Acoustic zero index metamaterials such as density-near-zero metamaterials have received increasing attention due to their potential applications on beam forming, cloaking, wave tunneling, and imaging. High transmission resulted by impedance matching of such zero index metamaterials and surrounding media requires the effective density and inverse bulk modulus to be simultaneously zero. Metamaterials possessing this property are called double zero index metamaterials. The design of double zero index metamaterials needs scatterers with sound speed lower than the background medium, which is extremely challenging for air acoustics because the air sound speed is among the lowest. This challenge can be solved for high order waveguide mode by designing structures with larger thickness. An experimental scan of the pressure field inside our design metamaterial excited by a point source reveals the existence of a Dirac cone at the Brillouin zone center. The measured envelope of the propagating wave inside the metamaterial shows double negative, double positive, and double zero properties below, above, and at the Dirac point,respectively. This result is confirmed by the measured acoustic beam out of the metamaterial. A gapless transition between double negative and double positive acoustic metamaterials is realized. The development of this double zero index metamaterial provides new routes to broaden practical applications of acoustic metamaterials.
Chengzhi Shi, Marc Dubois, Xuefeng Zhu, Yuan Wang, and Xiang Zhang, "Experimental observation of wave modulation and Dirac cone in acoustic double zero refractive index metamaterials (Conference Presentation)," Proc. SPIE 10170, Health Monitoring of Structural and Biological Systems 2017, 101700D (Presented at SPIE Smart Structures and Materials + Nondestructive Evaluation and Health Monitoring: March 26, 2017; Published: 11 May 2017); https://doi.org/10.1117/12.2258165.5427850806001.
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