Unidirectional amplification with acoustic non-Hermitian space-time varying metamaterial

Space-time modulated metamaterials support extraordinary rich applications, such as parametric amplification, frequency conversion, and non-reciprocal transmission. The non-Hermitian space-time varying systems combining non-Hermiticity and space-time varying capability, have been proposed to realize wave control like unidirectional amplification, while its experimental realization still remains a challenge. Here, based on metamaterials with software-defined impulse responses, we experimentally demonstrate non-Hermitian space-time varying metamaterials in which the material gain and loss can be dynamically controlled and balanced in the time domain instead of spatial domain, allowing us to suppress scattering at the incident frequency and to increase the efficiency of frequency conversion at the same time. An additional modulation phase delay between different meta-atoms results in unidirectional amplification in frequency conversion. The realization of non-Hermitian space-time varying metamaterials will offer further opportunities in studying non-Hermitian topological physics in dynamic and nonreciprocal systems. Exquisite control of loss and gain in non-Hermitian systems allows waves to propagate in unusual and useful ways. Here, unidirectional amplification is achieved in an acoustic metamaterial by dynamically varying the gain and loss with modulation phase delay between different metaatoms.

Automated summary

This study demonstrates the achievement of unidirectional amplification in a non-Hermitian system through the control of space-time varying metamaterials. By dynamically varying the gain and loss and introducing modulation phase delay, the efficiency of frequency conversion is increased. This experimental realization provides further opportunities to study non-Hermitian topological physics in dynamic and nonreciprocal systems.