Observation of Topologically Enabled Complete Polarization Conversion

Exploiting topological ideas has been a major theme in modern photonics, which provides unprecedented opportunities to design photonic devices with robustness against defects. While most previous works in topological photonics have focused on band theory, recent theories extend the topological concepts to the analysis of scattering matrices and suggest a topological route to complete polarization conversion (CPC). Here, the experimental observation of the topological CPC is reported. Using angle-resolved reflection measurements, it is unveiled experimentally that the CPC between arbitrary two polarizations occurs at vortex singularities of reflection coefficients in momentum space, reflecting its topological nature. Besides, it is visualized directly that for a given input polarization, the output one can cover the entire Poincare sphere over a wide frequency range by varying the incident angle, guaranteed by the topological nature of CPC, which is in sharp contrast to the conventional polarization-conversion approaches that usually suffer from the bulky volume, limited choice of eigen-polarization states, or narrow operation bandwidths. Remarkably, that BICs lie on the critical coupling curves that define the condition for CPC is experimentally demonstrated. This work paves the way to exploring the topological properties in scattering matrices for controlling light polarization and creating robust photonics devices.

Automated summary

Exploiting topological ideas in modern photonics has allowed for the design of robust photonic devices. Recent theories extend topological concepts to the analysis of scattering matrices, guiding the development of complete polarization conversion. This study experimentally observes topological polarization conversion at vortex singularities of reflection coefficients and demonstrates that it covers the entire Poincare sphere using the topological nature of CPC. Furthermore, the experiment confirms that BICs lie on the critical coupling curves defining the condition for CPC. This work opens up possibilities for controlling light polarization and creating robust photonics devices by exploring topological properties in scattering matrices.