Page 16 - Fall2021

TOPOLOGICAL ACOUSTICS
requirements for high fabrication tolerance. Topologi- cal properties also allow for phase control and latency, functionalities not available in acoustic devices today.
Nanoelectromechanical lattices (NEMLs) of resona- tors (Figure 3a) have demonstrated topologically robust waveguides using two-dimensional periodic arrangements of mechanically coupled, free-standing nanomembranes with circular clamped boundaries (Cha and Daraio, 2018). Such NEMLs form flexural phononic crystals with well-defined dispersion features, which can be used to tailor topological bandgaps (Figure 3b) offer- ing a pathway toward the miniaturization of even more complex acoustic topological insulators, like the ones in Figure 3b. An additional advantage arising from these miniaturized acoustic devices is the possibility to trans- duce energy between different physical domains (Hackett et al., 2021). For example, nanomembranes can convert optical, magnetic, or electrical signals into mechanical strains and vice versa. These couplings can, in turn, be used to introduce nonlinearities (Figure 2a), modulation, and tunability of the fundamental resonant frequencies and the dispersion of the devices (Cha et al., 2018). The functionalities of these new acoustic devices can extend beyond conventional filtering, enabling complete net- works and circuitry transporting pseudospins as a degree of freedom carrying information.
The potential of topological acoustics for RF communi- cation systems opens a path toward a new technological landscape with lower energy consumption, smaller form factors, and larger bandwidths. Such opportunities also come with challenges, including design and fabrication complexity. Miniaturized topological acoustic metamate- rials need to rely on advances in multimaterial fabrication capabilities to accomplish design flexibility, nonlinearity and dissipation control, and new strategies to impart the pseudospins of choice.
Information Science Based on Topological Sound
Sound is naturally used to encode and convey information. Human speech supported by sound carries information because our voice varies continuously in time and ampli- tude. Acoustic cues such as frequency and amplitude modulation allow communicators to derive meaning. Although this form of communication is based on analog signals, most information encoding, transmission, and processing techniques today are carried out in the digi- tal domain, where signaling cues are restricted to discrete values. Modern digital information processing relies on electronic digital logic circuits, whose elementary units are Boolean logic gates and use the binary numbers 0 and 1 to implement Boolean functions such as the NOT, AND, and OR gates. Consequently, processing of sound-encoded
Figure 3. a: Scanning electron microscope (SEM) image of a topological waveguide. Red and blue dots, lattice points of membranes with slightly different geometries. Flexural membrane motions (inset) were excited by simultaneously applying a DC/AC voltage (Cha and Daraio, 2018). b: Dispersion of topological edge modes experimentally measured in the geometry of panel a, where a is the lattice period. Yellow and red, modal resonances. c: SEM image of a nonlinear nanoelectromechanical lattice (NEML) (Cha et al., 2018). Red arrow, localized probe exciting the structure. Inset: geometrical nonlinearity induced by electrostatic softening. Red and blue, field maxima and minima, respectively. See text for further explanation.
16 Acoustics Today • Fall 2021