Mirror Symmetry Broken of Sound Vortex Transmission in a Single Passive Metasurface via Phase Coupling

Asymmetric transmission in a passive vortex system is highly desirable, as it enables the development of compact vortex-based devices. However, breaking the mirror symmetry of transmission via a single metasurface poses challenges due to the inherent symmetric transmission properties in reciprocity. Here, we theoretically propose and experimentally demonstrate a novel transmission-reflection phase coupling mechanism to achieve the broken mirror symmetry of sound vortex transmission. This mechanism establishes a special coupling link between transmission and reflection waves, superimposing asymmetric reflection phases on the transmission phases. By utilizing a single passive phase gradient metasurface with asymmetric reflection phase twists, distinct transmission phase twists for mirror-symmetric incident vortices can be achieved within a cylindrical waveguide. This is typically difficult to imple-ment in a reciprocal system. Numerical and experimental results both demonstrate the broken mirror symmetry of vortex transmission and reflection. Our findings offer a new strategy for controlling vortex wave propagation, which may inspire new directional applications and extend to the field of photonics.

Figure 1

Schematic of asymmetric generation and propagation of mirror-symmetric incident vortices. (a), (b) Transmission and reflection of mirror-symmetric vortex in a cylindrical waveguide. Red and blue spirals represent vortices with topological charges of $-1$ and 1, respectively. (c) Schematic diagram of a single TRI PGM, which is composed of groups of fanlike supercells. (d) Illustration of MIRs effect with normal and coupled processes in a 2D cross section of the unit cell for forward and backward incidences.

Figure 2

Design of the TRI PGM. (a) The TRI PGM comprises of six fanlike sections with double-opening resonators and perforated panels. (b) Azimuthal part of the fanlike sections. (c) The simulated transmission and reflection phase responses and transmission spectra for each unit cell. The corresponding wave propagation process of topological charge conversion for two pairs mirror-symmetric incidences are shown in (d) and (e), respectively.

Figure 3

Simulations of asymmetric vortex generation via PGM. (a), (b) The simulated scattered pressure fields for two pairs of mirror-symmetric incident vortices with the PGM placed at $z=0$. (c), (d) The simulated transmission phase and amplitude distributions of total pressure field patterns at $z=-2.6\lambda$ and $z=3.35\lambda$, respectively. The amplitudes are normalized by their maximum values.

Figure 4

Experimental demonstrations. (a) Cylindrical waveguide employed for experimental studies, with measurement areas indicated by red dashed lines. (b), (c) The experimentally measured phase and amplitude distributions of total pressure field patterns for transmission. The measured amplitudes are also normalized by the maximum values.