Zero refractive index in time-Floquet acoustic metamaterials

Abstract

Abstract:New scientific investigations of artificially structured materials and experiments have exhibit wave manipulation to the extreme. In particular, zero refractive index metamaterials have been on the front line of wave physics research for their unique wave manipulation properties and application potentials. Remarkably, in such exotic materials, time-harmonic fields have infinite wavelength and do not exhibit any spatial variations in their phase distribution. This unique feature can be achieved by forcing a Dirac cone to the center of the Brillouin zone ( G point), as previously predicted and experimentally demonstrated in time-invariant metamaterials by means of accidental degeneracy between three different modes. In this article, we propose a different approach that enables true conical dispersion at G with twofold degeneracy, and generates zero index properties. We break time-reversal symmetry and exploit a space-time modulation scheme to demonstrate a time-Floquet acoustic metamaterial with zero refractive index. This behavior, predicted using stroboscopic analysis, is confirmed by fullwave finite elements simulations. Our results establish the relevance of space-time metamaterials as a novel reconfigurable platform for wave control.

Active Acoustic Resonators with Reconfigurable Resonance Frequency, Absorption, and Bandwidth

“…We also show that the bandwidth and real impedance at resonance can be reconfigured by changing the edge frequency, B, of the low-pass filter and the value of the control coefficient at f 0 (which can be calculated directly from the desired target resonance impedance ratio of Z in /Z char for closed-box systems or Z/Z char for in-tube systems). Since control is operated in the time domain, and only involves time-domain operations on sensed signals, it can be changed dynamically faster than the acoustic signal and used to create resonators with time-varying properties; this is also a current topic of interest in the physical community [26][27][28][29][30]. We therefore believe that the present work can largely enrich the toolbox of acoustic engineers and applied physicists, not only by building lowloss acoustic metamaterials out of active resonators with calibrated robust resonance frequencies and programmable bandwidth, but also by helping construct a new generation of dynamic acoustic systems, such as fast switches or time-modulated artificial structures with novel physical properties.…”

Dirac-like cone-based electromagnetic zero-index metamaterials

“…In contrast, our review focuses on the fundamental physics of DCZIMs, comparison of DCZIM with zero-index materials based on volume plasmon, fishnet metamaterials and doped ENZ medium, classification of DCZIMs, demonstration of optical DCZIMs, and promising potential applications of DCZIMs. In addition to photonic metamaterials, the concept of DCZIMs has already been adopted by other kinds of metamaterials including acoustic [36][37][38][39][40][41][42][43][44][45][46][47] and elastic metamaterials 48,49 .…”

Time-dependent high-contrast subwavelength resonators

“…Moreover, the broken timereversal symmetry can be used to replicate spin effects from quantum systems. As an example, having phase-shifted ("rotation-like") modulations can provide a kind of "artificial spin" [14,18,32]. These ideas have been used to study classical analogues of the quantum Hall effect, and so-called Floquet topological insulators [14,27,29,31,32,35,36].…”