Observation of steerable polarization conversion in metasurfaces with parity-time phase transition

One of the primary objectives in the realm of light manipulation is the flexible and active control of polarization, which constitutes an essential degree of freedom of photons. Recently, it has been demonstrated that the polarization conversion is related to the parity-time (PT) symmetry in metasurfaces. Here, we propose a scheme for achieving near-perfect polarization conversions under unilateral linear polarized excitation, based on passive metasurfaces with an effective PT phase transition. The conversion from linear to circular polarization occurs at the exceptional point (EP), with the transmission polarization revealing the missing dimension of EP. To enhance linear polarization conversion efficiency, equivalent coherent excitation is induced, enabling near-perfect cross-polarization conversion. This effective manipulation also provides additional control over PT phase transitions and allows PT symmetry breaking at different coupling strengths. Experimental validation at microwave frequencies confirms the feasibility of this method, with potential extensions to terahertz and near-infrared bands. These findings not only introduce a PT-symmetric metasurface-inspired approach for exploring a wider range of phenomena in non-Hermitian physics but also hold great potential for scalable polarization and phase controlling, imaging, and other related areas based on a single-port configuration.