Non-Hermitian topological whispering gallery

Bolun Hu, Zhiwang Zhang (), Haixiao Zhang, Liyang Zheng, Wei Xiong, Zichong Yue, Xiaoyu Wang, Jianyi Xu, Ying Cheng (), Xiaojun Liu () and Johan Christensen ()
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Bolun Hu: Nanjing University
Zhiwang Zhang: Nanjing University
Haixiao Zhang: Nanjing University
Liyang Zheng: Universidad Carlos III de Madrid
Wei Xiong: Nanjing University
Zichong Yue: Nanjing University
Xiaoyu Wang: Nanjing University
Jianyi Xu: Nanjing University
Ying Cheng: Nanjing University
Xiaojun Liu: Nanjing University
Johan Christensen: Universidad Carlos III de Madrid

Nature, 2021, vol. 597, issue 7878, 655-659

Abstract: Abstract In 1878, Lord Rayleigh observed the highly celebrated phenomenon of sound waves that creep around the curved gallery of St Paul’s Cathedral in London1,2. These whispering-gallery waves scatter efficiently with little diffraction around an enclosure and have since found applications in ultrasonic fatigue and crack testing, and in the optical sensing of nanoparticles or molecules using silica microscale toroids. Recently, intense research efforts have focused on exploring non-Hermitian systems with cleverly matched gain and loss, facilitating unidirectional invisibility and exotic characteristics of exceptional points3,4. Likewise, the surge in physics using topological insulators comprising non-trivial symmetry-protected phases has laid the groundwork in reshaping highly unconventional avenues for robust and reflection-free guiding and steering of both sound and light5,6. Here we construct a topological gallery insulator using sonic crystals made of thermoplastic rods that are decorated with carbon nanotube films, which act as a sonic gain medium by virtue of electro-thermoacoustic coupling. By engineering specific non-Hermiticity textures to the activated rods, we are able to break the chiral symmetry of the whispering-gallery modes, which enables the out-coupling of topological ‘audio lasing’ modes with the desired handedness. We foresee that these findings will stimulate progress in non-destructive testing and acoustic sensing.

Date: 2021
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DOI: 10.1038/s41586-021-03833-4

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