摘要
增强可见-近红外光吸收在光电信号转换、探测、通信及传感等众多领域具有重要应用潜力。本文基于吉尔-图诺伊斯谐振腔(Gires-Tournois resonator)共振吸收原理,利用Al/Al2O3/Al三层膜结构制备了可见到近红外波段全铝基平面薄膜堆栈型超构吸收器。通过合适的参数优化选取,实现了吸收峰位连续可调。吸收峰值接近100%,变角度反射光谱显示器件对入射角度不敏感,理论数值模拟计算结果与实验结果相互吻合。完美吸收峰在500 nm附近的吸收器在532 nm激光照射下快速升温,最高温度可达55.4 ℃,表明该结构在光热转化领域的潜在应用。
关键词
可见-近红外波段的超吸收器在光电探
多层介质膜是光学器件中广泛应用的结构,把折射率不同的介质膜组合在一起,通过膜层设计可以实现特定波长的高反射率和透射率,这在各种激光器、探测器和其它光子器件中广泛地应用。这种平面多层膜结构制备过程无需复杂的微纳加工技术,可实现大面积制备。如Kat
本文基于Al/Al2O3/Al三层薄膜堆栈结构制备了可见到近红外波段的电磁吸收器,该器件可实现大面积、低成本制备。利用原子层沉积系统调控Al2O3的厚度,实现了可见-近红外波段连续可调的宽波段近完美吸收。不同偏振状态下变角度光谱显示器件对入射角度不敏感。实验测量了吸收器反射光谱,其结果与时域有限差分法(Finite Difference Time Domain,FDTD)仿真结果高度符合。最后的光照升温实验表明这种三层膜堆栈结构的吸收器在光照情况下升温快速,在光热转化领域具有广大的应用前景。
为了利用金属铝材料制备可见-近红外波段完美吸收波长可调的大面积吸收器,设计了Al/Al2O3/Al三层薄膜堆栈结构,其制备流程图如

图1 铝基电磁吸收器制备流程图 (a) 硅衬底,(b) 硅衬底/铝薄膜,(c) 硅衬底/铝/氧化铝薄膜,(d) 硅衬底/铝/氧化铝/铝薄膜
Fig. 1 Preparation flowchart of aluminum-based electromagnetic absorber (a) Silicon substrate, (b) Silicon substrate/aluminum film, (c) Silicon substrate/aluminum/alumina film, (d) Silicon substrate/aluminum/alumina/aluminum film
以单晶硅为衬底,制备流程如下:(1)清洗硅片衬底,先将硅片放入丙酮中超声10 min,取出放入乙醇中超声10 min,取出并用大量超纯水冲洗,放入浓硫酸/双氧水(比例1:4)混合溶液中浸泡20 min,取出并用大量超纯水冲洗后放入乙醇中浸泡后取出吹干;(2)用高真空电子束沉积系统在硅表面生长厚度约为100 nm的金属铝,作为结构中的金属反射层(
电子束沉积系统(High Vacuum E-beam evaporation system,Tri-Axis)用于底层和顶层金属铝的制备;原子层沉积系统(Atomic Layer Deposition,Picosun R200)用于氧化铝薄膜的生长;紫外-可见分光光谱仪(Lamda 950, PerkinElmer)用于测试器件的光谱特性;扫描电子显微镜(Scanning Electron Microscopy,SEM,FEI Siron 200)用于表征样品形貌,采用超高分辨模式(Ultra High Resolution,简称UHR);紫外-可见-近红外分光光度计(Cary 5000,Agilent Technologies)用于表征器件在不同偏振状态下样品的变角度反射光谱。
我们对制备的超吸收器进行形貌及结构表征。

图2 三层薄膜堆栈结构的吸收器 (a) 结构示意图, (b) SEM截面图
Fig. 2 An absorber with a three-layer film stack structure (a) Schematic structure, (b) SEM cross-sectional view
当电磁波入射到吸收器表面时,在界面处发生了反射、透射、吸收及散射,满足关系
用紫外-可见分光光度计Lamda 950对制备的吸收器进行了反射光谱的表征,入射角度为8°。如

图3 吸收器的反射光谱曲线 (a) Al2O3厚度分别为63 nm、73 nm、81 nm、90 nm样品的实验反射光谱曲线, (b) Al2O3厚度分别为63 nm、73 nm、81 nm、90 nm样品的模拟反射光谱曲线, (c) Al2O3厚度分别为105 nm、117 nm、136 nm、146 nm样品的实验反射光谱曲线, (d) Al2O3厚度分别为105 nm、117 nm、136 nm、146 nm样品的模拟反射光谱曲线
Fig.3 Reflectance spectrum curves of the absorber (a) The experimental reflection spectrum curve of samples with alumina thickness of 63 nm, 73 nm, 81 nm, and 90 nm, (b) The simulation of samples with alumina thickness of 63 nm, 73 nm, 81 nm, and 90 nm. Reflection spectrum curve, (c) The experimental reflection spectrum curve of samples with alumina thickness of 105 nm, 117 nm, 136 nm and 146 nm, (d) The simulated reflection spectrum curve of samples with alumina thickness of 105 nm, 117 nm, 136 nm and 146 nm
为了深入理解结构的特征,我们进行了理论分析。该三层膜结构的反射光谱符合斯涅尔反射定律。假定一束平行光从空气入射到结构表面(见

