Transparent Gradient-Index Lens for Underwater Sound Based on Phase Advance

Spatial gradients in a refractive index are used extensively in acoustic metamaterial applications to control wave propagation through phase delay. This study reports the design and experimental realization of an acoustic gradient-index lens using a sonic crystal lattice that is impedance matched to water over a broad bandwidth. In contrast to previous designs, the underlying lattice features refractive indices that are lower than the water background, which facilitates propagation control based on a phase advance as opposed to a delay. The index gradient is achieved by varying the filling fraction of hollow, air-filled aluminum tubes that individually exhibit a higher sound speed than water and matched impedance. Acoustic focusing is observed over a broad bandwidth of frequencies in the homogenization limit of the lattice, with intensity magnifications in excess of 7 dB. An anisotropic lattice design facilitates a flat-faceted geometry with low backscattering at 18 dB below the incident sound-pressure level. A three-dimensional Rayleigh-Sommerfeld integration that accounts for the anisotropic refraction is used to accurately predict the experimentally measured focal patterns.This work is supported by the Office of Naval Research.Martin, TP.; Naify, C.; Skerritt, E.; Layman, C.; Nicholas, M.; Calvo, D.; Orris, GJ.... (2015). Transparent Gradient-Index Lens for Underwater Sound Based on Phase Advance. Physical Review Applied. 4(3):034003-1-034003-8. doi:10.1103/PhysRevApplied.4.034003034003-1034003-843Naify, C. J., Martin, T. P., Layman, C. N., Nicholas, M., Thangawng, A. L., Calvo, D. C., & Orris, G. J. (2014). Underwater acoustic omnidirectional absorber. Applied Physics Letters, 104(7), 073505. doi:10.1063/1.4865480Li, R.-Q., Zhu, X.-F., Liang, B., Li, Y., Zou, X.-Y., & Cheng, J.-C. (2011). 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