Optical percolation and nonlinearity of sputtered Ag island films

The fractal dimension Df and the critical parameter nu of the sputtered percolation Ag island films are measured to be about 1.57 and 1.15, respectively. A modified Z-scan method including both transmittance Z-scan and reflection Z-scan is proposed to measure the pure nonlinear absorption of the semi-continuous Ag films near the percolation threshold. As the microstructure of the sputtered Ag films varies from the discontinuous to the continuous, the linear absorbance decreases from 0.97 to 0.58 while the nonlinear absorption coefficient changes from -1.4x10-5m/W to -4.6x10-6m/W at the wavelength of 800nm. The largest nonlinear absorption coefficient beta of the sputtered Ag films is obtained in the discontinuous film near the percolation threshold, which is mainly contributed by the large enhancement of local field of dis-continuous Ag particles.