Interference and diffraction in globular metal films

Abstract

The optical properties of globular Sn films on Al₂O₃ substrates are analyzed. These films are composed of Sn particles with sizes of the order of the wavelength of visible light, and are of interest because of their solar-selective properties. The experimental data show interference-film properties at long wavelengths, and a large diffracted component at short wavelengths superimposed on a Lambertian angular distribution. The calculated properties of finite-conductivity wire gratings in general agree well with the observed film properties; however, at short wavelengths a scalar diffraction model describes the angular distribution of the transmitted diffracted light. In this model the globular film appears to have film-thickness variations, and a coherence length of approximately two globule diameters. With an empirical “shadowing” term removed from the experimental data, this scalar model works well even at relatively long wavelengths (λ/4 ≤ globule radius).