Sapphire nanophotonics: Fabrication challenges and optical properties

Abstract

Sapphire has many optical applications in nanophotonics and optoelectronic devices due to its high index, broadband transparency, and chemical and physical stability. However, sapphire is notoriously difficult to process, cut, and micromachine. As a result, patterning high density and aspect ratio nanostructures in sapphire, such as those needed for nanophotonics, are challenging. Here, we demonstrate an effective fabrication approach to pattern high aspect-ratio sapphire nanostructures using a multilayer etching process. This approach is based on the concept of designing the etching selectivities of neighboring masks to significantly increase the overall etch selectivity. Using polymer nanostructures with initial height of 50 nm, the process is used to demonstrate antireflection sapphire nanostructures with feature width of 170 nm and depth of 345 nm, resulting in aspect ratio of 2.03. The optical properties of the sapphire nanostructures have been characterized and the surface reflectivity is reduced from 6% to 2% at near normal incidence and from 39% to 11% at 70° incident angle. The experimental data also indicates that the antireflection effect operates at broadband wavelength from 350 to 1000 nm. This work demonstrate that sapphire nanostructures can be patterned by using multiple masks to achieve etch selectivity over 7, and can find applications in sapphire-based nanophotonics, metasurfaces, and optoelectronics devices. • Sapphire is attractive for nanophotonic applications but is difficult to pattern with nanostructures. • A new method using multilayer etching mask is introduced to increase the etch selectivity of sapphire. • This process can be used to demonstrate higher aspect ratio nanostructures of over 2. • The fabricated sapphire nanostructures have antireflection properties over wide angle and broad wavelength ranges.