Abstract

Certain organic and molecular polymers have been shown to exhibit large, nonresonant optical nonlinearities that arise from pi-electron delocalization. Such materials offer novel features for applications in optical devices due to the intrinsic picosecond response associated with this delocalization. Side-chain liquid crystal polymers are particularly interesting, due to the potential for molecular design of desirable macroscopic properties. We report measurements of the nonlinear optical susceptibility of select side-chain polymer systems over a wide temporal and spectral range. The nature of the nonlinear optical susceptibility may be determined by degenerate four-wave mixing, using pulsed lasers and time delay between writing and readout pulses. At subpicosecond times, the electronic contribution to the susceptibility can be measured by direct determination of the conjugate reflectivity. As the pulse width is increased certain motional contributions, probably due to the rotation of large side-chain organic molecules, appear, which increases the overall conjugate reflectivity. Both electronic and motional contributions are exhibited for select materials, with extremely fast response already reported for other polymers, such as PBI. Both contributions are relatively wavelength independent, i.e., resonant. We also report on enhancements in the third-order response when the pulse length is increased to 10 ns, and the wavelength lies within the absorption tail of the LC polymer. In this regime, a thermal grating is formed in the material, which produces thermally induced degenerate four-wave mixing. This mechanism requires coherence in the two writing beams, and a delay of ~1–10 ns on readout to allow the grating to fully develop. All three contributions are described for several polymer systems.

© 1986 Optical Society of America