Nano-Engineered Film Has Refractive Index Near 1

A dielectric film with a refractive index close to that of air could be used to make photonic devices more efficient and mechanically stable.

Photonic devices require high contrast between their component materials, with some components having high refractive indices and others have low ones. The higher the contrast between those materials, the better the device performance.

Air has a low refractive index (n = 1) but it isn't mechanically stable. The lowest refractive index found in solid, naturally occurring materials is 1.39.

Now researchers from North Carolina State University have developed a film made of aluminum oxide that has a refractive index as low as 1.025 but that is mechanically stiff.

"By manipulating the structure of the aluminum oxide, which is dielectric, we've improved both its optical and mechanical properties," said professor Chih-Hao Chang.

"The key to the film's performance is the highly ordered spacing of the pores, which gives it a more mechanically robust structure without impairing the refractive index," said doctoral student Xu Zhang.

By manipulating the structure of aluminum oxide, a dielectric material, researchers were able to improve its optical and mechanical properties. The key to the film's performance is the highly ordered spacing of the pores, which gives it a more mechanically robust structure without impairing the refractive index. Courtesy of Chih-Hao Chang/North Carolina State University.

The pores are created in a polymer substrate using a nanolithography technique developed in Chang's lab. The porous polymer then serves as a template, which the researchers coat with a thin layer of aluminum oxide using atomic layer deposition. The polymer is then burned off, leaving behind a 3D aluminum oxide coating.

"We are able to control the thickness of the aluminum oxide, creating a coating between 2 nm and 20 nm thick," Zhang said. "Using zinc oxide in the same process, we can create a thicker coating. And the thickness of the coating controls and allows us to design the refractive index of the film."

Regardless of the how thick the coating is, the film itself is approximately 1 μm thick.

"The steps in the process are potentially scalable and are compatible with existing chip manufacturing processes," Chang said. "Our next steps include integrating these materials into functional optical and electronic devices."

The research was funded by the U.S. Air Force Research Laboratory and NASA. It was published in Advanced Functional Materials (doi: 10.1002/adfm.201502854).