Infrared durable protective/antireflection coatings with high performance on Ge and Si substrates

Firstly, a reverse synthesis method is outlined for the determination of the optical constants of infrared coatings from transmittance spectrum data by optimizing the physical thickness, refractive index and extinction coefficient, which can apply to all kinds of transmission spectra and do not rely on the existence of interference fringe patterns and the absorption of substrates. A lot of deposition experiments with the ion-assisted deposition (IAD) technique have firstly done about infrared optical materials ZnS, YF3, Ge and DLC (diamond-like-carbon), in order to obtain high dense and low absorbance layers in the 3-5μm and 8-12μm spectral windows. Using the reverse synthesis method, the refractive index and extinction coefficient of these materials have been obtained accurately. The appropriate deposition parameters and the problems from the deposition process are in detail discussed in this paper. YF3 has been then associated with ZnS and Ge layers so as to obtain antireflection coatings with high performance on Ge and Si substrates in 3-5μm and/or 8-12μm range. In addition, a novel multi-layer infrared AR coating utilizing DLC film as one of the low index films has also been developed for Si and Ge substrates. The optical performance is satisfied (average transmittance is no less than 97%, maximum transmittance is more than 98%, from 8μm to 11.5μm), which is better than that of a single film of one quarterwave DLC coating with interior surface multi-layer AR coated on Ge substrates. However, the adhesion between multi-layer AR coatings and DLC coating is not well, leading to coatings falling off from substrates after humidity test and moderate abrasion test, according to MIL-F-48497. Through a lot of experiments, a non-absorbing coating material has been found as bonding layer to solve this problem. The optical properties of three kinds of infrared coatings and the environmental and physical durability test results including high/low temperature test, abrasion test, salt solubility test, etc, are also in detail presented in this paper.