Enhanced acoustic sensing through wave compression and pressure amplification in anisotropic metamaterials

Abstract

Acoustic sensors play an important role in many areas, such as homeland security, navigation, communication, health care and industry. However, the fundamental pressure detection limit hinders the performance of current acoustic sensing technologies. Here, through analytical, numerical and experimental studies, we show that anisotropic acoustic metamaterials can be designed to have strong wave compression effect that renders direct amplification of pressure fields in metamaterials. This enables a sensing mechanism that can help overcome the detection limit of conventional acoustic sensing systems. We further demonstrate a metamaterial-enhanced acoustic sensing system that achieves more than 20 dB signal-to-noise enhancement (over an order of magnitude enhancement in detection limit). With this system, weak acoustic pulse signals overwhelmed by the noise are successfully recovered. This work opens up new vistas for the development of metamaterialbased acoustic sensors with improved performance and functionalities that are highly desirable for many applications.

Ultraslow waves on the nanoscale

“…The white scale bar shown in the top left panel corresponds to 10 µm (97). (A) Schematic of how the pressure-field of a sound wave is spatially compressed and enhanced while decelerated inside an acoustic metamaterial, before being detected by a sensor (61). (B) Photonic-acoustic metamaterial hybrid sensing system.…”

“…This last feature -that the bandwidth can be enhanced and engineered "by design" at the nanoscale -is a key characteristic of the herein reviewed (ultra)slow-light structures. Specific applications range from enhanced and more efficient nonlinear effects (68,73,74,108), to light-harvesting (64,83,89), bio-sensing (87,88), nano-imaging (80,111), optical and acoustic spectral demultiplexing (40-42, 55, 56, 61, 62, 64, 68, 75-81), on-chip spectroscopy (82), non-classical light sources (90,91), cavity-free plasmonic nanolasing (92)(93)(94), enhanced acoustic sensors operating beyond the noise-threshold limit (61,62), and tunable, deepsubwavelength, ultraslow guided Dirac fermions (102,103), plasmons and surface phononpolaritons in atomically-thin crystals and heterostructures (104)(105)(106)(107)(108)(109)(110). Broadband slow-light effects are also attained in other structures above the diffraction-limit, including photonic crystals and CROWs where broadband slow light is usually obtained with group indices of ~30-100 (21)(22)(23)(24)(25)(26)(27)(28), and PT-symmetric structures, which can be broadband and with the light speed reducing to zero at the exceptional point (58,114).…”

“…1, A and C), the wave deceleration in adiabatically tapered metamaterial waveguides can also be exploited for acoustic waves (61,62). Here, the concept of meta-materials (judiciously engineered media with desired wave responses) is especially fitting as it allows to design -in the deep-subwavelength, trueeffective-medium regime -structures with broadband slow-wave acoustic responses unattainable with natural media.…”

“…A universal characteristic of such negative-parameters wave-slowing structures -that one may also encounter for acoustic (61,62) ultraslow-wave structures and 2D materials -is that they allow for field accumulation and significant field-enhancement around the critical lightstopping point, deep below the diffraction limit, which can be exploited for a host of nanoscale applications. This is contrast to, e.g., slow light in atomic media where although the energy density U is indeed enhanced in the slow-light regime (U = P/υ g , P being the power density, and υ g →0), the electric-field amplitude E is not enhanced because most of the compressed energy is stored in the atomic polarization -not in the electric field (63).…”

“…1, B and C, for the photonic analogue), so that it can be detected with ultrahigh sensitivity by an acoustic sensor at the guide's end (Fig. 5A) (61). The acousticmetamaterial device is constructed by an array of stainless plates, spaced by air gaps, and behaves as a continuous medium.…”