We have observed double-peaked phase-conjugate pulses in the degenerate four-wave mixing of nanosecond laser pulses in solutions of diphenyl polyenes. We attribute this to the superposition of fast and slow gratings, where the slow grating is due to two-photon absorption. This is supported by the results of time delay and polarization experiments as well as evidence of a second spatial harmonic grating.

Recently, optical limiting has been demonstrated in solutions of diphenyl polyenes, and χ(3) values an order of magnitude larger than CS2 have been measured.1 In this report, degenerate four-wave mixing (DFWM) has been used to investigate the origin of the nonlinear response in solutions of diphenyl polyenes. The DFWM measurements were made using a frequency doubled Q-switched Nd: YAG laser producing 7 nsec pulses at 532 nm. The temporal profile of the phase-conjugate signal observed contains two features separated by ~5 ns. These results indicate that the nonlinear response of these materials contains both a fast and a slow component. The fast component is attributed to intrinsic nonlinearities. The slow component is attributed to slowly responding processes such as thermal effects and has been studied using polarization and time delay experiments. The results of these experiments indicate that the slow component is not simply explained and may consist of more than one nonlinear response. The molecules are known to have two-photon absorption near the laser wavelength that may be contributing to the slow component that is observed.