High Throughput Synthesis the Infrared Graded-index Antireflection Coatings

Bin Li^1*, Ping Xie¹, Weitao Su², Dingquan Liu¹

¹ Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai, 200083, China；

² Hangzhou Dianzi University, Hangzhou, Zhejiang Province, 310018, China

ABSTRACT: The lens and windows in the optical system operating in the spectral range of thermal infrared are commonly made of high-refractive-index materials, such as germanium. As a consequence, the resulting Fresnel reflections at the medium to their surface result into a lower net transmittance. For example, germanium with an index of around 4.0 will produce a reflection loss of around 36% per surface. Therefore, the broadband antireflection coatings are indispensable for optical components in the thermal infrared. Although thorium fluoride (ThF₄) is an ideal mechanically stable infrared coating material with a low refractive index, due to the detrimental property linked to its radioactivity, special handling and strict disposal are required whenever it is used. Depending on their structure, antireflection coatings can be classified as two types, namely, composed of homogeneous layers and inhomogeneous layers.

The antireflection coatings with a single inhomogeneous layer are shown advantageous with respect to omnidirectionality and broad-band behavior, in which a refractive index gradient is created along thickness.

In our investigation, the combinatorial materials libraries were synthesized of graded-index layers deposited from four rare-earth fluorides LaF₃, PrF₃, ErF₃ and SmF₃, admixed with a different amount of BaClF. FTIR spectrometer under Hyperion microscope was used to explore the antireflection characterization, EDX was used to determine the stoichiometry, μ-XRD was used to explore the crystallographic structure and trace phases, SIMS was used to in-depth chemical composition analyses. Finally, the libraries were subjected to the durability tests, including water solubility, adhesion and moderate abrasion, in accordance with the military specification MIL-C-4847A in the United States. In Figure 1, the combinatorial materials libraries were illustrated being subjected to the adhesion test.

It was demonstrated that the layers are robust enough to withstand normal handling and environmental conditions, furthermore, the best antireflection approach for the Ge surfaces can be achieved for the BaClF-LaF₃ layer with a BaClF concentration x=0.51, through the high-throughput characterization of the robustness and the spectral transmittance of the elements in the libraries. Because the optimized characteristics of a Gaussian index profile followed by a quintic-profile can be created in the layers, an effective refractive index around 1.2 can be achieved, which is lower than those of infrared low-index coating materials used commonly.

 Figure 1.  The libraries were subjected to the adhesion test.

Keywords: High throughput synthesis; infrared; graded-index; antireflection coatings.