  Vortex waves, which carry orbital angular momentum, have found use in a range
of fields from quantum communications to particle manipulation. Due to their
widespread influence, significant attention has been paid to the methods by
which vortex waves are generated. For example, active phased arrays generate
diverse vortex modes at the cost of electronic complexity and power
consumption. Conversely, analog apertures, such as spiral phase plates,
metasurfaces, and gratings require separate apertures to generate each mode.
Here we present a new class of metamaterial-based acoustic vortex generators,
which are both geometrically and electronically simple, and topologically
tunable. Our metamaterial approach generates vortex waves by wrapping an
acoustic leaky wave antenna back upon itself. Exploiting the antennas
frequency-varying refractive index, we demonstrate experimentally and
analytically that this analog structure generates both integer, and non-integer
vortex modes. The metamaterial design makes the aperture compact and can thus
be integrated into high-density systems.

Classical Physics;Mesoscale and Nanoscale Physics