Mathematics of effective $q$-plate in polarization optics

Abstract: The $q$-plate is a spatially inhomogeneous SU(2) birefringent optical element that has garnered significant interest due to its ability to mediate the spin-orbit interaction of light and facilitate the generation of optical vortices. The $q$-plate features a spatially varying fast axis orientation defined by two parameters: the topological charge $q$ and the offset angle $\alpha_0$. The notion of an effective waveplate arises when multiple waveplates, whether homogeneous or inhomogeneous, are aligned coaxially such that, under specific constraints, the composite system emulates the behavior of a single effective waveplate. This work presents a comprehensive mathematical formalism for realizing an effective waveplate through a cascaded configuration of three $q$-plates, each chosen as either a quarter-wave $q$-plate, a half-wave $q$-plate, or a combination thereof. This yields a total of eight distinct configurations. Some configurations result in an effective waveplate exhibiting a constant retardance, whereas others allow continuous modulation of the effective retardance over the full range from $0$ to $2\pi$ through the systematic variation of the relative offset angles between the constituent $q$-plates. This feature enables holonomic polarization transformations on the higher-order Poincar\'e sphere, making the concept of the effective waveplate applicable to topological index spaces. Moreover, tunable effective retardance holds significant potential for applications involving structured light corresponding to the higher-order Poincar\'{e} sphere, particularly in scenarios demanding controlled spatial modulation of polarization states or dynamic tailoring of polarization topologies.