Optical properties of nanostructured TiO_2 thin films and their application as antireflection coatings on infrared detectors

摘要

Oblique-angle deposited titanium dioxide (TiO_2) nanorods have attracted much attention as good antireflection (AR) coating material due to their low n profile. Therefore, it is necessary to better understand the optical properties of these nanorods. TiO_2 nanorods grown on glass and Si substrates were characterized in the visible (0.4-0.8 μm) and infrared (2-12 μm) regions to extract their complex n profiles empirically. Application of these nanorods in multilayer AR coatings on infrared detectors is also discussed. Optimization of graded index profile of these AR coatings in the broad infrared region (2-12 μm) even at oblique angles of incidence is discussed. The effective coupling between the incoming light and multiple nanorod layers for reducing the reflection is obtained by optimizing the effect from Fabry-Perot oscillations. An optimized five-layer AR coating on GaN shows the reflectance less than 3.3% for normal incidence and 10.5% at 60° across the whole 2-8 μm spectral range.