Narrowband directional chiral emission enabled by hyperbolic material α-MoO₃

Chiral emission plays a pivotal role in the design of advanced sensors and thermal detectors. However, the ability to control the chiral emission of far-field thermal radiation over a fixed narrow angular range is an important yet currently limited capability. In this paper, we investigate a lithography-free chiral thermal emitter based on the hyperbolic material α-phase molybdenum trioxide (α-MoO₃). Numerical studies demonstrate that the proposed chiral emitter is highly sensitive to the incident angle, showing narrowband chiral emission exclusively in the range of incidence angles from 75° to 89° at a wavelength of 10 μm. The directional chiral emission arises from the extrinsic chirality resulting from the mutual orientation of the α-MoO₃ film with in-plane anisotropy and the incident light. Furthermore, we elucidate the mechanism through polarization conversion and analyze the degree of polarization (DoP) of the thermal radiation. Our findings indicate that the DoP can reach 0.95, signifying efficient conversion of left-handed to right-handed circularly polarized light while maintaining perfect polarization at the resonant wavelength. Electric field distributions further validate the origin of directional chiral emission. This study not only advances the understanding of circularly polarized light manipulation but also promises significant applications in chiral sensing and thermal detection.