Large third-order electronic polarizability of a conjugated porphyrin polymer

Degenerate four-wave mixing measurements, using 45 ps pulses at 1064 nm, have been used to determine the magnitude of the third-order optical susceptibility tensor for thin films of a conjugated porphyrin polymer. The time dependence of the signals indicates that the dominant response is fast relative to the duration of the optical pulses. It is shown that a response on this time scale cannot be consistent with a mechanism in which resonant absorption is significant, and therefore that the primary component of the susceptibility must correspond to an instantaneous electronic polarization. The microscopic polarizability per macrocycle of the polymer is approximately 3 orders of magnitude greater than that of the monomer-a result that indicates the role of inter-macrocycle conjugation in the nonlinearity. This appears to be the largest one-photon-off-resonance third-order optical susceptibility reported for any organic material.