SiO2 Nanorod Thin Film Encapsulated by Al2O3 with Atomic Layer Deposition and its Optical Application

Low refractive index materials such as SiO
₂
nanorods or nanoporous SiO
₂
(np-SiO
₂
) and MgF
₂
have been attractive for use in various types of optical coatings. Due to the high porosity of the films, however, the material properties of these films are unstable. This phenomenon is confirmed in dry versus humid ambient where both the coefficient of thermal expansion (CTE) and refractive index change dramatically. We report here that the properties of np-SiO
₂
can be stabilized by depositing a cap layer of Al
₂
O
₃
using atomic layer deposition (ALD) on top. This stabilizing ability is demonstrated successfully for single layer and distributed Bragg reflectors (DBR). With this method, one can pattern nanoporous sensors so that some regions are stable for packaging while others are exposed to show enhanced interactions with environment for sensing.