基于纳米光腔的非线性光学超构表面

NONLINEAR METASURFACE BASED ON OPTICAL NANOCAVITY

张学才

11849195

硕士

材料物理与化学

李贵新

2020-05-27

2020-07-01

哈尔滨工业大学

深圳

近年来，光学超构材料作为一种新型人工材料引起了人们的广泛关注。光学超构材料在组分仍为传统材料的基础上，通过人工设计亚波长尺度的单元结构（或称“超构原子”）来控制材料的有效介电常数和磁导率，从而表现出超越传统材料的电磁响应。尽管超构材料为光学器件的设计提供了全新思路和实现方法，但由于其结构复杂受限于纳米加工技术，且在可见光范围内存在高损耗等问题，三维超构材料很难在光学波段得到实际应用。针对这些问题，超薄的光学超构表面应运而生。光学超构表面继承了超构材料亚波长单元设计的优点，仅利用一层超构原子即可灵活自由地调控光的振幅、相位、偏振、自旋和轨道角动量等性质。光学超构表面结构简单、设计直观、易于加工、利于集成，极大地推动了新型光学器件的发展。在线性光学领域已实现光学聚焦、全息成像、光束整形等形形色色的功能。超构表面在非线性光学领域也获得了广泛关注。非线性超构表面由于其超薄特性而免于苛刻的相位匹配条件。同时，非线性超构表面能够通过设计每个单元，实现对产生的非线性光的相位、振幅和偏振进行操控，实现多种多样的光学功能。该领域中，如何在超构表面有限的厚度内提高非线性转换效率一直是相关研究的热点。本论文设计了一个由亚波长尺寸的光学谐振腔组成的非线性等离激元超构表面，以实现在近红外波段对二次谐波产生效率的增强。同时，通过改变超构表面周期的方式来改变超构表面在所设计二次谐波频段的电磁环境，进而探究该变换对二次谐波效率的影响。本论文提出的非线性等离激元超构表面由亚波长尺度的金开口谐振环、二氧化硅间隔层、金反射面三层结构组成的微腔构成。金开口谐振环是一种具有优良二次谐波产生特性的超构原子，在和二氧化硅及金反射面构成微腔之后，微腔的能量局域特性和场增强效应可以显著地提高超构表面的非线性转换效率。本论文首先根据光学谐振腔理论设计了纳米腔超构表面，并利用基于有限元法的商业仿真软件 COMSOL Multiphysics，在频域模式下研究了上述超构表面的线性响应，并结合金属电子的非线性流体模型，对超构表面的二次谐波产生效率进行了数值仿真分析及优化。模拟结果表明，相比于只由单层开口谐振环构成的超构表面，本文所提出的纳米光腔构成的超构表面能提高二次谐波产生效率约 13 倍。本论文中通过先进的纳米加工及表征技术制备了该超构表面样品，进行了形貌表征，并详细介绍了制备流程和表征方法。本论文中也对该超构表面样品的线性及非线性光学特性进行了实验测量，测得其在基频波段的反射率和相应的二次谐波产生效率。实验和理论都证明，超构表面周期的变换能够改变二次谐波频率处电磁环境，进而显著改变二次谐波转换效率。同时，本论文的实验和理论表明，该纳米光腔超构表面能够有高的有效二阶非线性极化系数，这种腔结构的设计方案为提高二次谐波产生效率提供了一种行之有效的方法，在集成非线性光学器件方面具有潜在的应用价值。

In recent years, optical metamaterials have attracted widespread attention as a new type of artificial materials. Optical metamaterials are still composed of traditional materials, but its effective permittivity and permeability are controlled by the geometric shape of the subwavelength artificial unitcells (or “meta-atoms”). Thus, optical metamaterials can exhibit the electromagnetic response beyond the traditional materials. Metamaterials provide new ideas and implementation methods for the design of optical devices, but there are still some problems prevent them from practical from applications. For example, the complexity of the meta-atoms is limited by nano-fabrication technology, and there are high loss at the wavelength of visible light. For solving these problems, ultra-thin optical metasurfaces are created. The optical metasurfaces inherit the advantages of the metamaterials composed of subwavelength meta-atoms. With only a layer of meta-atoms, metasurfacees can flexibly and freely adjust the properties of light, such as the amplitude, phase, polarization, spin and orbital angular momentum. The optical metasurface is simple in structure, intuitive in design, easy for fabrication, and benefit for integration, which greatly promotes the development of novel optical devices. In the field of linear optics, various functions such as optical focusing, holographic imaging, and beam shaping have been realized on the platform of metasurfaces. Metasurfaces have also received extensive attention in the field of nonlinear optics. For its ultrathiness, nonlinear optical metasurfaces are free of harsh phase-matching conditions. Meanwhile, we can control every meta-atoms to control the generated nonlinear light’s phase, amplitude and polarization, and realizing various optical functions. In the field of nonlinear optical metasurfaces, how to improve the nonlinear conversion efficiency within the limited thickness of the metasurface has been the focus of related research. This thesis provides a nonlinear plasmonic metasurface composed with subwavelength-scale nano-cavities, for ehancing the second harmonic generation at the wavelength of near infrared. Also, the period of metasurface is adjusted for tailoring the electromagnetic environment at the frquency of the second harmonic, and the nonlinear conversion efficiency can be tailored.The nonlinear plasmonic metasurface proposed in this paper is composed of nanocavities, which is composed of a three-layer structure of a subwavelength-scale gold splitting-resonant ring, a silicon dioxide spacer layer, and a gold reflective surface. The gold splitting-resonant-ring is a meta-atom with excellent property in the field of the second harmonic generation. By forming a nanocavity with silicon dioxide and a gold reflective surface, the ability of energy localization and field enhancement of the nanocavity will significantly improve the nonlinear conversion efficiency of the metasurface. This thesis designs the nanocavities with the theory of cavities, and then uses the commercial simulation software COMSOL Multiphysics based on the finite element method for studying the linear response of the above metasurface in the frequency domain mode. Combined with the nonlinear hydrodynamic model of electron gas, the second harmonic generation efficiency of the metasurface also can be carried out and optimized by numerical simulation. The simulation results show that the metasurface consisting of the nano-cavities can improve the second harmonic generation efficiency by about 13 times compared to the metasurface consisting of only a single layer of splitting-resonant ring. Then, the metasurface samples were prepared by advanced nano-fabrication techniques and well characterized in morphology, and the preparation process and characterization methods were introduced in detail. Finally, the linear and nonlinear optical characteristics of the metasurface sample were measured experimentally. The reflectance spectrum at the wavelength of fundamental wave and the second harmonic generation efficiency were measured. It has been proved that the changing of period can change the electromagnetic environment at the frequency of the second harmonic, and the conversion efficiency of the second harmonic can be significantly tailored. Also, a high effective second order susceptibility has been demostrated in this thesis. And nanocavity metasurface provides a practical method for improving the efficiency of second harmonic generation, and has potential value in integrating nonlinear optical devices.

光学超构表面 非线性光学 二次谐波产生 等离激元 光学谐振腔

optical metamaterials optical metasurfaces nonlinear optics the second harmonic generation plasmonics optical cavities

中文

联合培养

南方科技大学

张学才. 基于纳米光腔的非线性光学超构表面[D]. 深圳. 哈尔滨工业大学,2020.