Revealing photonic Lorentz force as the microscopic origin of topological photonic states

Charged particles like electrons moving in a magnetic field encounter Lorentz force, which governs the formation of electronic topological edge states in quantum Hall effect systems. Here we show that photons transporting in magneto-optical materials and structures also encounter a physical effect called photonic Lorentz force via the indirect interaction with the magneto-optical medium assisted effective magnetic field. This effect can induce half-cycle spiral motion of light at the surface of a homogeneous metallic magneto-optical medium and inhomogeneous magneto-optical photonic crystals, and it governs the intriguing one-way transport properties of robustness and immunity against defects, disorders, and obstacles. Thus, photonic Lorentz force serves as the fundamental microscopic origin of macroscopic photonic topological states, much the same as classical Lorentz force does to electronic topological states.