OPTICAL-PROPERTIES AND MICROSTRUCTURE OF THIN SILVER FILMS

Several previous studies have shown that the optical transmittance of thin gold films at the metal-insulator transition is constant over a wide wavelength band. We show that this holds true for silver films in the spectral range from 1.5-mu-m to at least 43-mu-m. The optical properties of films near the transition are shown to be sensitive to variations in the microstructure of films grown on different amorphous substrates. The optical absorption near the transition is found to be 40% and essentially independent of wavelength. Narrow necks in the conducting pathways are argued to be an essential feature of the microstructure. In particular a correlation length cannot be defined for either silver or void clusters for films near the critical concentration. The optical absorption is evidently due to losses at the narrow necks in the conducting pathways. The scattering properties of films as a function of thickness are discussed, and the absence of any enhanced scattering at the transition is explained in terms of the particle size distribution.