Surface conductance and microwave scattering in semicontinuous gold films

Abstract

Semicontinuous gold films 4 nm to 12 nm thick were characterized using patterned coplanar waveguides over a frequency range of 100 MHz to 20 GHz. Such films can form two dimensional fractal aggregates mixed with dielectric voids with unusually large electromagnetic absorption. Surface conductance and microwave absorption were obtained from the measured scattering parameters using a microwave transmissionâ€reflection model. Within the percolation coverage of gold nanoparticles, when the surface conductance increases from 10^-5 S to 10^-3 S, the film properties transition from dielectric to metallic, and the corresponding microwave transmittance falls far more rapidly than the classical skin depth model would suggest. The resulting microwave absorption attains a peak value in this range, which results from an inhomogeneous localization of an electromagnetic field in fractal structures. The dielectric to metallic transition can be easily identified experimentally from an abrupt change in the phase of the reflection scattering parameter S₁₁. The results demonstrate a convenient easurement technique to study electromagnetic properties of surfaceâ€enhanced semicontinuous metallic films for thin broadband absorbers with minimized reflection, and for other microwave applications.