Ultrathin broadband solar absorbers for UV and UV-NIR based on a scalable plasmonic metamaterial

在联合实验和理论方法中，我们将研究可扩展等离子体超材料在紫外范围或从紫外到近红外的整个范围内作为超薄宽带太阳能吸收器的应用。目前的超材料由高度填充的颗粒等离子体纳米复合材料组成，具有广泛的颗粒尺寸分布，沉积在涂有介电间隔层的反射金属膜上。我们的概念是基于量身定制的长度、尺度、组成和其他参数的不均匀性。特别是，它结合了由于粒子之间的纳米间隙的强近场耦合而导致的单个纳米粒子的等离子体吸收的巨大展宽与其他展宽效应。对于UV-NIR吸收剂，这些范围从不同等离子体纳米粒子的组合到合金纳米粒子的阻尼和纳米级粗糙度到量子力学阻尼现象。对于紫外线吸收剂，只使用不同大小和形状的铝颗粒。我们的目标是在紫外范围和紫外到近红外范围内分别实现宽带吸收，同时在红外范围内具有低发射度，对光偏振和入射角不敏感。采用气相沉积和磁控溅射制备了纳米复合材料。簇源，除其他外，允许通过独立控制纳米颗粒归档因子和尺寸分布来精确剪裁纳米结构。与其他纳米结构等离子体吸收剂相比，纳米结构在沉积过程中通过自组织形成，不需要光刻技术。因此，这种方法适合大规模制造。我们最终的目标是对恶劣环境条件和热稳定性的鲁棒性。利用簇源，我们计划探索不同材料纳米粒子和合金纳米粒子结合的独特潜力。通过理论分析和仿真得到了吸波器的设计参数，如颗粒填充系数、介电间隔片的尺寸分布、厚度等。分析包括多重散射形式，基于时域有限差分法的大规模模拟，以及利用有限元方法获得纳米复合材料吸收光时的温度分布。模拟的输入参数将由各种表征技术提供，包括紫外-可见光谱和光谱椭偏法。我们已经在初步的实验工作中证明了我们的概念的总体可行性。虽然目前应用的重点显然是对宽带等离子体吸收体的基本理解，但我们希望我们的结果与未来在太阳能收集方面的应用高度相关。

In a joint experimental and theoretical approach, we will investigate scalable plasmonic metamaterials for applications as ultrathin broadband solar absorbers either in the UV range or in the entire range from UV to NIR. The present metamaterials consist of highly filled particulate plasmonic nanocomposites with a broad particle size distribution that are deposited on a reflective metallic film coated with a dielectric spacer layer. Our concept is based on tailored inhomogeneities in lengths scales, composition, and other parameters. In particular, it combines the huge broadening of the plasmonic absorption of the individual nanoparticles due to strong near field coupling across the nanogaps between the particles with other broadening effects. For the UV-NIR absorbers, these range from the combination of different plasmonic nanoparticles to damping in alloy nanoparticles and nanoscale roughness to quantum mechanical damping phenomena. For the UV absorbers, only Al particles of different size and shapes will be employed. We aim for broadband absorption in the UV range and from the UV to the NIR range, respectively, with simultaneous low emittance in the IR range and insensitivity to light polarization and incidence angle. The nanocomposites are fabricated by vapor phase deposition involving high-rate gas aggregation cluster sources and magnetron sputtering. The cluster sources, inter alia, allow precise tailoring of the nanostructure through independent control of the nanoparticle filing factor and size distribution. In contrast to other nanostructured plasmonic absorbers, the nanostructure forms during deposition by self-organization and does not require lithography techniques. The method thus lends itself to large-scale fabrication. We finally aim for robustness against harsh environmental conditions and thermal stability. Using cluster sources, we plan to explore the unique potential of combining nanoparticles of different materials and alloy nanoparticles. The design parameters of the absorber such as particle filing factor, size distribution, thickness of the dielectric spacer etc. will be obtained from theoretical analysis and simulations. The analysis includes a multiple scattering formalism, large scale simulations based on finite difference time domain method, and a finite element method to obtain the temperature profiles of the nanocomposites upon absorption of light. The input parameters for the simulations will be provided by various characterization techniques including UV-vis spectroscopy and spectroscopic ellipsometry. We already demonstrated the general feasibility of our concept in preliminary experimental work. While the focus of the present application is clearly on fundamental understanding of the broadband plasmonic absorbers, we expect our results to be highly relevant for future applications in solar energy harvesting.