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We discovered a new transmodal Fabry-Pérot resonance where one elastic-wave mode is maximally transmitted to another. It occurs when the phase difference of two dissimilar modes through an anisotropic layer becomes odd multiples of π under the reflection-free and weak mode-coupling assumptions. Unlike the well-established Fabry-Pérot resonance, the transmodal resonance must involve two coupled elastic waves between longitudinal and shear modes. The investigation into the origin of wiggly transmodal transmission spectra suggests that efficient broadband mode conversion can be achieved if the media satisfy the structural stability condition to some degree. The new resonance mechanism, also experimentally characterized, opens up new possibilities for manipulating elastic wave modes as an effective alternative to generating shear-mode ultrasound.

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Sub-wavelength imaging is possible if metamaterial lenses realizing hyperbolic or elliptic Equi-Frequency Contours (EFCs) are used. Theoretically, lenses exhibiting hyperbolic EFCs allow imaging with unlimited resolution, but only metamaterials of elliptic EFCs producing limited resolution have been so far realized in elastic field. Thus, an elastic metamaterial lens realizing truly hyperbolic EFCs can lead to superior-resolution ultrasonic imaging. This Letter presents the realization of an elastic lens exhibiting truly hyperbolic EFCs and its experimental verification.

Subwavelength imaging by metamaterials and extended work to pursue total transmission has been successfully demonstrated with electromagnetic and acoustic waves very recently. However, no elastic counterpart has been reported because earlier attempts suffer from considerable loss. Here, for the first time, we realize an elastic hyperbolic metamaterial lens and experimentally show total transmission subwavelength imaging with measured wave field inside the metamaterial lens. The main idea is to compensate for the decreased impedance in the perforated elastic metamaterial by utilizing extreme stiffness, which has not been independently actualized in a continuum elastic medium so far. The fabricated elastic lens is capable of directly transferring subwavelength information from the input to the output boundary. In the experiment, this intriguing phenomenon is confirmed by scanning the elastic structures inside the lens with laser scanning vibrometer. The proposed elastic metamaterial lens will bring forth significant guidelines for ultrasonic imaging techniques.