Experimental demonstration of Fresnel zone plate lens for robust subwavelength focusing at mega hertz.

Acoustic focusing with high energy intensity, broadband and few side lobes properties are highly demanded in many fields and can be achieved with Fresnel zone plate (FZP) lens as an artificial structure. However, FZP focusing lenses for high frequency range such as around 5 MHz with precise focusing effect has not been demonstrated and the dependence of the size, especially the full width at half maximum (FWHM), of the focal spot has not been well explored. In this work, we numerically and experimentally study the FZP lenses for robust and subwavelength underwater sound focusing effect around 5 MHz. Stable subwavelength FWHM of the focusing spot over a broad frequency range is achieved. It is found that the FWHM has a quasi-linear relationship with the ratio between the focal length and the diameter of the lens. A quasi-linear relationship between the focal length and working frequency is also validated theoretically, numerically and experimentally. Our study proposes broadband and robust FZP lenses paving the way for high quality acoustic focusing at Mega Hertz frequency range for the development of imaging, non-destructive evaluation, and so on.