High-index contrast silicon waveguides exhibit strong birefringence that hinders the development of polarization-insensitive devices, especially for sub-micrometer silicon layer thickness. Here a polarizationindependent 2 × 2 multimode interference coupler in a 220 nm silicon-on-insulator platform is designed and experimentally demonstrated for the first time. Leveraging the advanced control of electromagnetic properties provided by a subwavelength grating metamaterial topology, our multimode interference coupler operates for both TE and TM polarization states with measured polarization dependent loss, insertion loss and imbalance all less than 1 dB, and phase errors below 5◦ in the wavelength range from 1500 nm to 1560 nm. The device has a footprint of only 3.5 μm × 47.25 μm and was fabricated using a single etch-step process with a minimum feature size of 100 nm compatible with immersion deep-UV lithography.
