Measurement of the nonlinear refractive index of polydiacetylene using Michelson interferometry and z-scan

We have performed a detailed measurement of the sign of the nonlinear refractive index of PTS-polydiacetylene using Michelson interferometry and z-scan at 1.06 mu m wavelength. The measurement was performed using 60 ps laser pulses at 10 Hz repetition rate that avoided any significant thermal contribution. The sign of n(2) was found to be negative. The negative sign was explained considering saturation of the exciton absorption for off-resonant pumping. The saturation far outweighs two photon resonances in determining sign and magnitude of n(2) in the off-resonant domain within the wavelength range 700-1064 nm. (C) 1999 American Institute of Physics. [S0021-8979(99)04101- 8].