Metamaterials manipulate waves using a carefully crafted pattern of cavities, and waves traversing a metamaterial can be bent, focused and even amplified depending on the geometry of these internal structures. Up until now, work on metamaterials has primarily considered periodic arrangements of holes to allow for these wave-shaping properties. But such approaches suffer from frequency-dependent dips in transmission known as Wood’s anomalies.

Recent work by Amireddy and collaborators reported on a metamaterial lens capable of subwavelength imaging of ultrasonic waves — a common tool for medical diagnostics and even treatments. Now, the same authors have described a new multilayered metamaterial lens fashioned from an aperiodic pattern of holes capable of resolving structures less than 3 percent the size of an ultrasonic wave. The result, which improves the resolution of their prior method by 30 percent, also limits the impact of Wood’s anomalies.

To characterize their lens, the authors excited ultrasonic waves in two aluminum blocks, each featuring different subwavelength structures. By measuring the amplitude of waves transmitted from the block through the metamaterial lens, they were able to clearly image the small structures in their samples — features that didn’t appear when the metamaterial lens was absent.

They also compared the operation of their lens to a simulated six-layer metamaterial, finding a qualitative agreement between the transmission properties of the two. A closer analysis of the transmission through the simulated lens revealed different frequency dips in each layer, suggesting that the total transmission washes out the effect of Wood’s anomalies in individual layers.

Source: “Porous metamaterials for deep sub-wavelength ultrasonic imaging,” by Kiran Kumar Amireddy, Krishnan Balasubramaniam, and Prabhu Rajagopal, Applied Physics Letters (2018). The article can be accessed at https://doi.org/10.1063/1.5045087.