工字形椭圆纳米结构的吸收及其折射率敏感特性研究 下载: 1083次
1 引言
对于超材料完美吸收器,从频段来分,可以分为微波[1]、太赫兹[2-3],红外[4]、可见频带等频段的完美吸收器[5];从吸收带宽角度来分,又可分为宽带完美吸收器和窄带完美吸收器。其中,宽带完美吸收器一般可应用于太阳能电池[6-8],窄带吸收器可应用于传感器[9-10]、成像[11-12]和光探测器[13]等方面。对于光波段的完美吸收器,近年来,受到研究者们广泛关注的是基于表面等离激元共振的超材料完美吸收器,其主要用于光学隐身[14-15]和光学传感[16-18]等方面。表面等离激元(SPs)是贵金属表面自由电子在入射光子的激发下而产生的一种集体振荡的电磁模式[19]。SPs因具有局域电磁场的特性而使其具有突破传统光学衍射极限的能力和增强局域电磁场的特性[19-20]。
近年来,随着表面等离激元光子学研究的不断深入,SPs在生物学[21]、化学
本文提出一种工字形椭圆盘纳米超材料结构,该超材料结构由顶层的周期性阵列纳米结构、中间的介质层和底层的金属反射层所组成。其中顶层周期性结构单元结构由三个椭圆形金纳米盘以工字的形式排列构成,中间层为二氧化硅材料,底层为金膜。采用有限元方法研究了其吸收光谱特性、共振峰处的电场分布以及折射率传感特性,进而研究了结构参数变化对吸收光谱特性和折射率传感特性的影响,这些研究将为超材料结构在光学隐身和折射率传感器方面的应用提供参考。
2 结构与计算方法
本文设计的吸收器由厚度为t1的金作为基底,厚度为t2的二氧化硅作为介质层, “工”字型金椭圆盘周期性阵列作为顶层组成,椭圆盘阵列结构是周期性的,具有相同的周期p,如
图 1. 吸收器结构示意图。(a)椭圆盘完美吸收器三维示意图;(b)结构单元俯视图
Fig. 1. Schematic diagram of absorber structure. (a) Three-dimensional illustration of an oval disc perfect absorber; (b) top view of unit cell
3 结果与讨论
3.1 吸收特性
基于表面等离激元超结构的窄带吸收器的吸收机理主要由以下两方面决定:1)当入射光照射到结构表面时,尽量使反射率R为0,即调整阻抗匹配。由于R=|S11|2,S11越小,反射率越低。反射系数可表示为:
图 2. 有金基底(实线)和无金基底(虚线)时的吸收光谱
Fig. 2. Absorption spectra with gold substrate (solid line) and without gold substrate (dotted line)
为研究该结构产生吸收峰的物理机制以及基底对其吸收率的影响,分别计算了吸收谱中位置Ⅰ、Ⅱ、Ⅲ、Ⅳ处在xy平面和yz平面上的电场分布。当 λ=0.58 mm(Ⅰ)时,其电场辐射主要分布在竖直椭圆盘的两端、水平椭圆盘的上下两侧及它们之间的间隙,如
图 3. 不同波长处的电场分布。(a)~(d)对应波长Ⅰ、Ⅱ、Ⅲ和Ⅳ处在yz平面电场分布;(e)~(h)对应波长Ⅰ、Ⅱ、Ⅲ和Ⅳ处在xy平面电场分布
Fig. 3. Electric field distributions at different wavelengths. (a)--(d) Electric field distributions in the yz plane corresponding to the wavelengths I, II, III, and IV, respectively; (e)--(h) electric field distributions in the xy plane corresponding to the wavelengths I, II, III, and IV, respectively
吸收器的部分结构参数对吸收光谱的影响如
图 4. 不同结构参数对吸收光谱的影响。(a)不同间距d;(b)不同柱高h;(c)不同短轴w1;(d)不同长轴l1;(e)不同短轴w2;(f)不同长轴l2
Fig. 4. Effects of different structural parameters on absorption spectra. (a) Different distances d; (b) different column heights h; (c) different short axes w1; (d) different major axes l1; (e) different minor axes w2; (f) different major axes l2
改变吸收器旋转角度θ对吸收光谱的影响如
图 5. 不同角度和不同结构对吸收光谱的影响。(a)不同角度;(b)不同结构
Fig. 5. Effects of different angles and structures on absorption spectra. (a) Different angles; (b) different structures
3.2 折射率敏感特性
由于窄带超表面吸收器结构表面会产生等离子体,当自由电子和光子频率相同时,将产生等离激元共振现象,电场增强,此时入射光的反射率下降直至趋于0,且入射光均被结构吸收,因此,其会产生窄带吸收峰。表面等离子体对折射率变化十分敏感,吸收器的共振峰会随着折射率的变化而变化,因此,窄带超表面吸收器利用这一特性可以用于折射率传感器,以检测物体折射率的变化。为了研究完美吸收器的折射率传感特性,将外界环境折射率n分别设置为1.00,1.02,1.04,1.06,1.08,其中h=100 nm,d=30 nm,l1=l2=l3=120 nm,w1=w2=w3=50 nm,p=540,t2=30 nm,t1=50 nm,并依次得到对应的吸收光谱,如
图 6. 外界环境折射率对吸收光谱的影响。 (a)不同外界环境折射率n下椭圆盘的吸收光谱;(b)外界环境折射率n的变化与吸收光谱波峰位置的变化情况的关系图
Fig. 6. Effect of external environment refractive index n on absorption spectra. (a) Different external environment refractive index n; (b) relationship between the change of the external environment refractive index n and the change of the peak position of the absorption spectra
4 结论
设计了一种工字形椭圆纳米盘超材料结构窄带完美吸收器,用有限元方法研究了吸收器的光谱特性、共振峰处的电场分布以及其折射率传感特性,并分析了结构参数对吸收光谱和传感特性的影响。本文所设计的吸收器具有高吸收特性有以下三方面的原因:当入射光垂直照射到吸收器表面时,结构层的工字形椭圆盘之间的相互作用,使得局域电场增强;结构层和介质层之间产生局域等离激元共振,使得电场增强;吸收器的基底层能够对入射光进行反射,将光局域在介质层和基底之间,极大地提高了吸收器的吸收率。通过结构参数的变化可以调整吸收器的吸收率、波峰位置及其传感特性,本文设计的吸收器具有三个吸收峰,在波长分别为580 nm(Ⅰ)、670 nm(Ⅱ)和810 nm(Ⅲ)处,其吸收率分别达到91.06%、99.63%和97.26%,其中在Ⅲ处吸收器的灵敏度达到425 nm/ RIU,fFOM=14。因此,本文设计的工字形椭圆纳米盘超材料结构窄带完美吸收器,为等离激元超表面结构在光学隐身和折射率传感器方面提供了重要的参考价值。
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