Guided-mode resonance dispersion engineering on a SiN-based integrated photonic platform for diverse polarization state generation

By resorting to guided-mode resonances (GMRs), here we integrate distinct optical components, catering to the perpetual requirements of dense photonic integration. Specifically, we pursue an integration of three on-chip functionalities, namely, out-of-plane bi-modal coupling, narrowband spectral filtering, and diverse polarization state generation (PSG), using a GMR template fabricated on an ultra-low loss, transparent silicon nitride (SiN) platform and operating in near-infrared regime (∼850 nm). As a result, our platform can transform broadband unpolarized free-space light into an on-chip near-monochromatic signal with the desired state of polarization. To realize these objectives, we resort to high angular dispersion of orthogonally polarized GMRs in a classical mounting configuration (for TE 0 - and TM 0 -guided modes) and engineer a modal crossing point at a small off-normal incidence angle (∼3.3 deg). At this spectral degeneracy, the two orthogonal modes experience high-form birefringence in the sub-wavelength guiding region, allowing a full-cycle retardance within a guide length of just 5.3 μm. This enables PSG through the weighted superposition of orthogonal modes. The integration and polarization state generation capabilities demonstrated here are key to a wider adoption of SiN-based photonic integrated circuits.