OPTICAL-PROPERTIES OF THIN SEMICONTINUOUS GOLD-FILMS OVER A WAVELENGTH RANGE OF 2.5 TO 500 MU-M

The optical reflectance and transmittance of percolating gold films close to the metal-insulator transition were measured over an extended wavelength range: from 2.5 to 500-mu-m. It is shown that the inhomogeneous nature of such films controls the optical properties even at such long wavelengths as 500-mu-m, where the typical grain size is 10 nm. Effective-medium theory is shown to be invalid close to the percolation threshold even at 500-mu-m. Comparison of the measured data of all the samples with the scaling model of Y. Yagil et al. [Phys. Rev. B 43, 11 342 (1991)] yields excellent agreement over the entire wavelength range. In particular, the short length scale determined by the anomalous diffusion relation is shown to be the relevant length scale for the optical measurements. This scaling model for the optical properties of such films is thus experimentally justified, both qualitatively and quantitatively.