Polarization-independent multimode interference coupler with anisotropy-engineered bricked metamaterial

Many applications, including optical multiplexing, switching, and detection, call for low-cost and broadband photonic devices with polarization-independent operation. While the silicon-on-insulator platform is well positioned to fulfill most of these requirements, its strong birefringence hinders the development of polarization-agnostic devices. Here we leverage the recently proposed bricked metamaterial topology to design, for the first time, to our knowledge, a polarization-independent $2\times 2$ multimode interference coupler using standard 220 nm silicon thickness. Our device can be fabricated with a single etch step and is optimized for the O-band, covering a wavelength range of 160 nm with excess loss, polarization-dependent loss, and imbalance below 1 dB and phase errors of less than 5°, as demonstrated with full three-dimensional finite-difference time-domain simulations.