Graded-index thin-film stack for cladding and coupling

A graded-index multilayer thin-film stack is optimized to act as a cladding layer on top of a silicon (Si) nanowaveguide and also a collimator for chip coupling where the waveguide ends. The numerical example shows an optimized graded-index profile from 2.35 to 1.45 provides an optical coupling to the standard single-mode fiber with efficiency close to 90% while retaining tight light confinement for the Si nanowaveguide. The corresponding material realization of a graded-index profile with a Si-rich nitride SiN_x/SiON/SiO₂ system is explored using inductively coupled plasma chemical vapor deposition, and a SiN_x cladded Si waveguide is demonstrated.