Silver nanowire hyperlenses

由于电磁波的双曲色散关系，双曲超材料如银纳米线阵列允许将倏逝波转换为传播波。这对于光的亚波长成像和聚焦特别重要。虽然已经在理论上研究了几种超透镜的几何形状，但到目前为止，只有一种基于弯曲金属/电介质多层膜的早期设计已经在实验上实现。特别地，将金属纳米线阵列用于超透镜尚未在实验上得到证实，尽管理论表明这些纳米线透镜具有许多优点，例如大的工作带宽、低损耗、易于制造和平面表面。因此，我们打算从理论上研究和实验证明这种纳米线超透镜的亚波长聚焦和成像能力，使用两种不同设计的超透镜的基础上银纳米线阵列。通过将银电化学沉积到纳米多孔氧化铝模板中并结构化顶部表面，银纳米线超透镜将呈现用于成像和焦点产生的平坦底部表面。透镜允许入射平面波的聚焦，并通过反转光路，从近场亚波长大小的物体形成远场图像。预期吸收低于多层超透镜，多层超透镜主要在接近表面等离子体共振处操作。由于纳米线阵列的宽带双曲色散关系，可以实现用于不同波长选择的透镜，其不受特定材料组合给出的特定限制频率的约束。此外，纳米线超透镜的制造有望比超低粗糙度金属/电介质多层膜的沉积更稳健且要求更低。应用白色光SNOM和多光子光刻技术，研究了亚波长聚焦的形成。通过调节银纳米线的周期和直径，研究了空间色散对焦点尺寸和成像的影响。此外，吸收的影响，特别是对大波矢量，作为另一个限制参数将进行评估。除了纳米聚焦和亚波长成像的直接实验结果外，由于双曲超材料中提供的大光子态密度，这项工作对于新引发的增强热辐射的兴趣也很重要。这可能会进一步打开应用具有特定形状顶面的银纳米线超材料以增强辐射冷却的方法。

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.