Inhomogeneous polarization transformation revealing a PT transition in non-Hermitian optical beam shifts

Despite its non-Hermitian nature, the optical beam shift exhibits real eigenvalues and nonorthogonal eigenstates. To explore this unexpected similarity to typical PT (parity-time)-symmetric systems, we first categorize the entire parametric regime of optical beam shifts into Hermitian, PT-unbroken, and PT-broken phases. Besides experimentally unveiling the PT-broken regime, crucially, we illustrate that the observed PT transition is rooted in the momentum-domain inhomogeneous polarization transformation of the beam. The correspondence with a typical non-Hermitian photonic system is further established. Our work not only resolves a fundamental issue in the field of optical beam shift but also puts forward the notion of non-Hermitian spin-orbit photonics: a new direction to study non-Hermitian physics through optical beam shifts.