Subwavelength ultrasound focusing using negative index refraction metamaterials

SUMMARY/ABSTRACT Ultrasound is one of the most versatile tools in medicine for non-invasive imaging and therapies. As with any wave phenomenon, one of the limitations in ultrasound resolution is the diffraction limit. In this project, we propose the use of negative-index non-resonant acoustic metamaterials (NRAM) for ultrasound focusing exploiting their negative-index refraction and sub-wavelength focusing. Negative acoustic metamaterials have shown exciting properties such as amplification of evanescent waves,1–3 negative phase velocities (phase propagates in opposite direction to group propagation),4–7 impedance matching to enhance ultrasound transmission,7 and negative refraction angles.8,9 Using these properties, NRAM have demonstrated experimentally novel capabilities such as ultrasound sub-wavelength imaging, and imaging and focusing with rectangular slabs that produce mirroring images of ultrasound point sources.1–3 Many of these properties could bring major benefits for biomedical ultrasound.6,7,10,11 However, such capabilities have been shown only at low frequencies in the 1-60 kHz range,3,7,9 impeding their study and application at biomedical ultrasound frequencies. The low-frequency operation of metamaterials has been due mainly to technology limitations. However, recently our research group demonstrated a novel metamaterial technology achieving a 1-D negative-index behavior at 300 kHz in water incorporating <100 micrometer membranes and Helmholtz resonators,12 placing us in an ideal position to study the potential impact of NRAM for biomedical ultrasound. For this exploratory R21 project, we aim to build 2-D and 3-D negative electroactive metamaterials operating at 2 MHz to demonstrate negative refractive index and dynamic sub-wavelength focusing. Specifically, we aim to 1) demonstrate negative index and negative-angle refraction to achieve ultrasound focusing at 2 MHz frequency using negative metamaterial slabs, 2) demonstrate sub-wavelength focusing targeting 0.250.1 mm lateral focusing at 2 MHz, and 3) demonstrate electroactive dynamic focusing and acoustic index tuning at 2 MHz using piezoelectric-based NRAM. This exploratory project aims to demonstrate the technological capability of negative-index acoustic metamaterials to facilitate ultrasound focusing and improve resolution. The success of this exploratory project will provide experimental evidence to expand this effort and aim for higher frequency operation towards 5 MHz and to develop electro-active tunable metamaterials that can lead to active and adjustable focusing for ultrasound applications such as transcranial ultrasound imaging, therapies and stimulation.