Time-resolved degenerate four-wave mixing studies of solid-state poly( p -phenylene) oligomers

We have measured χ(3) for a series of solid-state samples of polyphenylene oligomers, (Ph)n, where n=4–8 is the number of phenyl ring units, using optical pulses of 140 fs duration and 650 nm wavelength. A degenerate four-wave mixing (DFWM) signal was only observable for n≥6, and thereafter the measured values of χ(3) increased monotonically with n. Measurements of the linear absorption are also reported. Although the DFWM measurements were performed at wavelengths far from resonance, the figure of merit χ(3)/α was essentially flat beyond n≥6. In contrast to previous studies of oligomers in solution, the principal nonlinear responses were instantaneous within the 140 fs temporal resolution.