Adjustable selective profiles using cermet absorbing films

Abstract

The optical and electrical properties of metal-dielectric composites (cermets) show large variations as a function of volume fraction, size, shape and the statistical distribution of metallic grains in the dielectric. These variations are of particular importance around the percolation threshold, where the so-called dielectric anomaly, attributed to the excitation of a resonant mode of the conduction electrons in the metallic grains, is observed. Therefore, cermet films close to, but below, the percolation condition are strongly absorbing in the visible range. In the infrared, they behave like a dielectric and are thus transparent. The flexibility of the optical properties around the percolation threshold can be used to develop new spectrally selective coatings totally absorbing in the visible range and highly reflecting in the IR, with an adjustable cut-off wavelength. We present optical simulations of some specially adjusted spectral profiles and their experimental realization using a three-layer coating consisting of a Pt-Al 2O 3 cermet absorbing film, an Al 2O 3 antireflecting film and an Al 2O 3 underlayer, deposited on a metallic substrate. These coatings are stable at high temperatures and exhibit a high mechanical resistance. They can be used as selective absorbers for many specific applications as a result of their cut-off wavelength: selective solar absorber surfaces, surface thermometers and flux meters made of thin film thermocouples, IR detectors, etc.