局部共振声子晶体的超分辨率成像和聚焦研究

The acoustic lenses have always lagged behind their optical counterparts for lacking the suitable materials in nature. However, this has begun to change ever since the appearance of acoustic lenses based on the phononic crystals (PCs). In this context, this subject will investigate the acoustic lenses based on the uniaxial resonant PCs, to achieve the imaging and focusing of elastic waves beyond the diffraction limit. To this end, a microscopic model is going to be proposed to interpret and predict the roles of resonators in controlling the lowest-order antisymmetric (A0) Lamb wave or Rayleigh wave, accompanying the numerical and experimental investigation of exotic features of the resonant PCs. Acoustic lenses based on the resonant PCs are designed accordingly with either the identical negative refraction index or the gradient refraction index to get the super-resolution imaging and focusing, respectively. Their background mechanism is further revealed together with the numerical and experimental observation of the superlensing phenomena. Further, the subject will design and use the same acoustic lens to get the superlensing imaging and focusing. A joint experimental platform, consisting both the air-coupled and laser ultrasonic technology, will be used to perform the experiments with both high speed and precision. This multi-disciplinary subject will bring new understandings in superlensing imaging and focusing, and the acoustic lenses can be used in many fields such as energy harvesting, non-destructive evaluations, etc.

自然界中材料的限制使声学透镜的研究一直于落后于光学透镜。近来，声子晶体声学透镜的出现使这一情况发生了改变。基于此，本项目将研究一种新型的单轴局部共振声子晶体透镜，从而实现弹性波的超分辨率成像和聚焦。项目将首先建立一个新的理论模型，从微观上解释并预测局部振子对声子晶体中A0波和瑞利波的调控，并从数值和实验上探索单轴局部共振声子晶体的新异特征。接下来，项目将分别设计负折射率和渐变折射率的声子晶体透镜，并探索其物理机制，分别实现超分辨率成像和聚焦，并从数值和实验上验证。之后，项目将设计并使用同一声透镜，实现超分辨率的成像和聚焦。项目将搭建空气耦合和激光超声的联合实验平台，并根据实验精度的要求对声透镜进行快速定量研究。这一多学科项目将增进对超分辨率成像和聚焦的认识，其成果可直接用于能量收集、无损检测等领域。

项目采用空气耦合与激光超声联合实验技术研究了A0波基于渐变折射率声透镜的聚焦现象，主要内容有：（1）搭建了空气耦合与激光超声联合实验技术平台。采用该技术分别研究了A0波和Rayleigh波在基于布拉格散射和局部共振机理的禁带内的传播行为，验证了该方法对30kHz - 1000kHz固体表面波传播测量的有效性。（2）研究了圆柱谐振子的共振模态，如圆柱零阶弯曲模态、零阶纵向模态、四极回音壁模态等对A0波的调控机理，发现弯曲和纵向模态可使A0波频散曲线分开，并使A0波的等效折射率增大，并基于此设计和制备了渐变折射率声透镜。回音壁模态会在禁带内产生新通带，并基于此实验观测了基于回音壁模态的波导、非对称传输、彩虹波导等现象。（3）项目实验观测了A0波在基于局部共振渐变折射率声透镜外部和内部的聚焦现象，测量了同一个透镜的多频率段聚焦现象，讨论了影响分辨率的关键物理量，发现基于共振结构的声透镜可实现超分辨率聚焦。