Thermochromic Infrared Metamaterials
Thermochromic Infrared Metamaterials
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DOI：
10.1002/adma.201504525
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发表时间：
2016-02-03
影响因子：
29.4
通讯作者：
Padilla, Willie J.
中科院分区：
文献类型：
--
作者：
Liu, Xinyu;Padilla, Willie J.
DOI：10.1002/adma. 201504525仍然受到Stefan-Boltzmann定律的限制。电磁超材料是工程复合材料，可以潜在地克服SB定律的T4依赖性，因为光学性质是从组成成分的几何形状获得的，而不是直接从它们的化学成分获得的。因此，超材料表面的温度相关光谱发射率可以经由结构的几何形状来定制和控制。虽然可调谐超材料在实现电磁波的实时控制方面取得了重大进展，[7，8]红外，[9-11] THz [12-15]和更低的频率，[16，17]还没有出现一个可行的解决方案来直接控制表面辐射的温度依赖性。在这里，我们提出并演示了一种人工热致变色材料，由超材料发射器[18]与双材料MEMS相结合，该超材料由金属接地层、介电层和顶部图案化金属层组成，（见图1），并已被证明产生波长相关的光谱发射率，峰值接近理想的黑体最大值。[18]高吸收和发射值源于磁共振和电共振的汇合，每个共振都类似于洛伦兹振荡器。因此，如果谐振中的任一个在振幅、中心频率或散射频率上改变，则这两个谐振之间的耦合可能偏离最佳，从而实现从1减小的值。我们设计了MEMS超材料（见实验部分），以在室温下获得相对较低的发射率。在这种配置中，顶部超材料图案悬挂在接地平面上方（图1a），并被设计为与吸收谐振具有非最佳耦合。由金和氮化硅制成的铋材料臂支撑顶部超材料层，并且当施加热量时，由于热膨胀系数之间的差异而膨胀并向下弯曲。结果，悬置超材料层与接地平面之间的距离减小。当顶部超材料层与接地层接触时（图1 B），实现了高红外发射状态（更多详细信息请参见支持信息的第2节）。因此，我们的人造热致变色材料遵循超级Stefan-Boltzmann曲线，并实现了单调增加的温度依赖性光谱发射率。相比之下，大多数材料仅实现适度的波长相关发射率，并且更重要的是，很少或没有温度相关发射率-仅如SB定律所描述的那样随温度的四次方变化。实验波长相关的光谱发射率显示在图2a中，对于我们的超材料的许多不同的几何配置，由温度决定。超材料层悬浮在室温（非优化）状态下，并实现峰值光谱
DOI: 10.1002/adma. 201504525 still limited by the Stefan–Boltzmann law. Electromagnetic metamaterials are engineered composites that can potentially overcome the T 4 dependence of the SB law, since the optical properties are obtained from the geometry of the constituent components, rather than directly from their chemical composition. Therefore, the temperature dependent spectral emissivity of metamaterial surfaces may be tailored and controlled via the geometry of the structures. Although tunable metamaterials have made significant progress in achieving real-time control of electromagnetic waves, in optical,[7, 8] infrared,[9–11] THz,[12–15] and lower frequencies,[16, 17] there has yet to emerge a viable solution to directly control the temperature dependence of surface radiation. Here we propose and demonstrate an artificial thermochromic material, fashioned from a metamaterial emitter [18] combined with bimaterial MEMS,(Figure 1), which is capable of achieving certain desired thermally dependent surface emission.The metamaterial consists of a metallic ground plane, a di electric layer, and a top patterned metallic layer,(see Figure 1), and has been shown to yield wavelength dependent spectral emissivity, with peak values approaching the ideal blackbody maximum.[18] The high absorption and emission values arise from the confluence of magnetic and electric resonances, each similar to a Lorentz oscillator. Thus, if either of the resonances is changed in amplitude, center frequency, or scattering frequency, the coupling between these two resonances may deviate from optimal, thus realizing values decreasing from unity. We have designed our MEMS metamaterial (see the Experimental Section) to attain relatively low emissivities at room temperature. In this configuration, the top metamaterial pattern is suspended above the ground plane (Figure 1 a) and designed to have non-optimal coupling to the absorptive resonance. Bi material arms, fashioned from gold and silicon nitride, support the top metamaterial layer and–when heat is applied–expand and bend down due to the difference between the thermal expansion coefficients. As a result, the distance between the suspended metamaterial layer and the ground plane is reduced. When the top metamaterial layer comes in contact with the ground plane (Figure 1 b) a state of high infrared emission is achieved (see Section 2 of the Supporting Information for more detail). Our artificial thermochromic material thus follows a super Stefan–Boltzmann curve and realizes a monotonically increasing temperature dependent spectral emissivity. In contrast, most materials realize only modest wavelength dependent emissivities and, more importantly, little or no temperature dependent emissivity–only varying as the fourth power of temperature as described by the SB law. The experimental wavelength dependent spectral emissivity is shown in Figure 2a for a number of different geometrical configurations of our metamaterial, as determined by temperature. The metamaterial layer is suspended in the room temperature (nonoptimized) state and achieves a peak spectral