Gold-black manufacture, microstructure, and optical characterization.

A previous contribution formulates a first-principle dipole antenna theory for predicting the polarization-sensitive directional spectral absorptance of gold-black in the near infrared. The current contribution chronicles a successful effort to validate that theory. After a brief review of gold-black history, we describe in some detail the design and construction of a vapor-deposition cell for laying down gold-black coatings on a mirrorlike gold substrate. The microstructure of 4- and 8-µm-thick coatings is revealed using scanning electron microscopy. An automated bench-level reflectometer has been used to measure the in-plane bidirectional reflectivity of the gold-black coatings in the visible (532 nm) and near-infrared (800 nm) for p and s polarization. Measurements are reported over incident zenith angles ranging between 10 and 50 deg. Results obtained using the apparatus are consistent with the dipole antenna theory in this range of incident zenith angles.

First-principle model for the directional spectral absorptivity of gold-black in the near infrared.

Gold-black coatings formed by evaporation and subsequent sublimation of pure gold in an inert low-pressure atmosphere are used to enhance absorption of thermal radiation. Previous attempts to predict the spectral absorptivity of gold-black coatings have typically assumed a granular continuum with effective bulk optical properties. In the current effort the principles of diffusion-limited aggregation (DLA) are applied to mimic the observed microstructure of actual gold-black layers consisting of a random fractal distribution of dendritic gold filaments that are postulated to behave as dipole antennas. Absorption of incident electromagnetic radiation by individual filaments is predicted using a unique time-dependent lossy antenna model drawing on the Drude-Sommerfeld free-electron theory. Single-filament spectral absorptivities are combined based on the DLA microstructure model to predict the spectral and, for the first time, the directional absorptivity of the gold-black layer. Results for normal spectral absorptivity are shown to be in good agreement with measurements reported in the literature.