Focusing Acoustic Beams with a Ball-Shaped Lens beyond the Diffraction Limit

We report on measurements of a subwavelength focusing of an ultrasound beam by a polymer ball-shaped lens (Rexolite) immersed in water at room temperature. The beam arises in the near field of the ball-lens shadow side. Considering a 8.2 lambda-diameter ball lens, with lambda being the wavelength, we find a superfocused beam with 14.4-dB-intensity gain, a full width and a full length at half maximum of lambda/2 and 2.2 lambda, respectively, and sidelobes under -10 dB. The observed phenomenon is in excellent agreement with the theoretical and computational predictions based on the partial-wave expansion and finite-element methods. Our results may foster the design of simple lens elements for super-resolution acoustic microscopy and ultrasound imaging.