Negative refractive index and acoustic superlens from multiple scattering in single negative metamaterials
Negative refractive index and acoustic superlens from multiple scattering in single negative metamaterials
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DOI：
10.1038/nature14678
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发表时间：
2015-09-03
期刊：
影响因子：
64.8
通讯作者：
Lerosey, Geoffroy
中科院分区：
文献类型：
--
作者：
Kaina, Nadege;Lemoult, Fabrice;Lerosey, Geoffroy
超材料是一种人造复合介质，其结构尺度远小于波长，为调控波的传播提供了令人惊喜的可能性（1 - 6）。其中最有趣的一点是能够实现超透镜效应——即聚焦或成像突破衍射极限（7）。这源于左手特性——即对波产生负折射的特性，这是负折射率超材料的典型特征（8 - 10）。然而，要实现这一目标需要设计“双负”超材料，它在电磁学中同时作用于介电常数和磁导率，在声学中则同时作用于密度和压缩性；这通常意味着要使用两种不同的构建单元（13，14）或呈现多重重叠共振的特定粒子（15 - 17）。这样的要求限制了双负超材料的适用性，例如，阻碍了利用左手声学超材料进行亚波长聚焦的任何演示（18）。在此我们表明，通过依靠仅由一种单共振单元构成的介质，这些严格条件可在很大程度上放宽。具体而言，我们通过一个简单但通用的半解析模型表明，巧妙地打破单负超材料的对称性足以将其转变为双负超材料。然后我们证明，这仅仅是因为波在超材料共振元件上的多次散射，由于超材料的亚波长图案化，这种现象在这些介质中常常被忽视。我们将我们的方法应用于声学，并通过数值模拟验证，它仅基于亥姆霍兹共振器就可实现负折射率声学超材料。最后，我们演示了负折射率声学超透镜的操作，实现了亚波长聚焦和成像，其光斑宽度和分辨率分别比衍射极限提高了7倍和3.5倍。我们的研究结果对超材料的物理性质具有深远影响，凸显了其亚波长晶体结构的作用，从而进入了超材料晶体的领域。这以一种比以往可能的方式更为简单的方式拓宽了设计具有新颖特性的复合介质的可能性范围。
Metamaterials, man-made composite media structured on a scale much smaller than a wavelength, offer surprising possibilities for engineering the propagation of waves(1-6). One of the most interesting of these is the ability to achieve superlensing-that is, to focus or image beyond the diffraction limit(7). This originates from the left-handed behaviour-the property of refracting waves negatively-that is typical of negative index metamaterials(8-10). Yet reaching this goal requires the design of 'double negative' metamaterials, which act simultaneously on the permittivity and permeability in electromagnetics(11,12), or on the density and compressibility in acoustics; this generally implies the use of two different kinds of building blocks(13,14) or specific particles presenting multiple overlapping resonances(15-17). Such a requirement limits the applicability of double negative metamaterials, and has, for example, hampered any demonstration of subwavelength focusing using left-handed acoustic metamaterials(18). Here we show that these strict conditions can be largely relaxed by relying on media that consist of only one type of single resonant unit cell. Specifically, we show with a simple yet general semi-analytical model that judiciously breaking the symmetry of a single negative metamaterial is sufficient to turn it into a double negative one. We then demonstrate that this occurs solely because of multiple scattering of waves off the metamaterial resonant elements, a phenomenon often disregarded in these media owing to their subwavelength patterning. We apply our approach to acoustics and verify through numerical simulations that it allows the realization of negative index acoustic metamaterials based on Helmholtz resonators only. Finally, we demonstrate the operation of a negative index acoustic superlens, achieving subwavelength focusing and imaging with spot width and resolution 7 and 3.5 times better than the diffraction limit, respectively. Our findings have profound implications for the physics of metamaterials, highlighting the role of their subwavelength crystalline structure, and hence entering the realm of metamaterial crystals. This widens the scope of possibilities for designing composite media with novel properties in a much simpler way than has been possible so far.