Journal
NATURE PHYSICS
Volume 14, Issue 6, Pages 608-612Publisher
NATURE PUBLISHING GROUP
DOI: 10.1038/s41567-018-0082-3
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Funding
- Hong Kong Research Grants Council [AoE/P-02/12]
- LABEX WIFI (Laboratory of Excellence within the French Program Investments for the Future) [ANR-10-LABX-24, ANR-10-IDEX-0001-02 PSL*]
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In causal physics, the causal Green's function describes the radiation of a point source. Its counterpart, the anti-causal Green's function, depicts a spherically converging wave. However, in free space, any converging wave must be followed by a diverging one. Their interference gives rise to the diffraction limit that constrains the smallest possible dimension of a wave's focal spot in free space, which is half the wavelength. Here, we show with three-dimensional acoustic experiments that we can realize a stand-alone anti-causal Green's function in a large portion of space up to a subwavelength distance from the focus point by introducing a near-perfect absorber for spherical waves at the focus. We build this subwavelength absorber based on membrane- type acoustic metamaterial, and experimentally demonstrate focusing of spherical waves beyond the diffraction limit.
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