Chiral structures are promising in many applications, such as biological sensing and analytical chemistry, and have been extensively explored. In this paper, we theoretically investigate the chiral response of twisted bilayer α-MoO₃. Firstly, the analytical formula for the transmissivity is derived when the structure is illuminated with circularly polarized plane waves. Furthermore, the results demonstrate that the twisted bilayer α-MoO₃ can excite the strong chirality with the maximum circular dichroism (CD) of 0.89. In this case, the chirality is due to the simultaneous breaking the rotational symmetry and mirror symmetry, which originates from the relative rotation of two α-MoO₃ layers. To better understand the physical mechanism, the polarization conversion between the left-hand circular polarization (LCP) and right-hand circular polarization (RCP) waves is discussed as well. Moreover, it is found that the structure can maintain the strong chirality (CD > 0.8) when the twisted angle varies from 69° to 80°, which effectively reduces the strictness in the requirement for rotation angle. In addition, the CD can be larger than 0.85 when the incidence angle of circularly polarized plane wave is less than 40°, implying that the chirality is robust against the angle of incidence. Our work not only provides an insight into chirality induced by the twisted bilayer α-MoO₃, but also looks forward to applications in biological sensing.

Chirality has always been a hot research topic because it possesses potential applications in polarization optics, chemical and biosensing. In the previous works, intrinsic chirality has been extensively explored, but its development is limited due to the complexity in fabrication of chiral metamaterials. Therefore, there is an urgent need to simplify fabrication and design of compact devices with chiral response. Extrinsic chirality has shown great potential because it can be realized in nonchiral anisotropic planar structures with low-cost fabrication techniques. In this paper, the extrinsic chirality of biaxial hyperbolic material α - M o O 3 with in-plane anisotropy has been investigated. By analyzing the effect of thickness of α - M o O 3 film, the angle of incidence, azimuth angle, and wavelength of incidence on the circular dichroism (CD), the maximum CD can reach 0.77. This strong extrinsic chirality of the α - M o O 3 film results from the mutual orientation of the α - M o O 3 film and the incident light. In addition, α - M o O 3 film can still maintain strong extrinsic chirality when the azimuthal angle ranges from approximately 20°–57° and the angle of incidence is from 55°–80°.

Since it is designable, metasurface was widely used in various fields, especially the design of polarization converters. However, most of the polarization converters currently designed can only work under normal incidence or small angle incidence, which hugely limits the application of the device. In this paper, a chiral metasurface based on the z-shaped unit cell, which can manipulate circular polarization wave, is proposed. The simulation result shows that this converter can maintain the polarization state of circular polarization after reflection from 8.18 to 13.988 GHz with a polarization conversion ratio of more than 90%. Moreover, the structure is insensitive to the incidence angle, which can keep a stable performance both for left-handed circularly polarized wave and right-handed circularly polarized waves as the incident angle increase to 75°. The proposed metasurface with simple structure and angular stability can be used in communication and polarization manipulating devices.