Picosecond nonlinear optical response of three rugged polyquinoxaline-based aromatic conjugated ladder-polymer thin films

We have fabricated the first optical quality polymer films of aromatic ladder polymers having an unbroken conjugated backbone, and find third-order nonlinear susceptibilities two orders of magnitude larger than CS2. The nonlinearities respond in less than 6 ps. Although we find their space symmetry to agree with that expected for a nonresonant electronic nonlinearity, the mechanism may well depend on lattice motion. The polymers are stable and rugged at room temperature. We also develop and analyze a simple form of degenerate four-wave mixing for the measurements, finding expressions for the effects of uncertainties in the spatial and temporal pulse profiles.