Optical properties of well-defined granular metal systems

Optical properties of granular metal systems prepared by alternate vacuum deposition of Ag and MgF₂ have been studied. Mean thickness of Ag island films and MgF₂ spacer layers were about 0.2 and 5 nm, respectively. Three samples consisting of 1, 10, and 50 island films were prepared, which are denoted as S₁, S₁₀, and S₅₀, respectively. Optical properties of S₁ were well explained by the simple formula for the two-dimensional (2D) system. Those of S₅₀ were well explained by the Bruggeman theory (BT) for the 3D system rather than by the Maxwell-Garnett theory (MGT). Those of S₁₀ could not be explained by BT as well as by MGT, because the thickness of the sample is so small that the influence of the sample surface cannot be ignored. In each case the dielectric constants contributed by bound electrons in silver fine particles were found to be modified from those of the bulk, which suggests also a modification of the energy-band structure.