Broadband mid-infrared thermal emission with large degree of circular polarization enabled by symmetry-broken metasurfaces

Mid-infrared (MIR) circularly polarized emission (CPE) is widely used in molecular sensing, information encryption, target detection, and optical communication. However, the generation and regulation of broadband MIR thermal emission with a large degree of circular polarization (DoCP) is still a major challenge. Here, we design a symmetry-broken chiral plasmonic metasurface consisting of asymmetric spilted-ring resonators (ASRRs) to emit broadband CPE with high purity in the MIR region of 3.4-5 mu m. The simulated results show that the DoCPs at the wavelengths of 3.74 mu m and 4.27 mu m are 0.7 and 0.71, respectively, and the DoCP is higher than 0.5 in the wide wavelength ranges of 3.5-4.83 mu m. According to the local Kirchhoff's law, the spin-dependent thermal emission originates from the strong inherent local chirality of the ASRR through the near-field distribution and the local emissivity density. Then, the effects of geometric parameters of meta-unit on the DoCP characteristics are studied in detail, which indicates that the geometric perturbation segments quantified by S result in the CPE regulation. Specifically, the DoCP decreases from 0.71 to 0 as the perturbation factor S increases from 0 to 5. Finally, we numerically demonstrate that the designed chiral plasmonic metasurface has potential applications in infrared circularly polarized light detection.