The nonlinear optical spectrum of polydiacetylene in the near infrared

The spectrum of the modulus and the imaginary part of the third order nonlinear optical susceptibility χ(3) of polydiacetylene was studied over the range 750–1000 nm by a combination of two‐photon absorption and degenerate four wave mixing measurements. A peak in the two‐photon absorption spectrum near 880 nm was found to be due to induced absorption, probably caused by direct optical excitation of the long‐lived triplet exciton. No clear evidence was observed for resonances due to two‐photon accessible Ag symmetry states in the spectral range studied.