图4 吸收器结构示意图
Fig. 4 Schematic diagram of absorber structure
根据文献[
, | (1) |
其中: 表示从介质i入射到介质j的反射系数,入射光为TE模式时,, 入射光为TM模式时,;Nm=n+ikm表示介质m的复折射率; 是在介质m中的传输相位,;R表示为结构的反射率,。入射光在该结构中传播,当相位移动累积到mπ时,便会增强上下两层铝和氧化铝之间界面区域的光吸
针对设计的器件结构,我们用时域有限差分法进行了数值仿真计算。首先对100 nm Al/ Al2O3/5 nm Al三层薄膜结构进行椭圆偏振测试,数值反演计算获得每层薄膜的光学参数。

图5 100 nm Al/ 63 nm Al2O3/5 nm Al样品中各层薄膜的光学参数 (a) 100 nm厚度的Al, (b) 5 nm厚度的Al, (c) 63 nm厚度的Al2O3。黑色曲线为折射率n,红色曲线为消光系数k
Fig. 5 Optical parameters of each layer of 100 nm Al/63 nm Al2O3/5 nm Al (a) 100 nm aluminum, (b) 63 nm Alumina, (c) 5 nm aluminum. The black curve is the refractive index n, and the red curve is the extinction coefficient k
仿真计算的结果如
为了更直观的描述超吸收器100 nm Al/Al2O3/5 nm Al的峰位与介质层Al2O3厚度之间的关系,我们从实验与模拟的反射光谱中提取两者的数据,作超吸收峰位与Al2O3厚度关系的折线图,如

图6 超吸收器中间Al2O3厚度与吸收峰位关系的折线图
Fig. 6 Line chart of the thickness relationship between the superabsorber cavity and the position of the absorption peak
以上讨论都基于入射光近似于垂直照射在超吸收器表面上的情况。在很多领域的应用中,超吸收器在变角度入射下的性质是一个非常重要的性能指标。如隐身领域中,飞行器在雷达不同角度照射下,需要保持较高的电磁波吸收率。如太阳能电池领域中,针对不同时间段太阳不同角度的照射,希望吸收体具有宽波段吸收的特性。因此,我们研究了超吸收器在变角度下对吸收性能的影响。利用Cary 5000型分光光度计对100 nm Al/90 nm Al2O3/5 nm Al吸收器分别在s偏振和p偏振态下进行了变角度反射光谱的测试,角度范围为6-6

图7 不同偏振状态下器件的变角度反射光谱 (a-c) 分别是实验测量的s,p及非偏振状态下的器件变角度反射光谱,(d-f) 分别是模拟计算的s,p及非偏振状态下的器件变角度反射光谱
Fig. 7 Variable angle reflection spectra of the device under different polarization states. Experimental (a-c) and calculated (d-f) reflectance spectra as a function of incidence angles under different polarization conditions
为了研究器件超吸收峰形成的物理机制,我们利用时域有限差分法对100 nm Al/90 nm Al2O3/5 nm Al吸收器的679 nm处吸收峰进行了电磁场分布模拟计算。设置单个监视器,为xz纵截面(y=0 nm),入射角度为8°。

图8 8°入射条件下样品共振波长处的电场及磁场分布 (a) xz纵截面(y=0 nm)处电场分布, (b) xz纵截面(y=0 nm)处磁场分布
Fig. 8 The electric and magnetic field distribution at the resonant wavelength of the sample at 8° incidence (a) The electric field distribution at the xz longitudinal section (y=0 nm), (b) The magnetic field distribution at the xz longitudinal section (y=0 nm)
由于超吸收器的吸收性能来自于金属的耗散,利用这个特性在超吸收峰附近可以实现光照升温。超吸收器100 nm Al/63 nm Al2O3/5 nm Al的吸收峰位为504 nm,其超吸收范围为417~648 nm,因此我们选择532 nm激光器对其光照升温。如

图9 (a)电磁吸收器进行光照升温的实验装置图, (b) 样品在240 mW激光功率下光照升温曲线, (c) 样品在300 mW激光功率下光照升温曲线, (d) 样品在440 mW激光功率下光照升温曲线,其中黑色曲线为室温变化曲线,红色曲线为样品温度变化曲线
Fig. 9 (a) Diagram of the experimental setup of the electromagnetic absorber for light heating, (b) The heating curve of the sample under 240 mW laser power, (c) The heating curve of the sample under 300 mW laser power, (d) The heating curve of the sample under 440 mW laser power , in which the black curve is the room temperature change curve, and the red curve is the sample temperature change curve
本文利用三层膜全铝基结构Al/Al2O3/Al,通过改变中间介质层Al2O3的厚度,制备了吸收波长可调的可见-近红外波段的吸收器。电磁场分布仿真模拟结果表明,该吸收器符合吉尔-图诺伊斯谐振腔共振吸收模型。光照升温实验表明其具有良好的光热效应。这种三层平面薄膜结构的电磁吸收器工艺简单、价格低、适合大面积制备,可应用在太阳能电池、探测器、光热转化等领域。
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