Microstructural and nonlinear optical properties of thin silver films near the optical percolation threshold

The microstructural and the nonlinear optical properties of thin silver films were investigated near the optical percolation threshold. Thin silver films were deposited by using a thermal evaporator. From the transmission electron microscopy measurement, the sub-15-nm-thick films were found to exhibit a discontinuous network composed of metallic islands and a supporting matrix. As the film thickness was increased, the size of the metallic islands increased. From the transmission spectra of silver films with various thicknesses, the optical percolation transition was found to occur near a film thickness of 9 mn. As the film thickness was increased, the transmittance variation over the wavelength range from 800 nm to 2400 nm changed from an increasing behavior to a decreasing behavior. From the nonlinear transmission experiment employing femtosecond-laser pulses for seven different volume fractions of silver islands, the maximum nonlinear absorption was found to occur at a film thickness of 7.5 nm, a little less than the critical thickness required for optical percolation. The enhanced nonlinear absorption is thought to be due to the local field effect caused by metallic islands.