Freeform Metasurface-Assisted Waveguide Coupler for Guided Wave Polarization Manipulation and Spin–Orbit Angular Momentum Conversion

Metasurface integrated waveguide couplers provide a promising pathway toward advanced manipulation of free-space light for guided wave coupling, such as polarization routing, mode-selective coupling, and wavelength (de)multiplexing. However, it is a great challenge to manipulate the guided wave polarization state to be completely decoupled from the polarization of incident light. Here, an adjoint-based topology optimization method is employed to inversely design a freeform metasurface for unidirectionally coupling free-space light into customized waveguide mode, with the capabilities of polarization manipulation and guided mode selection. An arbitrary transverse magnetic or transverse electric eigenmode can be unidirectionally excited for any incident polarization state conditions, with the maximum polarization-rotated mode purity and unidirectional coupling efficiency approaching 90.97 and 43%, respectively. Furthermore, by using multiobjective topology optimization, a meta-waveguide coupler with dual functions of generating polarization-controllable hybrid mode and realizing reconfigurable spin–orbit angular momentum conversion is inversely designed, leading to the increase of a mode (de)multiplexer channel number. Our work provides an effective strategy for multifunctional integrated on-chip devices and may have potential applications in optical communication, integrated optics, and quantum optics.