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  • Letter

Negative Transient Flux in the Near Field of a Subwavelength Source

Xiao Li1,2, Pengqi Li1,‡, Ming-Hui Lu3, Mathias Fink4,†, and Guancong Ma1,*

  • 1Department of Physics, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China
  • 2Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China
  • 3National Laboratory of Solid State Microstructures & Department of Materials Science and Engineering, Nanjing University, Nanjing, Jiangsu 210093, China
  • 4Institut Langevin, ESPCI Paris, PSL University, CNRS, 10 rue Vauquelin, Paris, France

  • *phgcma@hkbu.edu.hk
  • †mathias.fink@espci.fr
  • ‡Present address: School of Physics and Innovation Institute, Huazhong University of Science and Technology, Wuhan 430074, China
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Phys. Rev. Applied 16, L011004 – Published 23 July, 2021

DOI: https://doi.org/10.1103/PhysRevApplied.16.L011004

Abstract

The emission of waves by a small source is a generic wave problem that has long been thought to be well studied. Here, we report the experimental observation that the energy flux of an outgoing sound wave emitted by a deep-subwavelength-sized source can become negative, which indicates the backward flow of energy. Such a negative transient flux exists, even in a homogenous medium, but is only observable in the time domain and in the extreme near field of the source. By wave-impedance analysis, we show that such a phenomenon is fundamentally rooted in the geometry of the wavefield itself and, hence, is generic. Our findings have implications in the time-dependent emission, absorption, and scattering of waves.

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Phys. Rev. Applied 16, L011004– Published 23 July, 2021
  • Received 13 May 2021
  • Revised 3 July 2021
  • Accepted 6 July 2021
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