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Topological insulators, which possess robust topologically protected properties for manipulating the wave propagation against the disorder and defects, have grown into a large research field in photonic and phononic crystals. However, the conventional topological band theory is used to describe a closed photonic/phononic crystal that is assumed to be Hermitian system. In fact, practical physical systems often couple with outside environment, and induce non-Hermitian Hamiltonian with complex eigenvalues. Recently, many novel topological properties have been induced by the interacting between non-Hermitian and topological phases, a prominent example is non-Hermitian skin effect that all eigenstates are localized to the boundary in open system, which different from the conventional topological edge states. The unique physical phenomenon has stimulated various applications, such as wave funneling, enhanced sensing, and topological lasing. In this work, we describe the non-Hermitian skin effect using winding numbers. The sign of the winding number determines the rotation direction of the loops in the complex frequency plane, which the sign can be controlled by the nonreciprocal coupling direction. In this context, we designed different topological skin interface between different domains with opposite winding numbers to manipulate the energy focusing to middle or two-end of non-Hermitian 1D acoustic cavity chain. In experiment, we used an electroacoustic coupling method, employing a unidirectional coupler composed of microphones, speakers, phase shifters, and amplifiers, to introduce positive and negative non-reciprocal couplings between the two acoustic cavities and studied the characteristics of these non-reciprocal couplings. Then, the non-reciprocal coupling cavities were extended into a chain structure, and the magnitude and sign of the non-reciprocal couplings were flexibly controlled using phase shifters and amplifiers. Through this method, we successfully constructed interfaces between different winding numbers, achieving a one-dimensional non-Hermitian skin effect at various interfaces. The experimental results indicate that the sound can be focused at middle interface or two-end interfaces for different nonreciprocal coupling distributions, and the skin interface can also be switched from middle to two-end by exchanging the nonreciprocal coupling direction of the domains. Our research results offer greater flexibility in the design of acoustic devices and may provide a new platform for exploring advanced topological acoustic systems for controlling sound propagation.
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Keywords:
- non-Hermitian skin effect /
- interface state /
- phononic crystal /
- resonant cavity
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