Design and experimental demonstration of effective acoustic gain medium for PT-symmetric refractive index

The parity-time (PT)-symmetric system has been a hot research topic in recent years. Nevertheless, the corresponding gain part is hard to be realized experimentally in acoustic systems. In this paper, by combining the concept of injecting external energy and the soundwave-structure interaction, we propose a S-shaped waveguide model with a wave modulator, which exactly obtains the effective acoustic gain medium. We both numerically and experimentally show the implementation of the acoustic gain medium and calculate the effective parameter refractive index by the standard retrieval method. Crucially, the flexible reconstructions of the proposed structure can be easily conducted by merely changing one parameter of the proposed structure (e.g., height of the wave modulator). Moreover, to construct the on-demand PT-symmetric refractive index period, we further present a practical approach by applying the well-selected parameters of the model and analyzing the modulator's impedance. Our results are expected to be helpful for the design and potential applications of the acoustic PT-symmetric systems.