Realization of unidirectional zero reflection based on a loss–gain balanced metamaterial

In this work, we designed and fabricated an acoustic metamaterial with unidirectional zero reflection. The metamaterial was developed based on the concept of a parity-time symmetric material using one-dimensional (1D) orifice-type unit cells as a loss material and cylindrical closed-end tubes as a gain material. Twenty such composite unit cells were connected in series to construct a one-dimensional loss–gain balanced metamaterial. The characteristics of reflection and transmission were studied at frequencies between 1100 and 1800 Hz, ensuring that the metamaterial met the conditions of a homogeneous medium. From the experiments, it was found that the designed loss–gain balanced metamaterial had nearly complete unidirectional zero reflection (r≲1.6%) at f=1202 Hz; in addition, it was shown to exhibit bidirectional zero transmission (t≃0) at f=1501 Hz.