Unidirectional transmission of surface water waves based on evanescent wave modes

In this paper, the unidirectional transmission of surface water waves is experimentally observed by connecting the mode-selective channel and the spatial symmetry-breaking channel. The proposed mode-selective channel contains a symmetry structure but only allows the propagation of anti-symmetric modes in a specific frequency band, while the antisymmetric modes can be excited in the channel with spatial symmetry-breaking. Therefore, the surface water waves can only propagate through the channel when the fundamental plane wave mode is excited at the entrance of the spatial symmetry-breaking channel, but not vice versa. The results of theoretical and numerical analyses indicate that the evanescent wave mode caused by non-Bragg resonances is responsible for the emergence of the antisymmetric mode transmission. The non-Bragg evanescent wave mode, generated by the resonance between the fundamental and higher-order modes, widens the unidirectional transmission band of surface water waves. Limited by the small structure, the experimental observed unidirectional transmission has a wave extinction ratio of 20.49 dB. The simulations closer to the ocean situation show that the bandwidth can reach 0.18 Hz, and the extinction ratio is 46.09 dB. The realization of surface water wave transmission not only enriches our knowledge on ubiquitous wave phenomenon, but also benefits applications in ocean engineering, such as coastal protection, ocean wave control, green energy collection, and reef maintenance.