Cross-section mismatch: metamaterials to the rescue

Coupling light into and/or out of a photonic integrated circuit is often accomplished by establishing a vertical link between a single-mode optical fiber and a resonant waveguide grating, which is then followed by a tapered and a single-mode waveguides. The tapered waveguide operates as a spot-size converter, laterally expanding or contracting the light beam between the single-mode waveguide and the resonant waveguide grating. In this work, we propose using subwavelength structures to achieve tapering functionalities. To this end, we designed a metamaterial structure that enables the modulation of the refractive index necessary to either expand or focus a beam of light. Furthermore, we simulated the metamaterial structure through adequate numerical methods and the expanding, and focusing performances were analyzed in terms of efficiency and mode profile matching. We achieved over 43 % and 48 % for the integral overlap with the transverse magnetic fundamental mode for the focusing and expanding configurations, respectively, out of 49 % and 51 % of power transferred.