Scalable Mid‐Infrared Polarization Control via Shape‐Engineered Graphene‐Integrated Waveguides

ABSTRACT Graphene‐integrated waveguide polarizers are promising building blocks for mid‐infrared integrated photonics, yet simultaneously achieving high extinction ratio, low insertion loss, and compact footprint remains challenging. Here, we demonstrate a graphene‐assisted waveguide polarizer based on femtosecond‐laser‐written 3D waveguides with engineered cross sections. By comparing a conventional rectangular geometry with a tailored triangular geometry, we show that the triangular design significantly enhances polarization‐selective modal overlap with graphene. Simulations reveal that the triangular cross section increases both the birefringence and the differential overlap factors between the TM and TE modes, thereby providing a powerful degree of freedom to boost the polarization extinction ratio (PER). Experimentally, the graphene‐integrated triangular waveguide polarizer exhibits a high polarization extinction ratio above 18 dB with moderate insertion loss in the mid‐infrared region, outperforming the rectangular counterpart. To elucidate the underlying mechanisms, we develop a semi‐analytical model that links the PER to the polarization‐dependent graphene–mode overlap and geometry‐induced birefringence, yielding a compact scaling law that is in good agreement with both simulations and measurements. Our results establish geometry‐engineered graphene–waveguide coupling as an effective strategy for high‐performance polarization management and provide general design rules for 2D‐material‐assisted devices in mid‐infrared integrated photonic platforms.