Ultrahigh extinction ratio on-chip mid-infrared polarizers based on bound states in the continuum in etchless waveguides

Mid-infrared integrated photonics has attracted widespread attention due to its broad applications in free-space optical communications and spectroscopy. As a key component of integrated photonic systems, polarizers significantly impact overall functionality. However, developing efficient on-chip mid-infrared polarizers remains a challenge. Here, we designed an ultrahigh extinction ratio on-chip mid-infrared polarizer based on bound states in the continuum (BICs) in a graphene-on-silicon etchless waveguide. The bound zeroth transverse magnetic mode is selectively absorbed by precisely regulating the optical properties of graphene, achieving a precise polarization control for the two BICs-based modes. Specifically, a record-high polarization extinction ratio of 58.7 dB is achieved at a wavelength of 3.8 μm, with a 500-μm-long graphene-on-silicon etchless waveguide, while a low optical loss is only 2.3 dB for the BICs-based zeroth transverse electric mode. The work is expected to propel the leap-forward development of mid-infrared integrated photonic systems toward miniaturization, cost reduction, and mass production, advancing cutting-edge fields such as high-sensitivity trace sensing and emerging light sources.