Wave fields with a periodic orbital angular momentum gradient along a single axis: a chain of vortices

We present a theoretical analysis and an experimental demonstration of a wave field constituted by an axial array of vortices consecutively inverting the topological charge over distances of one-quarter of a wavelength. It exhibits a periodic gradient of the orbital angular momentum density along a single axis, although the field is created by the superposition of counter-propagating waves possessing no angular momentum themselves. The experiment is carried out with acoustic waves, whose wavelength is sufficiently large to allow a three-dimensional mapping of the field in amplitude and phase.