in the solubilization and processing of these polymers make optical- quality thin films available for NLO measurements. This paper reports χ⁽³⁾-Pi-characterization of thin films of poly(p-phenylene benzo-bisthiazole) (PBZT) and PBZT/nylon molecular composites by third-harmonic generation (THG). Solutions of PBZT or PBZT/nylon 66 in Lewis acid (GaCl₃ or AlCl₃-nitromethane were prepared as previously described.² Thin films of the pure PBZT or its molecular composites with nylon were prepared in a drybox by solution casting onto 1 mm optically flat fused-silica substrates, followed by washing in water and vacuum drying. The films, which were as thin as 0.4 µm, were of fair optical quality, as evidenced by weak optical scattering and by a uniform THG response over a large area of each sample. The films were slightly yellow, with a cutoff wavelength of 0.5 µm. Third- harmonic-generation experiments were performed with a picosecond laser system that was continuously tunable in the range 0.6-4 µm.³ At 1.8 µm, we measured a nonresonant χ⁽³⁾ = (1.0 ± 0.2) X 10⁻¹¹ esu for pure PBZT. The χ⁽³⁾ values obtained for 2:1, 1:1, and 1:2 PBZT/nylon composites correlate well with the pure material. No anisotropy in the films was detected. The large optical damage threshold of PBZT (>50 GW/cm² for 30 ps pulses at 1.8 µm) further adds to the attractiveness of this material. We have probed the effects of two- and three-photon resonances by performing THG studies over a large wavelength range. The results will be reported at the meeting.

© 1990 Optical Society of America