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以PDMS为基底的超表面制备主流上分为lift-off法与刻蚀法,由于PDMS基底的表面活性很低,光刻胶难以附着其上,lift-off方法需要先对PDMS基底进行表面处理以增强其表面活性,这无疑增大了实验难度。因此文中采用传统的紫外光刻和湿法腐蚀技术制作可拉伸超表面,制备过程展示在图1所示流程图中。首先在一个超声清洗干净的玻璃片上用真空旋涂机旋涂负性光刻胶层,烘干后通过紫外暴露作为牺牲层使用。实验中使用的有机弹性PDMS基底是美国道康宁公司生产的Sylgard 184硅胶,将Sylgard 184中的主剂与固化剂按照10∶1的重量比混合,磁力搅拌均匀后置于真空干燥器中脱气直到气泡完全去除。然后将液态的PDMS旋转涂布于带有牺牲层的玻璃片上,将玻璃片放入烘箱中在80 ℃下烘烤2 h,固化得到22 μm厚的PDMS薄膜。随后用磁控溅射镀膜机在PDMS表面沉积了一层200 nm厚的金属银。接下来在金属银上旋涂正性光刻胶,前烘后用光刻机紫外曝光将掩膜版上的十字结构转移光刻胶上,后烘显影后得到图案化的光刻胶层,将样品上暴露的银用Fe(NO3)3溶液腐蚀即可得到十字结构的金属银。最后在样品上旋涂覆盖一层14 μm厚的PDMS薄膜作为保护层以防止金属脱落,用丙酮溶解掉牺牲层,就能从玻璃片上剥离出具有银十字图案的PDMS薄膜。
制备的可拉伸超表面实物图展现在图2(a)中,此时样品处于未拉伸状态,x轴方向宏观长度为L0。当通过夹具向样品施加沿x轴方向的拉伸移动时,图2(b)中显示样品在拉伸方向的宏观尺寸发生明显增大,此时长度为L。据此我们用拉伸比例S来精确描述样品的形变S = (L−L0)/L0,实际涵义为长度增加量与样品初始长度的比例。
图 2 (a) 样品实物图;(b) 样品拉伸实物图;(c) 可拉伸超表面三维结构示意图;(d) 超表面拉伸形变模型
Figure 2. (a) Photograph of the sample; (b) Photograph of the stretched sample; (c) 3D schematic diagram of the stretchable metasurface; (d) Stretch deformation model of metasurface
超表面三维结构示意图如图2(c)所示,整体呈现为两层PDMS弹性薄膜夹着中间金属银十字阵列的三明治结构。文中设计了三种不同参数的样品作为对比来研究可拉伸超材料的物理机理,具体参数在表1中列出。拉伸时样品宏观尺寸的变化必然导致微观上单元周期的改变,图2(d)所示为沿x轴几何变形的结构单元拉伸模型。根据该模型,x方向的周期将按拉伸比例增加,而y方向的周期只会轻微减小,研究中可以视为保持不变。此外,由于金属缺乏弹性,银十字图案的长度也将保持不变。因此拉伸过程中结构参数只有拉伸方向周期变为:
表 1 不同样品的结构参数
Table 1. Structural parameters of different samples
Sample Period(P0)/μm Length(l)/μm Width(w)/μm Sample 1 110 80 10 Sample 2 110 90 10 μm Sample 3 120 80 10 μm $${P_x} = {P_0} + {P_0} \cdot S$$ (1) 在文中,可拉伸弹性超表面的太赫兹频率响应特性是通过德国BATOP公司生产的太赫兹时域光谱系统(THz-TDS)进行测量,如图3所示。时域光谱系统中太赫兹波源和探测器都是光电导天线,利用光电采样的方式采集太赫兹脉冲信号,所测试的弹性超表面被安装在定制的夹具上,该夹具一侧固定,另一侧可通过旋转旋钮移动,于是通过旋转旋钮即可控制样品的拉伸比例。在实验中,水平方向偏振的准直太赫兹波通过光阑后垂直入射到样品上,光阑确保了太赫兹波入射到样品的有效区域。
Terahertz flexible stretchable metasurface based on double resonance response (Invited)
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摘要: 可拉伸器件中太赫兹波特性的主动控制对于涉及大机械变形或拉伸的先进太赫兹应用至关重要。将金属超表面与弹性薄膜聚二甲基硅氧烷相结合,设计并研制了基于不同微观机理的双带太赫兹波段主动控制器件。利用金属与弹性薄膜在拉伸作用下的形变失配,通过周期敏感的十字结构超表面实现了双带调制效果。在36%的拉伸下,利用偶极子模式和晶格模式分别在1.26 THz和2.41 THz处实现了调制深度为90%和78%的双频带调制。通过晶格模式进行的调制工作频率具有较大的动态范围,可以从2.41 THz调谐到1.85 THz。由于电偶极子谐振模式和周期性晶格谐振模式的机制彼此独立,因此可以独立地设计两个谐振频率,从而在几何上调节双带调制器的频率间隔。提出的可拉伸超表面制备简单,具有强度调制深度大,频率调谐范围广的优点,既能用于太赫兹波的主动控制,也能用于被动式的位移传感。Abstract: Active control of terahertz wave characteristics in stretchable devices is essential for advanced terahertz applications involving large mechanical deformation or stretching. Here, a dual band terahertz active control device based on different mechanisms was designed and fabricated by combining metal metasurface with elastic film polydimethylsiloxane. Based on the deformation mismatch between metal and elastic film under tension, the double band modulation effect was realized by using the periodically sensitive cross structure metasurface. Under 36% deformation, the dipole mode and the lattice resonance mode were exploited to experimentally achieve dual-band modulation with a modulation depth of 90% and a modulation depth of 78% at 1.26 THz and 2.41 THz, respectively. The operating frequency through the lattice mode had a large dynamic range, which could be tuned from 2.41 THz to 1.85 THz. Since the mechanisms of the electric dipole resonance mode and the periodic lattice resonance mode were independent from each other, the two resonance frequencies were designed independently, which allowed the frequency interval of the dual-band modulation to be geometrically adjustable. The stretchable metasurface presented in this paper is simple to prepare, and has the advantages of large intensity modulation depth and wide frequency tuning range. It can be used not only in active control of terahertz wave, but also in passive displacement sensing.
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Key words:
- terahertz /
- metasurface /
- stretchability /
- modulation /
- tuning
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表 1 不同样品的结构参数
Table 1. Structural parameters of different samples
Sample Period(P0)/μm Length(l)/μm Width(w)/μm Sample 1 110 80 10 Sample 2 110 90 10 μm Sample 3 120 80 10 μm -
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