Photonic spin-Hall effect at the strong gain points in parity-time symmetric bilayer systems

• The longitudinal shift is enhanced in all three systems and the longitudinal Photonic spin-Hall effect is suppressed at the SGPs. • The transverse PSHE of the reflected beam is enhanced at the SGPs in systems within the mixed PT-symmetric phase (e.g., #PT1and #PT2). • The transverse PSHE is suppressed at the SGPs but enhanced in its vicinity in systems within the broken PT-symmetric phase (e.g., #PT3). • There are distinct OAM distributions and PSHE modulation mechanisms compared to those in conventional systems. Photonic spin-Hall effect (PSHE) at the strong gain points (SGPs) in parity-time (PT) symmetric systems exhibits intriguing features, yet its underlying physics and connection to the PT-symmetric phase are unclear. Here, we clarify this using rigorous full-wave theory calculations. At the SGPs, the significant phase gradient of the s- or p-wave reflected Fresnel coefficients ( r s or r p ) causes a large longitudinal shift while suppressing the longitudinal PSHE. In the mixed PT-symmetric phase, the transverse PSHE in the reflected beam is enhanced at the SGPs. However, in the broken PT-symmetric phase, the transverse PSHE is suppressed at the SGPs but enhanced in its vicinity when the SGPs of r s coincides with that of r p , with no topological phase transition unlike at the Brewster angle. The modulation mechanism is that PT-symmetric phase transitions at the SGP enable control over the intrinsic/extrinsic orbital angular momentum distributions, thereby regulating the transverse PSHE. These findings reveal the underlying mechanism and provide a new perspective for understanding the PSHE at the SGPs.