Multi-functional antireflective surface-relief structures based on nanoscale porous germanium with graded refractive index profiles.

We fabricated the multi-stacked graded refractive index (GRIN) porous amorphous germanium (a-Ge) films with linear, quintic, and Gaussian index profiles for their refractive indices at a wavelength of 1.33 μm by e-beam evaporation with a glancing angle deposition technique. The porous a-Ge films with inclined nanocolumnar structures were deposited on Ge substrates at incident vapor flux angles of 40, 55, and 75°. The surface wetting behaviour of the porous a-Ge films was explored, exhibiting the hydrophilic surfaces with water contact angles below 78°. Their optical reflectance properties were investigated, together with the theoretical analysis using the rigorous coupled-wave analysis simulation. For the GRIN a-Ge films with a quintic index profile, a low reflectance band of <2% was shifted towards the longer wavelength with increasing the total thickness (ttotal) of GRIN a-Ge layers. For the GRIN a-Ge films with optimized ttotal values of 350, 333, and 225 nm for linear, quintic, and Gaussian index profiles, respectively, the low reflectance spectra of <10% were obtained at wavelengths of 1.1-1.7 μm, which yielded the lowest average reflectance value (Ravg) of ∼2.2% for Gaussian index profile (compared to the Ravg ∼ 3.7 and 4% for linear and quintic ones, respectively). The angle-dependent reflectance characteristics were also studied at incident angles of 15-85° for s- and p-polarized lights at a wavelength of 1.33 μm. The calculated results showed similar trends to the experimental data.