THE THEORETICAL ANALYSIS AND EXPERIMENTAL INVESTIGATION OF SUB-WAVELENGTH FOCUSING VIA CONCAVE LENS

In order to improve working performance of traditional plane lens, i.e., increase energy at focal point, reduce focusing size, and achieve subwavelength focusing, a concave lens for focusing incident flexural wave is designed based on the plate thickness variation. Firstly, the transmission coefficient and phase variation are obtained by utilizing the Timoshenko beam theory, based on which the lens design is performed. After that, the working performance of concave lens designed is simulated via the frequency analysis in structural mechanics module of COMSOL multiphysics software, including the focal position, energy distribution, focusing size, and so forth, which are further compared with the traditional plane lens. Finally, the experimental measurements are carried out to further validate and conform the design scheme. It is demonstrated that the concave lens can focus the incident flexural wave at the specific position, with its performance better than the plane lens, because energy concentrated is higher and the focusing size is smaller. Specifically, it exhibits subwavelength focusing phenomenon with the focusing size smaller than half wavelength. Additionally, the concave lens is broadband, which can work at a frequency region centered at designed frequency even if the whole structure is maintained. The present design scheme is easy realized for engineering application, and the concave lens exhibits better performance, which can also provide guidance for the lens design in acoustics and optics.