Acoustic metasurfaces for efficient matching of non-contact ultrasonic transducers

Air-coupled ultrasonic techniques based on rapid and contactless inspection are useful for various applications, such as ranging finding, surface profile measurement, and non-destructive evaluation. However, impedance matching issues between air and piezoelectric materials lead to technical limitations, such as imaging resolution and penetration depth. In order to mitigate these problems, we utilize acoustic metasurfaces consisting of periodic concentric ring arrays with narrow slits and tiny air gap spacers. Narrow slits induce Fabry–Pérot resonance with respect to slit thickness, and tiny gap spacers generate half-wave resonance with respect to the period. Here, we utilize the coupled resonance combined with slit resonance and gap resonance. These coupled resonances lead to efficient impedance matching of airborne sound from ultrasonic transducers. Thus, these metasurfaces lead to a broadband sound pressure level gain in the frequency range of 0–80 kHz. Enhanced pulse signals using the pulse-echo and through-transmission ultrasonic techniques are achieved.