多功能变厚度宽带全向聚焦准直结构
Multi-Functional Variable Thickness Structure for Broadband and Omnidirectional Focusing and Collimation
摘要：
Luneburg透镜是一种对称的梯度折射率透镜，其折射率从外表面到中心以径向方式增加。它具有聚焦和准直波的能力，这使得它可用于能量收集，波导和传感器中的组件。理想的Luneburg透镜应该易于制造，具有宽带和全向特性，以及可以容易地调节的焦距。然而，现有的基于声子晶体的结构Luneburg透镜很难达到这些要求。在这里，我们提出了一种替代结构的Luneburg透镜，其折射率随着其径向距离的变化而平滑地变化，作为厚度变化的结果。通过对弯曲波在变厚度透镜中传播的理论计算、数值模拟和实验测量表明，对于不同的频率和传播方向，在变厚度透镜的内部、边缘和外部都可以获得弯曲波聚焦。弯曲波准直也被证明时，点源被放置在各自的焦点为每个镜头。此外，示出了通过Luneburg透镜聚焦到压电能量采集器上的弯曲波与没有聚焦的情况下获得的弯曲波相比可以导致所采集的电压的显著增加。
英文摘要：
Luneburg lens is a symmetric gradient-index lens with a refractive index that increases from the outer surface to the center in a radial manner. It has the ability to focus and collimate waves, which makes it useful for energy harvesting, waveguiding and as a component in transducers. An ideal Luneburg lens should be easy to fabricate, has broadband and omnidirectional characteristics, as well as a focal length that can be easily tuned. However, existing structural Luneburg lenses based on phononic crystals can hardly achieve these requirements. Here, we propose an alternative structural Luneburg lens which has a refractive index that varies smoothly with its radial distance as a result of a changing thickness. Theoretical calculations, numerical simulations and experimental measurements of flexural wave propagation through the lens showed that flexural wave focusing can be obtained inside, at the edge and outside of the variable thickness lens for different frequencies and propagation directions. Flexural wave collimation was also demonstrated when a point source was placed at the respective focal points for each lens. Furthermore, it was shown that flexural waves that were focused onto a piezoelectric energy harvester by the Luneburg lens can lead to a significant increase in the harvested voltage compared to that obtained without focusing.
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