Introductory Chapter: Phonons and Acoustic Metamaterials

1. Introduction

Phonons are quantum excitations in solids that describe the quantum properties of lattice vibrations. They are quantum descriptions of atomic or ionic vibrations in crystals [1]. Phononic crystals [2], on the other hand, are artificial periodic structures that manipulate the propagation of phonons through their periodicity, analogous to how electronic crystals control electrons.

Phononic crystals manipulate the propagation of phonons through periodic structures, thereby achieving control and modulation of sound waves. The design of phononic crystals can affect the phonon dispersion relations and bandgap structures, thereby influencing the propagation properties of phonons. The design and application of phononic crystals typically involve aspects such as frequency-selective phonon transmission, phononic waveguides, and phonon isolation. These applications rely on controlling the propagation of phonons. Therefore, research on phononic crystals is closely related to the properties of phonons.

Acoustic metamaterials represent a broader concept than phononic crystals. It is widely accepted that acoustic metamaterials are materials with properties that do not exist in nature. While some acoustic metamaterials still rely on periodic configurations of local resonators [3] and/or scatterers [4], or topological properties of supercells [5], their operating frequency range can be significantly lower than that of phononic crystals, especially in the sub-wavelength region. Furthermore, acoustic metamaterials can be aperiodic, achieved by artificially modifying structural or material properties. Examples include acoustic black holes (ABHs) [6], rainbow trapping [7], acoustic metasurfaces [8], and functionally graded materials [9].

2. Trends of phonons and acoustic metamaterials

Phonons and acoustic metamaterials represent a vibrant research area that has attracted substantial attention in recent years. The research trends in this field include, but are not limited to, the following:

• Exploration of nonlinear phenomena

  In the realm of acoustic metamaterials, there is a burgeoning interest in investigating nonlinear phenomena. This entails scrutinizing the nonlinear behavior of acoustic metamaterials, encompassing aspects such as nonlinear wave propagation, wave mixing, and frequency conversion. By delving into nonlinear acoustic metamaterials, researchers aim to unveil novel functionalities with implications for diverse applications, including signal processing, imaging, and nonlinear acoustics.

• Advancements in active and reconfigurable architectures

  Active and reconfigurable acoustic metamaterials have emerged as focal points of research endeavors. These metamaterials exhibit tunable properties that can be dynamically controlled or reconfigured in real time. Integrating actuators, sensors, or external stimuli, active acoustic metamaterials demonstrate potential for adaptive functionalities, including dynamic acoustic lenses, switches, and cloaks. Such developments hold promise for enhancing versatility and adaptability in acoustic wave manipulation.

• Pursuit of broadband and low-frequency capabilities

  Efforts to achieve broadband and low-frequency performance in acoustic metamaterials continue to drive research endeavors. Broadband and low-frequency acoustic metamaterials are essential for applications such as underwater acoustic cloaking, seismic wave control, and low-frequency sound absorption. Research in this domain focuses on overcoming technical challenges to realize effective manipulation capabilities across a wide frequency spectrum, thereby expanding the applicability and utility of acoustic metamaterials.

• Exploration of topological concepts

  Topological principles are increasingly being explored in the design and development of acoustic metamaterials. By harnessing topological concepts, researchers aim to achieve robust and protected sound wave propagation. Topological acoustic metamaterials may exhibit unique edge states or protected modes that are resilient to defects or disorder, offering potential applications in sound waveguiding, routing, and isolation. The pursuit of topological acoustic metamaterials represents a frontier in the quest for resilient and fault-tolerant acoustic wave manipulation.

• Advancements in acoustic black holes (ABHs)

  ABHs are engineered structures designed to concentrate and absorb sound energy effectively, mimicking the way gravitational black holes trap light. They typically feature a tapered geometry that gradually slows down and compresses incoming acoustic waves, leading to their efficient absorption and dissipation. ABHs are utilized in noise reduction and vibration control applications due to their exceptional energy-trapping capabilities. Research into ABHs is focused on optimizing their design and expanding their applications in various fields, including acoustics, materials science, and engineering.

In summary, the trends mentioned above underscore the dynamic landscape of research in phonons and acoustic metamaterials. Continued exploration of nonlinear phenomena, active and reconfigurable architectures, broadband and low-frequency capabilities, topological concepts, and ABHs promises to catalyze transformative breakthroughs in acoustic wave manipulation and pave the way for novel applications across diverse scientific and technological domains.