Nonlinear refraction and multiphoton absorption in polydiacetylenes from 1200 to 2200 nm

We report femtosecond measurements of the dispersion in the nonlinear refraction and multiphoton absorption in polydiacetylenes, specifically in PTS [polymer poly (bis para-toluene sulfonate) of 2,4-hexadiyne -1,6 diol] over the spectral range 1200 to 2200 nm. Various modifications of the Z-scan technique were used to make the measurements. The nonlinear refractive index coefficient n(2) decreased monotonically with wavelength and can be reasonably extrapolated to previous measurements at 1064 nm. It was found that multiple multiphoton absorption mechanisms contributed to the nonlinear absorption at most wavelengths so that the intensity dependence at each wavelength was needed to identify the different contributions. A two-photon absorption coefficient decreasing monotonically with increasing wavelength was identified with the long wavelength tail of the massive two-photon absorption peak measured previously at 930 nm. The three-photon absorption coefficient showed a weak resonance around 1850 nm associated with the one-photon absorption into the odd symmetry peak at 620 nm, but also exhibited larger values at shorter wavelengths whose assignment is not clear. The four-photon coefficient, measurable only around 1900 nm was associated with four-photon absorption into the even symmetry 465 nm state responsible for strong two-photon absorption measured previously at 930 nm. This resonance, normally much too weak to be observed, was measurable only because of the accidental degeneracy with the three-photon absorption resonance. This degeneracy also leads to a single photon excited state absorption into the 465 nm state via an initial three-photon absorption into the odd symmetry 620 nm state. It was shown that this (3+1) process is in the saturation regime over the intensity range of the measurements and does not contribute to absorption change proportional to the cube of the input intensity, which indicates the pure four-photon absorption process. Thus the measured four-photon coefficient was identified to be due to true instantaneous four-photon absorption. Finally, polarization-dependent Z-scan was used to evaluate the difference between the linear and nonlinear absorption coefficients parallel and perpendicular to the polymer's conjugation axis and to crosscheck the nonlinear measurements made via Z-scan. The differential linear absorption coefficient was found to be in a good agreement with that measured by a standard spectrophotometer measurement.