Monday, October 12, 2015

ALD Dielectric film with a refractive index close to air

From North Carolina State University, here is an amazing use of ALD Al2O3 and ZnO dielectric film that has optical and electrical properties similar to air, but is strong enough to be incorporated into electronic and photonic devices - making them both more efficient and more mechanically stable. Full report here and in the actual publication below as well in this supporting information file with free access :

Ordered 3D Thin-Shell Nanolattice Materials with Near-Unity Refractive Indices

Xu A. Zhang, Abhijeet Bagal, Erinn C. Dandley, Junjie Zhao, Christopher J. Oldham, Bae-Ian Wu, Gregory N. Parsons and Chih-Hao Chang
Advanced Functional Materials, DOI: 10.1002/adfm.201502854

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. You can see the structure here, on the micrometer scale.

The refractive indices of naturally occurring materials are limited, and there exists an index gap between indices of air and available solid materials. With many photonics and electronics applications, there has been considerable effort in creating artificial materials with optical and dielectric properties similar to air while simultaneously being mechanically stable to bear load. Here, a class of ordered nanolattice materials consisting of periodic thin-shell structures with near-unity refractive index and high stiffness is demonstrated. Using a combination of 3D nanolithography and atomic layer deposition, these ordered nanostructured materials have reduced optical scattering and improved mechanical stability compared to existing randomly porous materials. Using ZnO and Al2O3 as the building materials, refractive indices from 1.3 down to 1.025 are achieved. The experimental data can be accurately described by Maxwell Garnett effective media theory, which can provide a guide for index design. The demonstrated low-index, low-scattering, and high-stiffness materials can serve as high-quality optical films in multilayer photonic structures, waveguides, resonators, and ultra-low-k dielectrics.