Wave vortices around oscillating subwavelength holes: Water-wave observation

Abstract

We consider a two-dimensional wave system containing a subwavelength hole, such as an aperture in an interface supporting surface electromagnetic or acoustic waves or an island in a fluid surface sustaining gravity-capillary waves. Recent studies have revealed the emergence of pronounced wave vortices around such structures, termed type II vortices, in contrast to conventional (type I) vortices associated with phase singularities and intensity nulls. A notable natural manifestation of type II vortices occurs in ocean tides around islands such as New Zealand, Madagascar, and Iceland, where the tidal phase increases by ±2π around the island. Although this phenomenon is usually associated with the Coriolis effect from the rotation of the Earth, here, we demonstrate the controlled generation of type II vortices using a minimal and tunable setup: a dipole-oscillating subwavelength hole and a single incident plane wave. Using laboratory gravity-capillary waves and an oscillating subwavelength “island,” we directly measure the resulting phase structure, topological charge, and wave angular momentum. We show that the emergence and handedness of the vortices can be precisely controlled via the relative phase between the dipolar source and the incident wave. Our results offer a simple and versatile mechanism for engineering subwavelength wave vortices, with potential applications in a variety of two-dimensional wave systems.