Feng Peng, Li Wen-Di, Zhang Weihua
Opt Express. 2015 Feb 9;23(3):2328-38. doi: 10.1364/OE.23.002328.
We theoretically study the metal-insulator-metal (MIM) structure based ultrathin broadband optical absorber which consists of a metallic substrate, a dielectric middle layer, and a nanostructured metallic top layer. It is found that, there exists an effective permittivity, εnull, for the top nanostructured metallic layer which leads to unit-absorption (zero-reflection) of the MIM structure. Importantly, this εnull exhibits abnormal dispersion behaviors. Both its real and imaginary parts increase monotonically with the wavelength. To obtain such naturally non-existing permittivity, we investigate the optical properties of two typical types of metal-dielectric nanocomposites, namely, thoroughly mingled composites using Bruggeman's effective medium theory, and more realistic Au nanosphere-in-dielectric structures using numerical permittivity retrieval techniques. We demonstrate that the εnull-type dispersions, and consequently, perfect absorption can be obtained over a broad spectral range when the filling factor of the metal component is close to the percolation threshold. The result not only explains the recently reported broadband absorbers made of randomly deposited Au nanoparticles [M. K. Hedayati, et al, Adv. Mater. 23, 5410 (2011)], but also provides theoretical guidelines for designing ultrathin broadband plasmonic absorbers for a wealthy of important applications.
我们从理论上研究了基于金属-绝缘体-金属（MIM）结构的超薄宽带光吸收器，该吸收器由金属基底、电介质中间层和纳米结构金属顶层组成。研究发现，对于顶层纳米结构金属层存在一个有效介电常数εnull，它导致MIM结构的单位吸收（零反射）。重要的是，这个εnull表现出异常色散行为。其实部和虚部均随波长单调增加。为了获得这种自然界不存在的介电常数，我们研究了两种典型类型的金属-电介质纳米复合材料的光学性质，即使用布鲁格曼有效介质理论的完全混合复合材料，以及使用数值介电常数反演技术的更实际的金纳米球-电介质结构。我们证明，当金属组分的填充因子接近渗流阈值时，在很宽的光谱范围内可以获得εnull型色散，从而实现完美吸收。该结果不仅解释了最近报道的由随机沉积的金纳米颗粒制成的宽带吸收器[M. K. Hedayati等人，《先进材料》23, 5410 (2011)]，还为设计用于众多重要应用的超薄宽带等离子体吸收器提供了理论指导。
