Manipulating acoustic waves in wavevector domain by using nonlocal metasurfaces: Analog differentiation and extreme acoustic detection

Conventional acoustic metasurfaces have shown exceptional capability through spatial wavefront engineering, but the degrees of freedom in the wavevector domain remain to be unlocked. Here, we shift the manipulation paradigm from the spatial domain to the wavevector domain via nonlocal metasurfaces (NMSs). The proposed NMS consists of two sublayers of surface groove structures cascaded via a nonlocal channel, which enhances internal transverse energy flow. An inverse design framework integrating the genetic algorithm with numerical simulation is established for the NMS design. We find that the internal nonlocal energy flow dictates the wavevector-dependent transmissivity of NMSs. The nonlocal effect on the amplitude transmissivity results from the interaction between two sublayers, while the transmitted phase is dominated by the backward sublayer. Differentiations of 1st order on a Gaussian beam and a cylindrical wave are demonstrated and show agreement between theoretical and numerical results. Furthermore, we showcase the practical applications of NMSs in detecting deep subwavelength scatterers with diameters of 0.15 wavelengths, and isolating a weak source buried in a strong noise with a tenfold amplitude. Robustness analyses show that the transfer functions and detection and isolation effects of the proposed NMSs keep acceptably stable when thermoviscous loss, fabrication error, imperfect periodicity, structural defect, and background reverberation are considered. Our work elaborates the physical image of acoustic wavefront reshaping in the wavevector domain, paving the way for advanced applications in signal processing, imaging, and sensing and communication.