Recently, the team led by Professor Chen Shuqi and Professor Cheng Hua from Nankai University has made important progress in breaking the mirror symmetry of sound vortex transmission. Based on the orbital angular momentum (OAM) diffraction law of acoustic phase gradient metagratings (PGM), a new mechanism for transmission phase control is put forward, and a single metasurface is used to break the mirror symmetry of sound vortex transmission in a reciprocal acoustic system for the first time. The findings of the research were published in Physical Review Letters under the title “Mirror symmetry broken of sound vortex transmission in a single passive metasurface via phase coupling”. 

Schematic diagram of sound vortex transmission with broken mirror symmetry

Reciprocal modulation with a passive and simple structure provides greater flexibility and applicability for compact acoustic systems than non-reciprocal modulation that requires active devices or external intervention. However, due to the constraints of reciprocity, the transmission phase provided by a single metasurface for the forward and backward incidences is fixed. The phase twist attached to the transmitted vortex is characterized by mirror symmetry. If the backward incident vortex irradiated on the metasurface adopts the equivalent cases (mirror-symmetric) with the forward one, then the corresponding transmitted vortex will also be mirror-symmetric. Therefore, for the scalar wave of acoustic wave, it is difficult to use a single passive metasurface to break the mirror symmetry of vortex transmission in the reciprocal system. This has high value for theoretical research as well as practical application prospects. A special PGM that can simultaneously control the transmission and reflection of sound waves is designed in this study. Based on the sound vortex diffraction law, it proposes the coupling mechanism for the transmission and reflection phases of sound waves for the first time. When the incident sound vortex undergoes multiple internal reflections (MIRs) inside the PGM, the transmission phase will couple with the asymmetric reflection phase, thereby achieving the asymmetric phase control of the transmitted sound wave. Under the proposed phase coupling mechanism, this study breaks the mirror symmetry of sound vortex transmission in a cylindrical waveguide, offering a novel paradigm for the realization of asymmetric signal transmission in the reciprocal acoustic system.

Tang Yugan, a doctoral student at Nankai University, is the first author, Professor Chen Shuqi and Professor Cheng Hua are the co-corresponding authors, and the School of Physics and the School of Materials Science and Engineering of Nankai University are the authors’ affiliation. 

URL:

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.132.177001

（Edited and translated by Nankai News Team.)