Wave Vortices Around Oscillating Subwavelength Holes: Water-Wave Observation

TL;DR

This study demonstrates controlled generation and manipulation of wave vortices around subwavelength structures using a simple setup, with potential applications across various two-dimensional wave systems.

Contribution

We introduce a minimal experimental setup to generate and control type-II wave vortices around subwavelength holes using a dipole oscillation and incident plane wave.

Findings

Type-II vortices observed around subwavelength holes in water waves.

Vortex properties can be tuned by adjusting the phase between source and incident wave.

Direct measurements of phase structure, topological charge, and wave angular momentum were performed.

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 striking 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 $\pm 2\pi$ 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.