声单向操控研究进展

电子二极管的发明标志着现代电子学的诞生, 在整个人类社会中引起了科技的深刻变革. 声波是一种具有非常悠久的研究历史的经典波, 却始终被认为仅具有对称的传播形式. 若能制造出可像电子二极管控制电流般实现声波单向导通的声学器件, 显然将对整个声学研究领域产生重大影响, 具有重要的科学意义及应用价值. 第一个基于非线性媒质与声子晶体的声二极管利用非线性突破声学互易原理的局限, 首次实现了将声能流限制在单一方向上的声整流效应. 针对非线性系统转换效率低下的固有缺陷, 在线性体系内围绕声单向传播这个重要科学问题开展了一系列理论和实验研究, 设计与制备了多种具有特殊结构和性能的线性声学单向结构, 在器件的效率、带宽及尺寸方面产生了突破. 在声二极管研究的基础上, 第一个可以像电子三极管操控电流般对声流进行操控与放大的声三极管理论模型也被提出. 本文介绍了声单向传播这一新兴且富有蓬勃生机的研究领域中的主要进展.

Recent advances in acoustic one-way manipulation

Realizations of one-way manipulations in various kinds of energy flux are always highly desirable. The most famous example should be the invention of electric diodes which marked the emergence of modern electronics and resulted in worldwide technology revolutions. Acoustic wave, albeit a classical wave with much longer reflearch history in comparison with the electricity, has long been thought to propagate easily along two opposite directions in any path. Hence it should be intriguing to realize the one-way transmission of acoustic waves by designing the acoustical analogy of electric diodes, which would have deep implications in all the acoustics-based applications and the field of acoustics in general. In this review, we briefly describe reflent advances in acoustic one-way manipulation which has become a new frontier of science and is of remarkable significance in both the physics and engineering communities. The emergence of the first “acoustic diode”, formed by coupling a phononic crystal (PC) with a nonlinear medium, offers the possibility of rectifying acoustic energy flux by breaking through the barrier of reciprocity principle via the introduction of nonlinearity. Despite of the efforts in enhancing the performances of nonlinear acoustic diodes by updating their structures, the inherent shortcomings in nonlinear systems such as low efficiency and narrow bandwidth still attract considerable attentions on the potential of linear structures, aiming at constructing a one-way manipulation on particular modes of an acoustic wave without breaking the reciprocity principle. A series of linear acoustic one-way devices have already been designed and fabricated with significantly improved performances. On the basis of asymmetric mode conversion, a linear one-way plate for Lamb waves is designed. High efficient one-way transmission for plane waves propagating along two opposite directions is realized by coupling a PC and a diffraction structure. Unidirectional waveguide is designed and fabricated which only allows for a plane wave incident from one of the two openings to pass. A unidirectional structure with a total thickness as thin as the wavelength is realized by reconstructing the otherwise plane wavefront with acoustic gratings. An acoustic gradient-index structure is proposed that can directly manipulate the wave trajectory asymmetrically and then yield asymmetric acoustic transmission within a considerably broad band. Acoustic metamaterials with near-zero indexes have also been employed to realize unidirectional transmission with a controllable transmitting angle and consistent wavefront. These advances are important steps towards the practical applications which generally require integration and minimization of devices having high efficiency and broad bandwidth. The reflently emerged “acoustic transistor” has been described as well, which can be regarded as the acoustical counterpart of an electric transistor and enables the amplification and switch of acoustic waves by an acoustic wave, or by exploiting the three-wave mixing effect. We also discuss the challenge and promise of the usage of acoustic one-way devices in controlling acoustic waves.