Fabricating optical waveguide based on porous silicon structures

In this paper, first single-layers porous silicon (PS) was formed on p+-type silicon substrate using electrochemical method. We characterized the fabricated PS layers by gravimetric method and FESEM analyses. Vertically aligned nanocolumns of Si were observed in the layers perpendicular to the initial Si surface. After investigating control samples, we have fabricated two-layer structures with different refractive indices by controlling the fabrication parameters such as current density, anodization time, and electrolyte composition, on the basis of the fabricated single-layers. A gradient-porosity PS layer was also fabricated by modulating applied current density during anodization. The elaborated structure is consisted of porous layers with modulated refractive indices and could be used as a graded-index waveguide. The refractive indices of each layer were calculated using Bruggeman's effective medium approximation. In two-layer structures, the lower layer with lower refractive index (fabricated in higher current density) is the clad and the upper layer with higher refractive index (formed in lower current density) plays the role of core. Moreover, we utilized oxidation process to expand relevant range for the waveguide to the visible wavelength. We have also demonstrated that the porosity will decrease and the thickness of the layers will increase after oxidation.