Luneburg lens design enhances broadband and omnidirectional acoustic collimation and focusing

A spherically symmetric gradient-index lens with a refractive index that varies as a function of radial distance from its center, the Luneburg lens has long been used for electromagnetic wave transportation and communication systems applications. With the relatively recent development of graded index acoustic metamaterials, the lens is increasingly being used for manipulating elastic and acoustic waves.

“There has been a lot of work on acoustic Luneburg lens for broadband audible frequency range of up to 15 kilohertz or a single ultrasonic frequency of 40 kHz,” said author Liuxian Zhao. “However, until now, no work has been conducted for broadband acoustic Luneburg lens at an ultrasonic frequency range”

Zhao et al. build on previous modified optical Luneburg lens designs to demonstrate an acoustic version with a tailorable focal length that functions in the ultrasonic frequency range of 20 to 40 kHz. Because significant ray aberrations compromise acoustic collimation and focusing efficacy, the researchers introduce a governing equation to minimize the aberrations.

The research analytically, numerically, and experimentally investigates collimation and focusing performance of the ultrasonic waves and demonstrates the reduced-aberration acoustic Luneburg lens’ (RAALL’s) broadband and omnidirectional characteristics.

“Based on trajectories obtained with the ray-tracing technique, the RAALL can achieve better acoustic focusing in comparison to other modified Luneburg lens designs,” said Zhao.

The RAALL has promising implications in ultrasonic imaging, diagnosis, treatment, sonar systems, and energy harvesting. In the future, the scientists plan to investigate echolocation based on RAALL sonar systems to improve the performance of signal-to-noise ratio and resolution.

Source: “Broadband acoustic collimation and focusing using reduced aberration acoustic luneburg lens,” by Liuxian Zhao, Timothy Horiuchi, and Miao Yu, Journal of Applied Physics (2021). This article can be accessed at https://doi.org/10.1063/5.0064571 .