Measurement of a Broadband Negative Index with Space-Coiling Acoustic Metamaterials

We report the experimental demonstration of a broadband negative refractive index obtained in a labyrinthine acoustic metamaterial structure. Two different approaches were employed to prove the metamaterial negative index nature: one-dimensional extractions of effective parameters from reflection and transmission measurements and two-dimensional prism-based measurements that convincingly show the transmission angle corresponding to negative refraction. The transmission angles observed in the latter case also agree very well with the refractive index obtained in the one-dimensional measurements and numerical simulations. We expect this labyrinthine metamaterial to become the unit cell of choice for practical acoustic metamaterial devices that require broadband and significantly negative indices of refraction.

Figure 1

Labyrinthine unit cell: (a) Designed dimensions of the unit cell. (b) Photograph of a fabricated unit cell.

Figure 2

Retrieved refractive index: (a) Retrieved index from experiment (pink circles for real part, red asterisks for imaginary part). (b) Retrieved index from simulation (solid blue line for real part, dashed blue line for imaginary part).

Figure 3

Experimental setup: (a) Photograph of the assembled prism with 55 identical labyrinthine unit cells. (b) Two-dimensional measuring platform that was used to perform the field mapping.

Figure 4

Simulation and measurement showing negative refraction at the prism-air interface: (a) Simulation of prism made of the real labyrinthine structure at 2100 Hz. (b) Simulation of the prism made of a homogeneous material having the effective parameters retrieved from one-dimensional reflection and transmission measurements at 2100 Hz. (c) Zoomed-in plot of the highlighted region in (b). (d) Measured field pattern at 2100 Hz.

Figure 5

Dispersion characteristics of the prism: Measured field pattern at (a) 2000 Hz, (b) 2300 Hz, and (c) 2600 Hz, respectively.