Page 22 - Summer 2015

Nonreciprocal Acoustics
magnetic circulator to an acoustic transducer is not a viable solution for full duplex acoustic operation because there are no efficient electromagnetic circulators that can handle the required levels of power at such low frequencies (circula- tors based on transistors would completely break down due to the nonlinear behavior of transistors at higher powers). For these reasons, acoustic circulators appear to be an ideal technology for underwater communications and imaging systems.
Another promising application field for nonreciprocal acoustics is the manipulation of vibrational energy for sys- tem protection or energy harvesting. Nonreciprocal devices completely redefine the common paradigm of wave propa- gation according to which waves have to travel in both direc- tions and, as a result, according to which reflections always exist at device interfaces or in the presence of defects. Non- reciprocal materials may be used to force acoustic waves to go one way along a predefined path from one point to an- other. This could eliminate Fabry-Pérot resonances, match- ing problems, and sensitivity to defects or disorder. In this vein, nonreciprocal materials are promising in the devel- opment of the acoustic equivalent of topological insula- tors (Khanikaev et al. 2015), with application potentials in energy harvesting and vibrational energy isolation. Due to their large physical size, such artificial nonreciprocal materi- als may lead to broader bandwidths than the ones reported for subwavelength nonreciprocal devices and find applica- tions in noise control and management. Finally, the concept of nonreciprocal phononic propagation may lead to several exciting possibilities in the field of heat management via the nonreciprocal control of thermal phonons. This application area has the potential to create a novel class of highly asym- metric thermal conductors that easily conduct heat in one direction but insulate in the other (Maldovan, 2013).
Summary
Nonreciprocal devices that break the symmetry of sound transmission between two points in space have been recent- ly demonstrated using various approaches, thereby opening exciting new directions in acoustics research. Such systems let sound propagate in one direction and completely block any transmission in the reverse direction, violating one of the most basic principles of acoustics, Rayleigh reciproc- ity. They can be achieved by using either nonlinearities or a parameter that is odd-symmetric under time reversal for linear systems. The latter can be achieved by imparting an- gular momentum via a moving fluid. Nonlinear systems
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that achieve highly nonreciprocal response are relevant for high-power routing and manipulation of sound, such as the protection of systems from incident acoustic power beyond a given threshold. Linear systems that break nonreciprocity are relevant in different scenarios, such as signal manipu- lation and processing, and hold significant promise in un- derwater acoustic communication systems and sonic and ultrasonic imaging devices. Moreover, violation of Rayleigh reciprocity may lead to a novel class of artificial materials that overcome the fundamental limitations of conventional materials by completely modifying the common paradigm of wave propagation within the medium. These materials will not exhibit reflections at defects or impedance-matching is- sues arising from multiple reflections between interfaces. More futuristic applications for these nonreciprocal devices may also be envisioned, including defect and disorder-free, topologically protected sound propagation in sonic or pho- nonic crystals as well as control of thermal heat transfer in nonreciprocal phononic systems. It is our opinion that non- reciprocal acoustics has a bright future as a new frontier in acoustical device engineering. It introduces a new degree of freedom in system design by relaxing the usual constraint of time-reversal symmetry in wave propagation and scatter- ing and therefore considerably extends our ability to control acoustic waves.
Acknowledgments
This work was partially supported by the Air Force Office of Scientific Research and the National Science Foundation. MRH acknowledges the support from the Office of Naval Research.
Biosketches
Romain Fleury received a MS degree in micro- and nanotechnology from the University of Lille and the National Engineering Diploma from Ecole Cen- trale de Lille, France, in 2010. In 2015, he received a PhD degree in Electrical and Computer Engineering from the
University of Texas at Austin. His research interests focus on novel interdisciplinary wave phenomena in acoustics, elec- tromagnetics, and condensed matter physics. His PhD work on artificial acoustic materials with broken time-reversal symmetry has led to two patent applications, three best stu- dent papers award (including two from ASA), and several high-impact publications including the cover of Science in 2014.