Metamaterial-engineered fiber-chip couplers in silicon and silicon nitride waveguides

Integrated photonics has become a mainstream technology driven by advances in optical communications and leveraging mature processing infrastructure of silicon microelectronics. In practical applications of photonic integrated circuits (PICs), the presence of low-loss off-chip optical coupling interfaces is of key importance. Photonic chips require optical connection to the external world, facilitating both multi-channel fiber connections and free-space multi-port optics. Optical coupling in and out of the planar waveguide circuits is still a critical challenge in integrated photonics due to limitations caused by geometrical, material, and modal mismatches. The utilization of subwavelength grating (SWG) metamaterials, i.e. nano-structured waveguide segments with structural periodicity smaller than the wavelength of the propagating light, is often harnessed as an effective design tool to improve the performance of fiber-chip optical couplers, without compromising the fabrication simplicity. In this work, we present recent advances in the development of low-loss photonic chip interfaces based on surface grating couplers with SWG metamaterials. In particular, we report on advanced design solutions of surface gratings realized on silicon and silicon nitride waveguide platforms, facilitating effective control of polarization and enhanced fiber-chip coupling performance with losses down to 1dB.