Investigation of the nonlinear optical coefficients of gold nanoparticles by Z-scan method using the femtosecond laser pulses

The present work measured the sign and the amount of nonlinear optical absorption coefficient and nonlinear refractive index of gold nanoparticles by Z-scan method. The Ti: sapphire laser is used in two modes: continuous wave and pulsed with 60 fs duration and a repetition rate of 80 MHz at the wavelength of 800 nm, with an average power of 28 mW. Gold nanoparticles were obtained by the laser ablation method with 20 ns pulses and a repetition rate of 10 Hz in SDS solution. Due to using femtosecond laser pulses with a high repetition rate, thermal and electronic nonlinear effects appeared. The results were compared with a continuous wave Z-scan at the same wavelength and exposure power to eliminate the cumulative thermal effect and separate the electronic and thermal nonlinear coefficients. The electronic and thermal nonlinear absorption coefficients were obtained 15×10-5 and 125×10-5cm.W-1, respectively. Kerr de-focusing effect occurred in both electronic and thermal nonlinearity, and the nonlinear electronic and the thermal refractions were calculated -41×10-9 and -34×10-9cm2W-1, respectively. The experiments show that the nonlinear coefficients of the sample prepared in this research are larger than the values obtained for the samples made by the chemical method.