Subwavelength twin ultrasound focused (STUF) beam generated by shear-to-longitudinal mode conversion in a triangular prism

Abstract

Focusing ultrasound fields in regions below the incident wavelength using elementary objects (such as spherical and cylindrical) as acoustic lenses have been successfully demonstrated over the last five years. This unique way to tightly concentrate the energy of acoustic fields is interesting for the development of new high resolution ultrasound systems. Usually, an incident longitudinal beam interacts with these lenses to produce a subwavelength beams at its shadow region. However, no shear-wave beam has been reported to produce subwavelength beams using objects as acoustic lenses. In this work, we numerically and experimentally report the generation of a subwavelength twin ultrasound focusing (STUF) beam using a 1 MHz shear transducer coupled to a Rexolite triangular prism. Numerical simulations were performed in order to study the generated field as a function of the apical angle of the prism and to find the mechanical configuration for the experimental validation. The results show that by changing the apical angle of the triangular prism, the main features of the STUF beams can be changed. To validate the numerical model, a prism with apical angle of 90o was built and the acoustic field distribution was measured by a needle hydrophone. A STUF beam with width of 0.8λ and depth of focus (DOF) 3λ was generated. A good agreement between numerical and experimental results was reported. The proposed system open new possibilities to design and built a simple and low cost acoustic system for microparticle trapping applications.