Silicon nitride TM-pass polarizer using inverse design

Integrated silicon nitride polarizers play a critical role in the design of complex integrated devices such as filters, switches, and large Mach-Zehnder interferometer networks. These devices require precise control of both polarizations on a single circuit. In addition, polarizers are essential to accurately characterize these devices, primarily due to the low efficiency and polarization extinction ratio (PER) of the surface coupling gratings used in CMOS-compatible silicon nitride platforms for test-specific optical I/O. In this article, we present the design and experimental performance of six prototypes of TE-reflector/TM-pass polarizers specifically optimized for the C-band. These prototypes resemble subwavelength gratings with several additional intricate aspects. In particular, the longer prototypes feature two distinct regions, one representing non-intuitive tapers and the other showcasing a more distinct subwavelength grating. We achieve a high TM transmission efficiency of -0.28 dB along with a PER of 18.2 dB. These results are obtained with a device occupying an area as low as 11 µm × 2 µm, setting a new performance benchmark for compact polarizers compatible with standard silicon nitride platforms.