Acoustic lens: A thin plate with quasi-periodic array of holes

A thin steel plate with quasi-periodic array of holes is fabricated, which is demonstrated to be able to function as an acoustic lens. With either a plane wave or a point source in object space, hotspots with the smallest size up to one-fifth wavelength are observed in the post-evanescent field regime in image space. Imaging of a point source with good resolution at different positions is also exhibited. The singular field distributions originate from the interference of the diffractive beams specific to quasi-periodic structures, which generates distinct and plentiful diffractive patterns.