,还通过尺寸参数 x = a 2 π / λ 与入射光波长有关。为此我们选择 λ = 450 nm , 630 nm 这两种不同的波长来考察这个不对称二聚体的散射特性。这里入射光偏振仍设定为沿y方向。

Figure 4. Far field intensity distribution of scattered light from a non-Symmetric nanodimer under the incidence of a y polarized light. The direction of ϕ = 0 coincides with the x axis. The radii of the two spheres are respectively a 1 = 75 nm , a 2 = 115 nm . The centers of the two spheres are located on the x axis with coordinates x 1.2 = ± 99 nm . The refractive indices are the same n = 4 + 0 i . The wavelengths of the incident light are (a) λ = 450 nm and (b) λ = 630 nm

图4. y偏振光照射下非对称纳米二聚体散射光在xy平面的远场光强分布。 ϕ = 0 方向与x轴重合。两个球形例子的半径分别为 a 1 = 75 nm , a 2 = 115 nm 。两个球的球心位于x轴,坐标为 x 1.2 = ± 99 nm 。两个球的折射率相同,为 n = 4 + 0 i 。入射光波长为(a) λ = 450 nm (b) λ = 630 nm

图4给出了远场散射光强分布。首先比较(a)图与图3(b),两者除了二聚体的结构不同之外,其他参数都相同。从中不难发现空间相干性与二聚体结构之间也有比较独特的相互作用。具体来说,当空间相干度 η = 0.5 i 时,对称二聚体形成关于y轴非对称的散射分布花样,而非对称二聚体却得到一个几乎对称的散射分布花样。而对照 η = 0.8 i η = 0.8 e i 2 π / 3 连个参数,发现本来散射强度不同的两个散射分布变成了散射强度几乎相等、并且关于y轴对称的分布。而对照图4(a)和图4(b)可以看出,由于入射波长不同,干涉效应造成的结果也显著不同,在 λ = 630 nm 时,相干入射和部分相干如何得到的散射强度呈现出巨大反差。

5. 结论

本文在耦合偶极子模型下,通过解析推到和计算研究了由两个电介质球体构成的对称和非对称结构的纳米二聚体的散射特性。通过在入射光中引入部分空间相干性,我们发现光的空间相干度对于这种结构的散射强度、远场散射分布和散射方向性都有显著影响。这种相干性效应的机理进一步表现为它与入射光的偏振态和波长混合为一种复杂的调制作用。特别的,研究发现通过调节空间相干度的幅值和相位可以明显的散射方向性,这在一般的耦合电偶极子 [20] 散射问题中是较难实现的。本研究的方法和结果对于发展利用纳米结构控制光场的技术具有潜在的价值。

基金项目

本研究得到广东省自然科学基金项目(2016A030307026),国家基础研究计划项目(22078072)和广东省教育厅特色创新项目(2021ZDZX4060)的资助。

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