Picosecond phase conjugation in polydiacetylene gels

A detailed analysis of picosecond time-resolved phase conjugation at 1.06 μm, in the transparency region of soluble polydiacetylenes is presented. Different grating contributions are isolated. It is shown that in red and blue gel forms, phase conjugation at high pump energies is driven by two-photon absorption degenerate six-wave mixing, superimposed on degenerate four-wave mixing. The third-order nonlinear susceptibility of these polymers is reported: It reaches 10−11 esu (30 times CS2 value) for the pure blue amorphous polymer. Two-photon excited species increase phase conjugation at 1-GW cm−2 pump power by one order of magnitude.