Ultra-compact ultrasonic metalens for underwater focusing

There are significant challenges to design acoustic metasurface in the ultrasonic regime where the wavelength is small. Moreover, the routine design rule based on effective medium approximation is invalid for underwater ultrasound. Our recent development of metasurface provides a great opportunity to solve above-mentioned challenges and the results demonstrate the great potential to apply this metasurface in the field of ultrasonic focusing. Here, we presented a focusing metasurface lens with extreme simplicity (slot structure) and ultra-compact size (deep-subwavelength spacing and thickness). Instead of tuning the acoustic path length individually, we exploited the strong wave couplings between the deep-subwavelength-spaced units for the phase modulation. A microscopic coupled-wave theory was used to predict the phase profile, based on which the metasurface lens for ultrasonic focusing was optimized. Particularly, broadband focusing was also exhibited due to the non-resonant arrangement of the proposed metasurface lens. The numerical and experimental results agreed well with each other, effectively validating the feasibility of the proposed metasurface lens. The proposed approach provides a feasible avenue for the design of simple and ultra-compact ultrasonic lenses that would be suitable in various fields of ultrasonics.