Degenerate four-wave mixing measurements, using 45 ps pulses at 1064 nm, ha
ve been used to determine the magnitude of the third-order optical suscepti
bility tensor for thin films of a conjugated porphyrin polymer. The time de
pendence of the signals indicates that the dominant response is fast relati
ve to the duration of the optical pulses. It is shown that a response on th
is time scale cannot be consistent with a mechanism in which resonant absor
ption is significant, and therefore that the primary component of the susce
ptibility 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 indicat
es the role of inter-macrocycle conjugation in the nonlinearity. This appea
rs to be the largest one-photon-off-resonance third-order optical susceptib
ility reported for any organic material.