Subwavelength Focusing Beam and Superresolution Ultrasonic Imaging Using a Core-Shell Lens

The ability to manipulate acoustic fields beyond the diffraction limit offers possibilities for many applications, including ultrasound imaging and nondestructive testing. We numerically and experimentally report a simple method for subwavelength ultrasound focusing and superresolution imaging (i.e., images produced with a subwavelength spatial resolution) using a core-shell shaped lens. By changing the mechanical properties and size conditions of the lenses, we can significantly enhance the subwavelength properties of the focusing beams. These properties have a major effect on the image quality of ultrasonic systems. Using a carbon steel core and a Rexolite shell of total diameter of 12.8 mm positioned at a 2 MHz ($\ensuremath{\lambda}=0.74$ mm) plane wave incident field, we show that a beam with intensity gain of 20 dB, full width at half maximum (FWHM) of 0.44 mm ($0.6\ensuremath{\lambda}$), and full length at half maximum (FLHM) of 2.74 mm ($3.7\ensuremath{\lambda}$) can be achieved. Moreover, we experimentally demonstrate that a superresolution imaging system using the core-shell lens can scan objects with subdiffraction information. The results show that the superresolution method can double the spatial resolution and the focal depth can be increased using an appropriate size of lens. The proposed design system can be easily applied to assisted superresolution acoustic microscopy devices.