Large third-order electronic polarizability of a conjugated porphyrin polymer

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.