Silver nanowire hyperlenses

由于电磁波的双曲色散关系，像银纳米线阵列这样的双曲超材料允许将逝去的波转变为传播的波。这对于光的亚波长成像和聚焦特别重要。虽然已经从理论上研究了几种超透镜的几何结构，但到目前为止，只有一种基于弯曲金属/介质多层膜的早期设计在实验上实现。特别是，金属纳米线阵列用于超透镜还没有实验证明，尽管理论表明这些纳米线透镜具有许多优点，例如，大工作带宽、低损耗、易于制造和平面表面。因此，我们打算使用两种不同设计的基于银纳米线阵列的超透镜，从理论上研究并实验证明这种纳米线超透镜的亚波长聚焦和成像能力。通过在纳米多孔氧化铝模板中电化学沉积银，并构建顶表面，银纳米线超透镜将呈现平坦的底面，用于成像和聚焦。这种透镜可以聚焦入射的平面波，并通过反转光路，从近场亚波长大小的物体形成远场图像。预计吸收率将低于多层超透镜，后者主要工作在接近表面等离子体共振的地方。由于纳米线阵列的宽带双曲线色散关系，可以实现不同波长选择的透镜，其不受由特定材料组合给出的特定限制频率的限制。此外，纳米线超透镜的制造有望比沉积超低粗糙度金属/电介质多层膜更坚固，要求更低。利用白光近场光学显微镜和多光子光刻技术，研究亚波长聚焦的形成。通过调整银纳米线的周期和直径，研究空间色散对聚焦尺寸和成像的影响。此外，还将评估作为另一个限制参数的吸收的影响，特别是对于大波矢。除了纳米聚焦和亚波长成像的直接实验结果外，这项工作对于新引发的对增强热辐射的兴趣也将是重要的，因为双曲超材料中提供了高的光子态密度。这可能会进一步开辟将银纳米线超材料应用于增强辐射冷却的途径，这些材料具有特殊形状的顶面。

Due to the hyperbolic dispersion relation for electromagnetic waves, hyperbolic metamaterials like silver nanowire arrays allow the transformation of evanescent waves into propagating waves. This is of special importance for subwavelength imaging and focussing of light. Although several hyperlens geometries have been theoretically studied, only one early design based on bent metal/dielectric multilayers has experimentally been realised up to now. Particularly, the use of metal nanowire arrays for hyperlenses was not experimentally demonstrated yet although theory suggests a multitude of advantages for these nanowire lenses, e.g. large operating bandwidth, low loss, easy fabrication, and plane surfaces. We therefore intend to theoretically investigate and experimentally demonstrate the subwavelength focusing and imaging ability of such nanowire hyperlenses using two different designs of hyperlenses based on silver nanowire arrays. By electrochemical deposition of silver into nanoporous alumina templates and structuring of the top surface, the silver nanowire hyperlenses will exhibit a plane bottom surface for imaging and focus creation. The lenses allow the focusing of incident plane waves and, by reversing the light path, the formation of far-field images from near-field subwavelength-sized objects. Absorption is expected to be lower than for multilayer hyperlenses, which mainly operate close to the surface plasmon resonance. Due to the broadband hyperbolic dispersion relation of the nanowire arrays, lenses for different wavelengths of choice can be realised, which are not bound to a specific limiting frequency, given by a particular material combination. Moreover, the fabrication of the nanowire hyperlenses promises to be robust and less demanding than the deposition of ultra-low-roughness metal/dielectric multilayers. Applying white light SNOM and multi-photon lithography, the subwavelength focus formation will be studied. The influence of spatial dispersion on focus size and imaging will be investigated by adjusting period and diameter of the silver nanowires. Furthermore, the impact of absorption, especially for large wave vectors, as another limiting parameter will be evaluated. Besides the direct experimental results on nanofocusing and subwavelength imaging the work will be important for the newly sparked interest in enhanced thermal radiation due to the offered large photonic density of states in the hyperbolic metamaterial. This might further open ways to apply silver nanowire metamaterials with specifically shaped top surfaces for enhanced radiative cooling.