Chirality-induced tunable quasi-Brewster angle and anomalous photonic spin-Hall effect

The photonic spin Hall effect (PSHE) has demonstrated significant application potential in precision metrology, sensing, edge imaging, and spin-optical devices. The majority of these applications have focused on optical systems exhibiting either the Brewster or quasi-Brewster effect. Consequently, the flexible adjustment of the quasi-Brewster angle has emerged as a pivotal concern in manipulating the PSHE and expanding its applications. Here, by studying the scattering of a linearly polarized beam incident on an air-chiral interface, we find that the quasi-Brewster angle can be tuned by the chirality parameter κ and induces an anomalous PSHE. This anomalous PSHE manifests as both asymmetric out-of-plane and additional in-plane spin-Hall shifts. Specifically, the real part of κ, which is related to optical activity, results in a separation of the quasi-Brewster angles for left- and right-handed circularly polarized components, leading to asymmetric out-of-plane shifts. The additional in-plane shifts are induced by the circular dichroism of the chiral materials, which is dictated by the imaginary part of κ. This anomalous PSHE can be attributed to the destructive interference between normal and abnormal modes within the scattered beam under linear polarization incidence. Furthermore, we illustrate potential applications of chirality-modulated PSHE in encoding and encryption. Our findings offer additional degrees of freedom for controlling the PSHE, paving the way for advancements in spin-related photonics applications.