When we press and pluck the strings of a guitar, it generates different types of acoustic waves. How those waves sound is decided by how we pick those strings and the strings’ nature properties such as length and thickness. The principle here is that wave radiation depends on both the intrinsic properties of the source and the eigenstates of its surrounding environment. This has laid the foundation to explore and exploit various physical phenomena in a wide range of wave systems. The evolving understanding of this paradigm has inspired countless breakthroughs in wave-matter interaction related fields ranging from mechanics and acoustics to optics and photonics. A long-held belief of wave-matter interaction is that an emitter always radiates into and interacts with the eigenstates that exclusively define the surrounding environment. Even in non-Hermitian systems featuring exceptional point(s) where two or more eigenstates coalesce leading to an incomplete eigen-basis in the Hilbert space, this was still deemed to be valid previously as the wave function associated with the missing dimension of the Hilbert space has not been observed in any physical system.

Research team led by Dr Jie Zhu, Associate Professor of the PolyU Department of Mechanical Engineering, conducted collaboration with Prof. Ren-min Ma’s team from Peking University, Prof. Li Ge’s team from City University of New York and other colleagues. They show that the above-mentioned century-old tenet can surprisingly break down at an exceptional point. With investigation on difference classic wave systems, the researchers experimentally demonstrated a chirality-reversal phenomenon in a whispering gallery mode cavity where the excited unidirectional wave circulation exhibits opposite handedness to the coalesced eigenstate. This striking yet extensively existed phenomenon were confirmed in both acoustic and electromagnetic wave systems.

Their finding, for the first time, reveals that the radiation field of an emitter can become fully decoupled from the eigenstates of its environment. Such counter-intuitive phenomenon transforms the fundamental understanding of wave-matter interaction and enriches the intriguing physics of exceptional points hidden behind source-eigenstate interplay. In acoustics, it could contribute to a board range of research fields, including non-Hermitian acoustics, noise control and abatement.

This work has been recently published online on Nature Physics [ “Revealing the missing dimension at an exceptional point”, https://www.nature.com/articles/s41567-020-0807-y ]. Dr Tuo Liu, Postdoctoral Fellow of the PolyU Department of Mechanical Engineering (also a PolyU ME PhD graduate) is the co-first author.

Read the full text of paper at https://rdcu.be/b2JlC.