Lab researchers discover novel approach to antireflection coating

Schematic of the metamaterial antireflection coating, where the front is air, the blue region is the spacer, and the grey region is the substrate to be coated.

September 23, 2010—Houtong Chen, Jiangfeng Zhou, John O'Hara, Frank Chen, and Abul Azad of the Center for Integrated Nanotechnologies (MPA-CINT) and Antoinette Taylor of Materials Physics and Applications Division (MPA-DO) have discovered a novel approach to antireflection coating. The Laboratory researchers use metamaterials that dramatically reduce reflection and greatly enhance transmission near a specifically designed frequency. The metamaterials operate over a wide range of incidence angles for both transverse magnetic and transverse electric polarizations.

When propagating electromagnetic waves encounter an interface of two media with different refractive indices, some portion of energy is reflected back, and the rest is transmitted through. This well-known phenomenon is the basis of numerous optical technologies. However, in many applications, such as solar cells, reflection is undesirable.

Conventional single or multi-layered antireflection coating technology has been a great success in the optical frequency range. However, the technology is limited by natural material properties and is not suitable for far infrared and microwave bands. Semiconductors and ceramic substrate materials often have large refractive indices, which make it difficult to find appropriate coating materials in far infrared and microwave bands. In addition, fabricating the high-quality and relatively thick films required for long wavelengths is difficult.

The scientists developed a novel approach of metamaterial antireflection coating. A relatively thin coating fabricated in a simple process is used. The metamaterial antireflection coating consists of an array of gold electric split-ring resonators and a gold mesh patterned using conventional photolithography methods, and separated by a spacer layer of spin-coated and thermally cured polyimide on intrinsic gallium arsenide substrates. The researchers investigated the terahertz (THz) frequency range where antireflection coatings are challenging. The metamaterial antireflection coating demonstrated almost zero reflection and significantly enhanced transmission over a wide range of incidence angles. The scientists identified a classical interference mechanism that is responsible for the antireflection property of metamaterials. Multiple reflections and transmissions in the metamaterial coatings cause destructive and constructive interferences that reduce reflection and enhance transmission. The research suggests a general rule in designing metamaterial antireflection coatings. This mechanism explains other related phenomena observed in metamaterials, such as metamaterial perfect absorbers and electromagnetic wave tunneling.

The research was published in Physics Review Letters and funded by the LANL Laboratory Directed Research and Development (LDRD) Program.