Full-polarization and high-coherence thermal radiation from magnetic photonic crystals realized by anisotropic saddle-band design at terahertz frequencies

The polarization control and coherence of thermal emission are crucial for enhancing its application performance. However, existing solutions primarily rely on supercell expansion or periodic perturbation to introduce band folding, which complicates the structure, reduces fabrication tolerance, and complicates dynamic control. This study proposes a method for regulating thermal emission based on magneto-optical photonic crystals. By introducing periodic anisotropy in the orthogonal directions of the lattice, a saddle-shaped band that can be tuned by magnetic fields is naturally formed in the terahertz frequency range. This process does not require supercell expansion or periodic perturbation and can achieve strong anisotropic dispersion solely through periodic regulation at the single-cell scale. By introducing magnetic optical materials and applying an external magnetic field, the active control of chirality and polarization state was achieved, and the full polarization-switchable thermal radiation from linear polarization to circular polarization was accomplished. Numerical results demonstrate that this structure can achieve circular dichroism exceeding 0.985, a quality factor exceeding 500, and a radiation divergence angle of less than 0.75°, while exhibiting good robustness to perturbations in key geometric parameters. This work provides an approach for realizing a dynamically reconfigurable, structurally simple, and highly coherent terahertz polarization thermal radiation source.