Subwavelength imaging with a zero-mass sonic meta-atom

Acoustic metamaterials offer powerful solutions for manipulating sound at subwavelength scales. One important application is super-resolved acoustic imaging, which relies on access to evanescent waves beyond the diffraction limit. Near-field techniques using subwavelength probes can capture these waves, revealing fine object details. Here, we introduce an experimental platform that harnesses airborne extraordinary transmission to couple evanescent acoustic waves into a subwavelength, zero-mass sonic meta-atom probe. By mounting a circular membrane at the tip of an air-filled waveguide with a conical tip, we exploit a modification of the acoustic inertance—caused by an object’s proximity—via the sonic Drexhage effect, leading to a downshift of the resonant frequency in the kilohertz range. Experimental results, supported by numerical and theoretical models, demonstrate that extreme subwavelength imaging is enabled by measuring the waveguide’s acoustic reflection coefficient, with lateral and depth resolutions of approximately λ / 20 and λ / 650 , respectively (where λ is the acoustic wavelength). The platform’s capabilities for texture measurement and noncontact scanning are also demonstrated.