title: Three-dimensional ultrathin planar lenses by acoustic metamaterials.
abstract: Acoustic lenses find applications in various areas ranging from ultrasound imaging to nondestructive testing. A compact-size and high-efficient planar acoustic lens is crucial to achieving miniaturization and integration, and should have deep implication for the acoustic field. However its realization remains challenging due to the trade-off between high refractive-index and impedance-mismatch. Here we have designed and experimentally realized the first ultrathin planar acoustic lens capable of steering the convergence of acoustic waves in three-dimensional space. A theoretical approach is developed to analytically describe the proposed metamaterial with hybrid labyrinthine units, which reveals the mechanism of coexistence of high refractive index and well-matched impedance. A hyperbolic gradient-index lens design is fabricated and characterized, which can enhance the acoustic energy by 15 dB at the focal point with very high transmission efficiency. Remarkably, the thickness of the lens is only approximately 1/6 of the operating wavelength. The lens can work within a certain frequency band for which the ratio between the bandwidth and the center frequency reaches 0.74. By tailoring the structure of the metamaterials, one can further reduce the thickness of the lens or even realize other acoustic functionalities, opening new opportunity for manipulation of low-frequency sounds with versatile potential.
authors.affiliation: Key Laboratory of Modern Acoustics, MOE, Department of Physics, Collaborative Innovation Center for Advanced Microstructures, Nanjing University, Nanjing 210093, China.
authors.forename: Yong
authors.initials: Y
authors.lastname: Li
authors.affiliation: Department of Marine Technology, Ocean University of China, Qingdao 266100, China.
authors.forename: Gaokun
authors.initials: G
authors.lastname: Yu
authors.affiliation: 1] Key Laboratory of Modern Acoustics, MOE, Department of Physics, Collaborative Innovation Center for Advanced Microstructures, Nanjing University, Nanjing 210093, China [2] Imaging Technology Group, Beckman Institute, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA.
authors.forename: Bin
authors.initials: B
authors.lastname: Liang
authors.affiliation: Key Laboratory of Modern Acoustics, MOE, Department of Physics, Collaborative Innovation Center for Advanced Microstructures, Nanjing University, Nanjing 210093, China.
authors.forename: Xinye
authors.initials: X
authors.lastname: Zou
authors.affiliation: China Shuangxing Technology Co., Ltd., Beijing 100142, China.
authors.forename: Guangyun
authors.initials: G
authors.lastname: Li
authors.affiliation: China Shuangxing Technology Co., Ltd., Beijing 100142, China.
authors.forename: Su
authors.initials: S
authors.lastname: Cheng
authors.affiliation: Key Laboratory of Modern Acoustics, MOE, Department of Physics, Collaborative Innovation Center for Advanced Microstructures, Nanjing University, Nanjing 210093, China.
authors.forename: Jianchun
authors.initials: J
authors.lastname: Cheng