Extraordinary sound transmission and collimation effect through a subwavelength slit with surface engineering of phase gradient

Subwavelength slit diffraction, as a fundamental diffraction phenomenon, is of great significance in various fields. However, governed by the symmetry of coupling modes and diverse diffraction orders, designing a device that can independently achieve excellent extraordinary transmission performance and a desirable collimation effect remains a challenging task. Here, we propose a new paradigm for manipulating acoustic slit diffraction by engineering a reversed phase gradient on one side of the structural surface. For instance, under forward incidence, the extraordinary transmission efficiency can be significantly enhanced, where the phase gradient plays a key role in exciting unidirectional evanescent waves along the surface; under backward incidence, the beam collimation effect can be well maintained, as the phase gradient functions to suppress unwanted diffraction orders. We further experimentally demonstrate this novel phenomenon. Our work enriches the methods for manipulating acoustic single-slit diffraction and shows potential applications in sensors and holography.