Achromatic gradient-index phononic crystal lenses with unique acoustic applications

Gradient index (GRIN) phononic crystal (PC) structures can be used to control waves in both optical and acoustic applications. Unfortunately, operation of acoustic GRIN PC lenses is often limited by chromatic performance. To solve this issue, Hyun et al. designed achromatic GRIN PC lenses that can operate with the same focal length over a range of frequencies due to the addition of thin coating layers.

The authors’ achromatic acoustic GRIN PC lens has ability to focus acoustic wave energy toward a targeted location for imaging applications and even energy harvesting. It only requires the use of one lens, making it ideal in terms of size and volume. Additionally, it can focus sound energy without any chromatic aberration.

“The primary advantage of our proposed design is that we only need to add simple thin coating layers to the existing GRIN PC design to realize an achromatic GRIN PC lens system,” said author Miso Kim. “This means that we don’t need any bulky use of multiple lenses.”

To achieve this design, the authors used a method known as topology optimization to design thin 3D printed layers of coating. During the optimization procedure, the coating components and gap size between grooved cells on the coating surface are tuned to improve broadband function. This method allowed them to distribute the coating inside designated areas of the lens depending on the target performance desired.

“The methodology we used in this work can be extended to any acoustic lens designs other than the GRIN PC, thus providing great design flexibility and versatile applicability,” said Kim.

Source: “Achromatic acoustic gradient-index phononic crystal lens for broadband focusing,” by Jaeyub Hyun, Wan-Ho Cho, Choon-Su Park, Ji-Ho Chang, and Miso Kim, Applied Physics Letters (2020). The article can be accessed at https://doi.org/10.1063/5.0009799 .