Graded Refractive Index Optics based on Dual-layer Ultrathin Films: Theory, Design and Applications in Integrated Photonics

摘要

This paper presents an overview of graded refractive index optics based on dual-layer ultrathin film technology and its application in integrated photonics as an on-chip lens for optical coupling of nano-waveguide. The theoretical derivation and discussion shows the equivalence between the dual-layer ultrathin film and a negative birefringent thin film and the influence of thin film thickness and light incident angle on this equivalence. For experimental verification, a set of dual-layer ultrathin films of titanium dioxide (TiO_2) and aluminium oxide (Al_2O_3) are deposited on silicon with different volume fractions. Characterization of refractive index and reflectance shows a good agreement between the experiment and theoretical design, and suggests the graded refractive index profile can be achieved via varying the volume faction of the dual-layer ultrathin film stack. As an application example, a graded refractive index lens with aberration-correction based on the above dual-layer thin film stack is designed and optimized for optical coupling between silicon waveguide and optical fiber. The optical design indicates the multilayer thin-film stack with a proper graded refractive index profile can serve as the passivation cladding on top of silicon waveguide but collimate the light at the waveguide ends for optical coupling to fiber with an estimated efficiency～89%.