Ultrasonic imaging is widely used in various fields, including nondestructive evaluation, medical diagnostics, and underwater inspection, due to its versatility and safety. However, conventional ultrasonic imaging methods suffer from poor resolution, which limits their ability to capture fine features within the near field. Metamaterials, including acoustic metamaterials, have gained significant research interest in recent years, offering benefits such as super-resolution imaging, vibration damping, and cloaking. Acoustic metamaterials can transfer the information carried by evanescent waves to the far field by amplifying or converting them into propagating waves, thereby overcoming the resolution limit. This work explores the use of resonant and non-resonant metamaterial concepts to improve the resolution of ultrasonic NDE imaging systems, as well as a novel waveguide-based reception technique to capture the evanescent field.
One approach involved designing and developing a Structured Channel Metalens using the Fabry Perot resonances principle to transfer evanescent fields to the far field. However, extending such metamaterials to an ultrasonic domain is challenging due to the shorter wavelength, which requires spatially narrow-band receiving techniques to capture wavefields past fine features of the metamaterial. Numerical simulations and experiments were performed on structure channel metamaterial and a thin stainless steel waveguide attached to a commercial transducer, demonstrating practical super-resolution ultrasonic imaging down to one by 2.5 times the operating wavelength.
Another approach involved demonstrating a novel non-resonant acoustic radially symmetric cylindrical hyperlens concept for ultrasonic NDE imaging, enabling subwavelength resolution and spatial magnification by transforming evanescent waves into propagating waves. Finally, non-resonant planar hyperlens concepts were demonstrated in the context of ultrasonic NDE super-resolution imaging with planar inner and outer surfaces. These advancements hold great promise for high-resolution ultrasonic imaging in industrial and biomedical applications, providing a low-cost, portable alternative.