Wavelength-scale gas-filled cuboid acoustic lens with diffraction limited focusing

Per the laws of geometrical optics, a plane-parallel medium does not possess focusing properties. However, in this paper, we demonstrate for the first time that an acoustic cuboid particle filled with CO2 is able to focus sound despite its flat surface. It is worth noting that traditional lenses focus sound through their curved surfaces. We report both numerically and experimentally the acoustic focusing of a cuboid of 2λ side, where λ is the wavelength in air. From these results, it can be derived that its focusing capabilities are close to the diffraction limit: at frequency of 5000 Hz, acoustic beam waist is about 0.5 wavelength and the gain is 8.8 dB. The flat structure lens proposed open new possibilities to design and built new lightweight and low cost acoustic lenses for different applications.