Optimization of Subwavelength Ultrasonic Focusing Beams in Core-shell Lenses through Numerical Simulations

In this study, we numerically examined the correlation between the mechanical parameters of core-shell lenses and the main characteristics of subwavelength ultrasonic focusing beams generated by these lenses. These beams possess unique properties that can be utilized in high-resolution imaging systems. We conducted numerical simulations for various core materials and different core and shell sizes of the core-shell lenses. Our simulations revealed that the full width at half maximum (FWHM) and depth of focus ( z₀ ) of the beams can reach 0.437 and 6.44 wavelengths, respectively. Additionally, we discovered that the intensity of the generated beams can be optimized by selecting a ratio of 0.43 between the inner and outer core, regardless of the core material of the lens. This work provides a deeper understanding of the design of lenses that produce beams below the diffraction limit, which can be applied in the development of high-resolution single element transducers.