Femtosecond measurement of the third order nonlinear optical coefficients of CuPc (copper phthalocyanine), CoPc (cobalt phthalocyanine) and ZnPc (zinc phthalocyanine) thin films using Z-scan measurement method

The Z-scan measurement method possessing the simplicity of the experiment set-up as well as the simplicity of the interpretation separately determines the nonlinear changes in index and absorption and has the high sensitivity to all nonlinear optical mechanisms. Thus we performed the experiment, varying a repetition rate at 82MHz, 4MHz, 800kHz and 400kHz with a pulse selector in the laser system. We found that the larger the repetition rate was, the more dominant the thermal nonlinearity was in both the nonlinear changes in index and absorption. At the 4OOkHz and 800kHz repetition rate where the samples didn't show thermal nonlinearity, we compared the experiment results with the theory derived from GD(Gaussian decomposition)method[ 11. At the 4MHz and 82MHz repetition rate where the thermal nonlinearity was dominant we explain the experiment results both qualitatively and quantitatively. The samples were irradiated by Ti-sapphire femtosecond laser with pulse widths of FWHM 100-200 fs. The samples employed for the measurement were CuPc(Copper Phthalocyanine), CoPc(Coba1t Phthalocyanine) and ZnPc(Zinc Phthalocyanine) , fabricated in a thin film by a vacuum evaporation. These have a response time of f s order due to electronic distortion and have widely used as a saturable absorber. Near the resonant absorption of &-band(& 750nm) in CuPc molecule the nonlinear absorption coefficient is121 α=α01+1/1=α0-α0/1_s1=α0+β1(-α0/1_s=β).