Topology optimization of broadband polarization conversion metasurfaces in microwave region

Polarization conversion metasurfaces are compact and efficient devices that manipulate the polarization state of electromagnetic waves, offering significant advantages in applications such as antennas, imaging systems, and optical communication. Traditional design methods often struggle to achieve high polarization conversion efficiency across wide bandwidths. To overcome this limitation, we present a gradient-based topology optimization method that uses material distribution approach for the design of broadband polarization conversion metasurfaces. In the broadband optimization process, we adopt a max-min-type objective function, where multiple frequency points within a given frequency band are selected, and the minimum objective value among them is maximized to ensure that the optimized structure performs effectively across the entire frequency range. We applied this method to the inverse design of polarization conversion metasurfaces targeting the X-band, Ku-band, K-band, and Ka-band. Our results demonstrate that the metasurfaces achieve polarization conversion ratios exceeding 90% across these broad frequency bands, significantly enhancing their bandwidth performance. Experimental validation was performed to verify the topologically optimized metasurfaces, where the measured results exhibits remarkable agreement with their numerical counterparts. This approach offers a powerful tool for the design of broadband polarization conversion metasurfaces and holds great promise for various applications, including frequency-selective surfaces, absorbing metasurfaces, and other advanced electromagnetic devices. • A novel polarization conversion metasurface design method based on topology optimization is presented. • The rational design space for the unit cell period and dielectric thickness of polarization conversion metasurfaces is studied. • Four broadband polarization conversion metasurfaces are inversely designed for different target frequency bands. Within the target operating frequency bands, the PCR of the metasurfaces consistently exceeds 90%. The mechanism of broadband polarization conversion is further investigated.