The use of pulsed DC magnetron sputtered niobium pentoxide as a high index material in optical devices as Anti-Reflecting Coating (ARC) and optical filters has been investigated. When deposited on a silicon substrate with native oxide, the few nanometers of niobium pentoxide were optically distinct from the rest of the niobium pentoxide layer, effect not observed on thermally oxidized silicon substrates. The dispersion curve of this interfacial layer was determined and used to simulate the reflectometry curves. Next, an ARC consisting of a bilayer of niobium pentoxide and silica was fabricated and its optical response was simulated with this interfacial layer. Finally, a Bragg mirror consisting of seventeen layers of niobium pentoxide and silicon dioxide was realized, underlining that this interfacial layer does not affect the optical response when several layers were at stake.