The optical properties of ultrathin Au films on silicon have been studied in the infrared over a wide frequencyrange from 200 to 10,000 cm~(-1). Thick films show a Drude behavior; i.e., with increasing frequency the transmission increases; for films below the percolation threshold at about 5 nm a negative slope for the frequency-dependent transmission is observed. When the thickness is further reduced, between 1 and 3 nm an anomaly occurs: the relative transmission reaches maximum values above 100percent compared with the bare substrate, indicating an antireflection coating of nanometer thickness for light of 5 (mu)m wavelength. This anomaly can be explained in the framework of effective-medium theories.
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