Active acoustic metamaterials reconfigurable in real time

A major limitation of current acoustic metamaterials is that their acoustic properties are either locked into place once fabricated or are only modestly tunable, tying them to the particular application for which they are designed. We present a design approach that yields active metamaterials whose physical structure is fixed, yet their local acoustic response can be changed almost arbitrarily and in real time by configuring the digital electronics that control the metamaterial acoustic properties. We demonstrate this approach experimentally by designing a metamaterial slab configured to act as a very thin acoustic lens that manipulates differently three identical, consecutive pulses incident on the lens. Moreover, we show that the slab can be configured to simultaneously implement various roles, such as that of a lens and a beam steering device. Finally, we show that the metamaterial slab is suitable for efficient second harmonic acoustic imaging devices capable of overcoming the diffraction limit of linear lenses. These advantages demonstrate the versatility of this active metamaterial and highlight its broad applicability, in particular, to acoustic imaging.

Figure 1

Reconfigurable metamaterial unit cell. (a) The unit cell consists of a piezoelectric membrane controlled by electronics. The cell acoustic response is controlled by a digital electronic circuit that can be reconfigured in real time. (b) Photograph of fabricated unit cell. (c) Metamaterial slab consisting of ten unit cells.

Figure 2

Experimental setup. (a) Top-view sketch of the 2D acoustic waveguide showing the metamaterial slab positioned in the middle of the waveguide. The incident sound consists of a series of three identical, closely spaced pulses. The acoustic field is measured in the highlighted waveguide region behind the metamaterial in which the metamaterial response to the first pulse is shown. (b) Time domain variation of the incident sound field.

Figure 3

Lens acoustic response to three closely spaced identical pulse. (a) Functional diagram of the reconfigurable electronics block. The rectifier generates the second harmonic, and the phase delay line configures the metamaterial to behave as a lens. (b)–(d) Time instances of the acoustic responses to each one of the three pulses. For each pulse the metamaterial slab behaves as a lens whose parameters are reconfigured in between pulses. The time instance, focal spots, and the direction of propagation of the beam leaving the lens are specified on each plot.

Figure 4

Second order harmonic lens performance. (a) Complex amplitude of the lens' second harmonic response (3000 Hz) to the third pulse shown in Fig. 3. The dotted line marks the focal plane. (b) Comparison between the field measured on the focal plane (circles) and an ideal lens imaging at the second harmonic (dotted curve) and the fundamental (solid curve). The second harmonic image is twice narrower than the image at the fundamental frequency.

Figure 5

Metamaterial lens configured to multiplex functionality. (a) Diagram of the reconfigurable electronics showing two phase delay lines that implement the two desired behaviors. (b) The first functionality is that of a focusing lens identical to the one shown in Fig. 3. (c) Metamaterial configured to behave as a beam steering device. (d) Metamaterial configured to behave as a focusing lens and beam steering device simultaneously.