Subwavelength beam focusing using a ball-shaped lens for ultrasound imaging

Ultrasonic superresolution images can be generated by means of (super) focusing acoustic beams to subwavelength dimensions or using algorithm-based methods. Here, we demonstrate that ultrasonic pulses which are superfocused by a ball-shaped lens can be used to produce superresolution images. The imaging system is comprised of a circular flat transducer with an operation frequency of $1$ MHz, and a ball lens centered in the beam axis of symmetry. The corresponding wavelength in water is $λ_0 = 1.53$ mm. The system resolution is $0.6λ_0$ in the focal plane at one wavelength away from the lens. The superresolution method is compared with a conventional ultrasonic system based on a spherically focused transducer. Our method presents twice more resolution with a shorter depth-of-field of $2λ_0$. Possible applications that take advantage of these features are discussed, as well as some limitations of the proposed technique.