Figure 3. Acoustic plane wave generation from a point source in the zero refractive index metamaterial.

(a) Measured pressure field radiated by acoustic point source embedded in zero refractive index metamaterial. The zero refractive index metamaterial collimates the emission from a point source to a plane wave. (b) Measured pressure field radiated by an acoustic point source in the empty waveguide. Without zero refractive index material, a cylindrical wave pattern is obtained in the waveguide. (c) Directivity of a collimated plane wave. The blue circles and red curve are directivities corresponding to the pressure field radiations shown in a,b, respectively. The black curve is the calculated directivity of an ideal line source with the same size (10 unit cells, that is, 6.5 wavelengths). The zero index metamaterial collimates the acoustic pressure field within 11±1°, close to 10.6° given by the theoretical limit of the line source. The noise observed on both experimental results is due to the spurious reflection from the waveguide edges.