Towards anti-causal Green’s function for three-dimensional sub-diffraction focusing
Title of Paper:
Towards anti-causal Green’s function for three-dimensional sub-diffraction focusing
Summary:
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.
Co-author:
Guancong Ma, Xiying Fan, Fuyin Ma, Julien de Rosny, Ping Sheng and Mathias Fink
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