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 large degree of circular polarization (DoCP) is still a major challenge. Here, we design a symmetry-broken chiral plasmonic metasurface consisting of asymmetric spilt-ring resonators (ASRRs) to emit broadband CPE with high purity in the MIR region of 3.4-5μm. The simulated results show that the DoCPs at the wavelengths of 3.74μm and 4.27μm are 0.7 and 0.71, respectively, and the DoCP is higher than 0.5 in the wide wavelength ranges of 3.5-4.83μm. 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 details, 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 increasing from 0 to 5. Finally, we numerically demonstrate that the designed chiral plasmonic metasurface has potential applications in infrared circularly polarized light detection.