Melanin, with its high refractive index (RI) and broadband absorption, is an important biomaterial responsible for many of the vibrant structural colors observed in nature and for UV protection. Even though the RI plays an important role in the function of melanin, there is an ambiguity in its reported complex RI and a lack of understanding of whether and how the UV radiation, these materials are likely to experience under normal use, will affect the complex RI. Here, we measured the wavelength-dependent (360–1700 nm) complex RI of synthetic melanin films before and after in situ UV treatment using ellipsometry. We modeled the ellipsometric data using a modified Tauc-Lorentz dispersion model and measured the thickness independently using atomic force microscopy. The UV radiation reduces the film thickness. Interestingly, we find that both the real and imaginary terms of the RI increase upon UV radiation. These experiments provide accurate measurements of the optical properties of melanin and a surprising result that synthetic melanin absorbs more light (∼25% increase in extinction coefficient) below 600 nm after UV exposure.

中文翻译：

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合成黑色素涂层宽带复折射率的表征及其紫外线照射后的变化

黑色素具有高折射率 (RI) 和宽带吸收，是一种重要的生物材料，负责在自然界中观察到的许多鲜艳的结构颜色和紫外线防护。尽管 RI 在黑色素的功能中起着重要作用，但其报道的复合物 RI 存在歧义，并且缺乏对这些材料在正常使用下可能经历的紫外线辐射是否以及如何影响复合物的了解RI。在这里，我们使用椭圆光度法测量了原位紫外线处理前后合成黑色素膜的波长相关（360-1700 nm）复合 RI。我们使用改进的 Tauc-Lorentz 色散模型对椭偏数据进行建模，并使用原子力显微镜独立测量厚度。紫外线辐射降低了薄膜厚度。有趣的是，我们发现 RI 的实部和虚部都随着紫外线辐射而增加。这些实验提供了黑色素光学特性的准确测量结果，令人惊讶的结果是合成黑色素在紫外线照射后吸收更多低于 600 nm 的光（消光系数增加约 25%）。