【作者】 郝志强；

【导师】 孙骞；

【作者基本信息】 南开大学 ， 光学， 2013， 博士

【摘要】 表面等离激元(Surface Plasmon Polaritons,简称SPPs)是由金属表面自由电子和光子相互作用形成的一种表面电磁模式。由于其性质独特,已经在纳米光子集成、高密度存储、生物化学检测和传感等很多领域展现出巨大的应用前景。SPPs的激发和控制均来自光与亚波长金属结构的相互作用。这种介观和微观尺度上光与物质的相互作用,不仅具有改善传统光学元件性能的作用,而且还有可能产生一些新的物理现象。本文以亚波长金属狭缝阵列、光栅为主要研究对象,采用数值模拟的方法,研究此类结构中表面等离激元光场调控特性,设计微纳尺度光学元件实现光场的调控。本论文的主要内容如下：1.内嵌凹槽的亚波长金属狭缝共振透射特性的研究自金属薄膜上的亚波长孔径及其阵列结构的增强透射现象发现以来,亚波长金属结构广泛地应用于纳米光学器件的设计。因为表面等离激元对结构信息十分敏感,所以可以通过改变结构特征的方法来调控亚波长孔径的透射特性。在本文的研究中,我们详细讨论了内嵌凹槽深度变化对狭缝透射特性的影响,并使用类法珀共振理论确定狭缝透射特性变化的机理。研究结果显示,嵌入凹槽后,狭缝透射共振峰的峰位出现了不变、红移和蓝移等一系列变化。按照共振模式数的奇偶将凹槽对狭缝透射性质的影响分成两类：偶数模式,SPPs不能有效耦合进入凹槽,可认为共振腔的有效长度几乎没有改变,故共振峰的峰位保持不变。奇数模式中,SPPs能够耦合进入凹槽传输,造成共振腔有效长度增大,共振峰出现红移现象。而看似蓝移的共振峰实际为有效长度增大较多时,共振模式数变化的结果。此外,由于结构的变化,狭缝中还会形成其他的共振模式对狭缝的透射性质产生影响。2.基于亚波长金属狭缝阵列的光场调控研究在亚波长金属狭缝阵列中,透射光的相位与亚波长金属结构的几何参量和材料的光学属性之间存在密切关联,通过结构及介质属性的调整对出射面相位的调节,可以实现光场的调控。基于内嵌凹槽的亚波长金属狭缝共振透射特性和金属表面凹槽反射特性的研究结果,我们提出了两种相位调节的方法：一是在狭缝阵列中狭缝的内部嵌入凹槽。由于在狭缝内部嵌入凹槽实际上相当于增加了狭缝的有效长度,因而在狭缝阵列中,通过在狭缝内部安排不同深度的凹槽可以控制出射面的相位延迟。利用这一原理我们分别设计了两种不同结构参数的阵列,分别实现了光束的聚焦和偏转操作。在远场范围内得到了亚波长尺度的焦斑,光束偏转的角度可达6o。二是在狭缝阵列的入射表面嵌入凹槽。表面嵌入凹槽的方法实现相位调节的机理是凹槽与狭缝所激发SPPs之间的干涉作用。在本文的研究中,我们固定了凹槽与狭缝之间的距离,分析凹槽深度变化对狭缝透射相位的调节作用。根据研究结果,通过在亚波长金属狭缝阵列的表面排列不同深度的凹槽,实现了光束聚焦和偏转的操作。同样在远场的范围内得到了亚波长尺度的焦斑,并有10o左右的偏转结果。3.双层亚波长金属光栅中莫尔条纹形成的研究莫尔条纹技术广泛应用于传感测量领域,利用两个光栅之间表面等离激元的耦合作用,可以形成一种近场高衬比度的莫尔条纹,在亚波长尺度上扩展莫尔条纹的应用空间。在近场区域内,双层金属光栅中层间的耦合作用会受到光栅之间相对位置变化的影响,因而在研究近场莫尔条纹的形成时,务必要考虑由于光栅间位置变化所产生的影响。我们研究发现,当两光栅间距变化时,出射场空间中除了形成正常的莫尔条纹外,还会形成一种空间周期性仅为正常莫尔条纹一半的光场分布,我们将其定义为二阶的莫尔条纹。由于SPPs耦合作用的存在,亚波长光栅的通光特性不同于传统光学中的光栅,除了前后狭缝同轴的情况外,层间狭缝的横向位移为半个周期时,也会有较强的透光现象。上述两个位置的层间耦合作用随着间距的变化而变化,当这两个位置透光情况相差不多时,出射光场分布的周期刚好是几何关系所决定周期的一半,这就是二阶莫尔条纹的成因。可见,在双层亚波长金属光栅中莫尔条纹的分布,是由光栅层间耦合作用所影响的透光位置的周期所决定,而不再像传统光学中由实际的几何结构的透光性来决定。更多还原

【Abstract】 Surface plasmon polaritons (SPPs) are electromagnetic surface modes by interaction of free electrons of metallic surface with environmental photons. It has shown a great prospect on many areas, such as integrated nano-photonic, high-density storage, biochemical detection and sensing owing to its shorter effective wavelength and near-field enhancement features.The excitation and modulation of SPPs are all based on the interaction of light with subwavelength metallic structures. In this thesis, the main research object is two-dimensional subwavelength metallic slit arrays and gratings.Numerical methods are adopted to study the mechanism of how to regulate SPPs in such structures, and some functional components are designed to achieve the regulation of the light field. The main contents are as follows:1. Analysis of the resonance transmittance properties of metallic sub-wavelength slit with an embedded grooveIn this part, we study the dependence of the transmission spectrum of slit on the depth of the embedded groove. Comparing with the results from cavity resonance mode theory, we find that the shift situation of the subwavelength metallic slit comes from two reasons, namely increase in the cavity length and the increase in the resonance mode number. The spectrum changes, such as blue-shift, are the hybrid results of the increase in both of the mode number and the cavity length.2. The study of beam manipulation by metallic nanoslit arrays with grooves inside slits or on the input planeThe manipulation of light on a subwavelength scale is typically achieved through the regulation of the phase on the outgoing surface. Usually, the phase shift experienced by light is sensitive to the geometry of structure and the materials surrounding. In our study, we proposed two methods to regulate the phase delay by different mechanism. The first one is adding grooves inside the slit array, this method can be thought of as a result of the increased effective length of slit, and the second one is notch grooves on the input plane of slits array, which is based on the interference effect between the slits and grooves. Based on them, metallic nanoslit arrays were designed for beam focusing and deflection. The simulated results indicate that the two methods are effective mechanism to tune the phase retardation at transmitted field.3. Second harmonic moire effect in a double-layer metallic nano-gratingMoire effect has been used in many metrical areas for a long time. A near-field moire effect between two subwavelength gratings was reported recently and extended the existing moire techniques to subwavelength characterization of nanostructures. It is clearly the coupling of SPPs between the two gratings would play a key role in the form of moire fringes.The results show that second harmonic moire fringes, which period is only a half of normal moire fringes, can be produced behind the double-layer nano-grating due to the change of interlayer space. In the subwavelength metallic grating, the transmitted properties are quite different with the case in traditional optics because of the presence of SPPs coupling. When the lateral shifts between the slits in two gratings are equal to zero and a half of periodicity of grating, there could be a strong pass of light at these two positions with the change of interlay space. It is obviously that the periodicity of these two positions is only a half of periodicity determined by the geometry. And that is reason why the second harmonic moire effect can be produced in the subwavelength double layer metallic grating.更多还原