Department of Electrical and Computer Engineering, The University of Texas at Austin, TX 78712, USA.
Nat Commun. 2012 May 29;3:870. doi: 10.1038/ncomms1877.
Optical metamaterials are usually based on planarized, complex-shaped, resonant nano-inclusions. Three-dimensional geometries may provide a wider set of functionalities, including broadband chirality to manipulate circular polarization at the nanoscale, but their fabrication becomes challenging as their dimensions get smaller. Here we introduce a new paradigm for the realization of optical metamaterials, showing that three-dimensional effects may be obtained without complicated inclusions, but instead by tailoring the relative orientation within the lattice. We apply this concept to realize planarized, broadband bianisotropic metamaterials as stacked nanorod arrays with a tailored rotational twist. Because of the coupling among closely spaced twisted plasmonic metasurfaces, metamaterials realized with conventional lithography may effectively operate as three-dimensional helical structures with broadband bianisotropic optical response. The proposed concept is also shown to relax alignment requirements common in three-dimensional metamaterial designs. The realized sample constitutes an ultrathin, broadband circular polarizer that may be directly integrated within nanophotonic systems.
光学超材料通常基于平面化的、复杂形状的、共振纳米包含物。三维几何形状可以提供更广泛的功能，包括宽带手性，以在纳米尺度上操纵圆偏振，但随着尺寸的减小，它们的制造变得具有挑战性。在这里，我们引入了一种实现光学超材料的新范例，表明可以在不使用复杂包含物的情况下获得三维效应，而是通过调整晶格内的相对取向来实现。我们应用这一概念，通过堆叠的纳米棒阵列实现了平面化、宽带双各向异性超材料，其具有定制的旋转扭曲。由于紧密间隔的扭曲等离子体超表面之间的耦合，使用传统光刻技术实现的超材料可以有效地作为具有宽带双各向异性光学响应的三维螺旋结构来操作。所提出的概念还表明，它放宽了在三维超材料设计中常见的对准要求。所实现的样品构成了一种超薄、宽带圆偏振器，可以直接集成在纳米光子系统中。
