Center for Nanoscale Science and Technology, National Institute of Standard and Technology, Gaithersburg, Maryland 20899, USA.
Nature. 2013 May 23;497(7450):470-4. doi: 10.1038/nature12158.
Decades ago, Veselago predicted that a material with simultaneously negative electric and magnetic polarization responses would yield a 'left-handed' medium in which light propagates with opposite phase and energy velocities--a condition described by a negative refractive index. He proposed that a flat slab of left-handed material possessing an isotropic refractive index of -1 could act like an imaging lens in free space. Left-handed materials do not occur naturally, and it has only recently become possible to achieve a left-handed response using metamaterials, that is, electromagnetic structures engineered on subwavelength scales to elicit tailored polarization responses. So far, left-handed responses have typically been implemented using resonant metamaterials composed of periodic arrays of unit cells containing inductive-capacitive resonators and conductive wires. Negative refractive indices that are isotropic in two or three dimensions at microwave frequencies have been achieved in resonant metamaterials with centimetre-scale features. Scaling the left-handed response to higher frequencies, such as infrared or visible, has been done by shrinking critical dimensions to submicrometre scales by means of top-down nanofabrication. This miniaturization has, however, so far been achieved at the cost of reduced unit-cell symmetry, yielding a refractive index that is negative along only one axis. Moreover, lithographic scaling limits have so far precluded the fabrication of resonant metamaterials with left-handed responses at frequencies beyond the visible. Here we report the experimental implementation of a bulk metamaterial with a left-handed response to ultraviolet light. The structure, based on stacked plasmonic waveguides, yields an omnidirectional left-handed response for transverse magnetic polarization characterized by a negative refractive index. By engineering the structure to have a refractive index close to -1 over a broad angular range, we achieve Veselago flat lensing, in free space, of arbitrarily shaped, two-dimensional objects beyond the near field. We further demonstrate active, all-optical modulation of the image transferred by the flat lens.
几十年前，Veselago 预测具有负电和磁极化响应的材料将产生“左手”介质，其中光以相反的相位和能量速度传播-这种情况由负折射率描述。他提出，具有各向同性折射率为-1 的扁平左手材料平板可以像自由空间中的成像透镜一样作用。左手材料不会自然出现，直到最近才有可能使用超材料实现左手响应，即通过亚波长尺度上设计的电磁结构来引起定制的极化响应。到目前为止，左手响应通常是使用由包含电感-电容谐振器和导电丝的单元的周期性阵列组成的谐振超材料来实现的。在具有厘米级特征的谐振超材料中，在微波频率下在两个或三个维度上各向同性的负折射率已经实现。通过自上而下的纳米制造将关键尺寸缩小到亚微米尺度，可以将左手响应扩展到更高的频率，例如红外线或可见光。然而，这种小型化是以单元对称性降低为代价的，从而产生仅沿一个轴为负的折射率。此外，光刻缩放限制迄今为止排除了在可见光以外的频率下制造具有左手响应的谐振超材料。在这里，我们报告了实验实现的用于紫外光的具有左手响应的块状超材料。该结构基于堆叠的等离子体波导，对于横向磁场极化具有各向同性的左手响应，其特征为负折射率。通过将结构设计为在宽角度范围内具有接近-1 的折射率，我们在自由空间中实现了 Veselago 平板透镜，用于对任意形状的二维物体进行近场之外的平场成像。我们进一步演示了通过平板透镜传输的图像的主动全光学调制。
