We observe a resonant transition in microwave absorption in thin thermally deposited Au nanoparticle films near the geometrical percolation transition pc where the films exhibit a fractal heterogeneous geometry. The absorption of incident microwave radiation increases sharply near the microwave percolation threshold near the geometric pc, consistent with effective medium theory (EMT) predictions. Both EMT and experiment indicate that the hierarchical structure of these films makes their absorption insensitive to the microwave radiation wavelength so that the singular absorption of microwave radiation pc is observed over a broad frequency range between 100 MHz and 20 GHz. Previous observations indicate that the interaction of electromagnetic radiation with randomly distributed conductive scattering particles gives rise to localized resonant modes and our measurements indicate that this adsorption process is significantly enhanced for microwave radiation in comparison to ordinary light. In particular, near the percolation transition a portion of the injected microwave power is stored within the film until dissipated. Finally, we find that the measured surface conductivity s of our Au films can be quantitatively described at all Au concentrations by generalized effective medium (GEM) theory, where the fitted conductivity percolation exponents, and pc itself, are consistent with known two-dimensional estimates. Our results demonstrate that microwave measurements provide a powerful means of remotely measuring the electromagnetic properties of highly heterogeneous conducting films, enabling the purposeful engineering of the electromagnetic properties of thin films in the microwave frequency range through the fabrication of disordered films of conducting particles such as metal nanoparticles, carbon nanotubes, etc.
Pub Type: Journals
semi-continuous films, microwave conductance, absorptance, percolation