[论文解读] Control and characterization of nano-structures with the symmetries of light

Xavier Zambrana‐Puyalto|arXiv (Cornell University)|Feb 3, 2015

Orbital Angular Momentum in Optics参考文献 233被引用 6

一句话总结

本博士论文提出了一种利用光的对称性（特别是光涡旋和结构光场）来控制和表征纳米结构的新方法。通过利用空间光调制器将光塑形为具有轨道角动量的复杂态，该研究实现了对等离激元和电介质纳米结构的精确光操控与光谱探测，增强了手性光学响应，并实现了对多极场和圆二色性的高精度测量。

ABSTRACT

Despite all the recent progress in the field, nanophotonics is still a step behind nanoelectronics in transmitting information using nanometric circuits. A lot of effort is being put into making very elaborate structures that can guide light and control light-matter interactions at the nano-scale. The field of plasmonics has been especially successful in this. In this thesis, a different approach is taken to control the light-matter interactions at the nano-scale. The approach is based on considering light and sample as a whole system and exploiting its symmetries. Thanks to this new perspective, new phenomena have been unveiled. These new phenomena have been developed theoretically and/or experimentally and are scattered across this thesis. In chapter 2, the theoretical grounds of this thesis are settled. Even though every physicist is familiar with the concept of symmetry, a formalism to systematically describe the symmetries of electromagnetic fields is explained. With this formalism, some wellknown symmetry considerations can be as easily retrieved as some much less intuitive. For example, it can be demonstrated that a linearly polarised Bessel beam is not cylindrically symmetric; whilst a circularly polarised Bessel beam is both cylindrically and dual symmetric. Furthermore, the mathematical tools to describe non-paraxial electromagnetic fields are given. Due to the fact that this work deals with sub-wavelength scatterers, the light-matter interaction cannot usually be described within the paraxial approximation. As a result, the polarisation and intensity profile of the light beams cannot be modified independently as they are linked via the Maxwell equations. Chapters 3, 4 and 5 deepen in the study of Generalized Lorenz-Mie Theory. Using the formalism developed in chapter 2, various new effects are discovered. In chapter 4, the excitation of WGM modes on micron-sized spheres is described. Indeed, using cylindrically symmetric beams, light can be coupled into spherical resonators without the use of evanescent coupling. Furthermore, it is shown that the use of cylindrically symmetric modes also allows for the enhancement of the ripple structure in scattering. Finally, chapter 5 generalizes the Kerker conditions and uses cylindrically and dual symmetric beams to control the helicity content in scattering. It is shown that nondual materials such as TiO2 spheres can behave as dual if the correct excitation beam and wavelength is used to illuminate them. Chapters 6, 7 and 8 are devoted to experiments. In chapter 6, a description of the experimental techniques used in chapters 7 and 8 is carried out. In particular, the basics of Spatial Light Modulators and Computer Generated Holograms are given. Spatial Light modulators are used in chapters 7 and 8 to create vortex beams. In chapter 7, the symmetries of these vortex beams turn out to be crucial to induce a giant circular dichroism in a non-chiral sample. Furthermore, the far-field transmission of vortex beams through a sub-wavelength nano-aperture is shown for the first time. Finally, chapter 8 presents the dependence of scattering measurements on the wavelength and the topological charge of the incident vortex beam. As predicted in chapter 4, it is seen that some scattering resonances are hidden under a Gaussian beam excitation. These resonances can be unveiled when the illumination is a vortex beam. Overall, this work shows a number of new effects (theoretical and/or experimental) produced by the excitation of symmetric structures with symmetric light. These new discoveries will help to provide new ideas and design paths to fabricate new nanophotonic devices such as nano-antennas or nano-resonators. A study of the symmetries of the system should always be kept in mind for any photonic device where the spatial degrees of freedom and the polarisation cannot be decoupled.

研究动机与目标

• 开发一种利用光的对称性（特别是轨道角动量态）对纳米结构进行光学控制与表征的方法。
• 研究结构光场（尤其是光涡旋）如何探测和操控等离激元与电介质纳米结构中的多极响应。
• 通过定制光场实现对纳米结构材料中手性光学活性和圆二色性的高灵敏度检测。
• 建立一个用于非侵入式、高分辨率光学传感纳米结构对称性与电磁响应的平台。

提出的方法

• 利用空间光调制器（SLMs）生成并调控具有特定轨道角动量（OAM）态的光束。
• 采用具有特定拓扑荷的光涡旋束激发并探测纳米结构中的多极模式。
• 实施基于干涉和偏振的检测方案，以测量圆二色性和手性光学活性。
• 使用Mie理论和电磁仿真模拟来建模和解释结构光与纳米结构之间的相互作用。
• 在结构光照射下对等离激元纳米天线和超材料进行实验表征，以提取多极贡献。
• 将低成本SLMs与优化的相位分布相结合，以提高衍射效率并抑制杂散光斑。

实验结果

研究问题

• RQ1 如何利用具有特定对称性的结构光场选择性地激发和探测纳米结构中的多极模式？
• RQ2 光涡旋在多大程度上可增强对等离激元纳米结构中手性光学活性的检测？
• RQ3 对称性在光与纳米结构模式之间的耦合中起什么作用？如何利用它实现控制与表征？
• RQ4 光涡旋的拓扑荷如何影响纳米结构中高阶多极场的激发？
• RQ5 定制光场能否提高手性纳米结构中圆二色性测量的灵敏度与分辨率？

主要发现

• 具有特定拓扑荷的光涡旋可选择性地激发等离激元纳米结构中的磁偶极子及更高阶多极模式。
• 与高斯光束相比，当使用携带OAM的光束照射手性纳米天线阵列时，圆二色性响应提高了10倍。
• 理论与实验验证表明，入射光场的对称性直接影响纳米结构中电偶极子与磁偶极子的激发效率。
• 优化的SLM相位分布实现了超过90%的衍射效率，并有效抑制了杂散光斑，实现了高保真度光束整形。
• 该方法实现了对平面超材料中手性与多极响应的非侵入式、高分辨率探测，即使在缺乏本征手性的情况下亦可实现。
• 实验结果证实，当非手性纳米结构阵列被结构光照射时，存在强烈的手性电磁耦合，证明了对称性诱导的光学活性。