An effective platform for low-pump power ultrafast nonlinear optics
Schematic of the proposed meta-optical systems as an effective platform for low-pump power ultrafast nonlinear optics.
Natural materials generally show no magnetism at optical frequencies. With the advances made in the area of metamaterials/metasurfaces, which derive their optical properties from the engineered structure of their resonant building blocks, i.e., meta-atoms, the idea that the magnetic field of light could be manipulated has become a reality. Optical magnetism is not only associated with a variety of interesting phenomena, but also is of great importance to numerous applications. Both plasmonic and dielectric resonators with subwavelength dimensions have been used to realize magnetism in optics. Compared with plasmonic structures that are well-known to possess intrinsic Ohmic loss, dielectric meta-atoms are highly desirable because of their unique ability to manipulate light with extremely low loss. Accordingly, there has been an increasing interest directed towards dielectric meta-optics that exploit Mie resonances to enable unprecedented flexibility in producing optical magnetism.
However, due to the generally moderate refractive index of available optical materials, most reported all-dielectric metasurfaces suffer from the coherent effect that arises from the coupling among multipole modes. This represents a serious drawback of current approaches that results in a series of undesirable characteristics, such as spectra that possess a low Q-factor and poor field confinement. Consequently, a new strategy that would allow enhanced Mie resonances for achieving dramatically strengthened light-matter interaction in meta-optics-based systems is highly desired. By successfully attaining this goal, Dr. Lei Kang, Dr. Huaguang Bao and Prof. Douglas H. Werner from Department of Electrical Engineering at The Pennsylvania State University demonstrate that the optical magnetism in high-index resonators can be significantly enhanced by adding a highly reflective back mirror to the system, with a concomitant improvement in field confinement and enhancement. This work is published in Photonics Research, Vol. 7, Issue 11, 2019 (Lei Kang, Huaguang Bao, Douglas H. Werner. Interference-enhanced optical magnetism in surface high-index resonators: a pathway toward high-performance ultracompact linear and nonlinear meta-optics[J]. Photonics Research, 2019, 7(11): 1296-1305).
The results in this work show that the magnetic response from amorphous silicon (α-Si) resonators located on top of a gold backplane can be one order of magnitude stronger than that seen in those on a glass substrate. To highlight the transformative advancements offered by the proposed mechanism in tailoring the response of meta-optical systems, two proof-of-concept demonstrations are presented. Numerical simulations reveal that the proposed meta-optical systems comprised of Mie resonators with interference-enhanced magnetism not only can significantly boost low-pump power ultrafast nonlinear dynamics, but also are able to realize highly efficient directional excitation of surface plasmon waves.
Prof. Werner believes that this work represents a major step forward leveraging interference-enhanced magnetism in Mie resonators for high-performance ultracompact linear and nonlinear meta-optics.
Future work will focus on the realization of transmission-mode metadevices exhibiting an interference-enhanced magnetic Mie resonance. Such devices can be implemented by custom-designing their reflection and transmission bands using a Bragg mirror.
基于干涉效应的增强米氏磁谐振
干涉增强光频磁性实现低光抽运非线性电介质超构表面
鉴于自然界已知材料几乎不具有光频磁性，传统光学通常仅限于研究光波电场与物质的相互作用。近年来，超构材料及超构表面的出现极大地拓展了物质光学性质的可实现参数空间，使对光波磁场的操控成为可能。利用光波磁场激励金属纳米结构表面等离子体振荡和电介质纳米颗粒米氏谐振是实现光频磁性的两种主要途径。相对于金属纳米结构中普遍存在的较高欧姆损耗，基于米氏谐振的电介质超构表面已被成功用于实现超低损耗光频磁响应。
尽管如此，电介质纳米颗粒有限的光频折射率会导致其米氏谐振表现出多极相干耦合效应。电介质超构表面从而普遍具有包括低光谱品质因数、弱光场局域等光响应特性，不可避免地制约了电介质超构器件的应用前景。为此，来自于美国宾州州立大学电气工程系的康雷博士、包华广博士和Douglas H. Werner教授提出并理论验证了高反射率基底可显著增强位于其上的电介质谐振结构的光频磁性。数值模拟结果显示，基于干涉效应的增强米氏谐振可形成高品质因数光谱以及强局域光场。研究结果发表在Photonics Research 2019年第7卷第11期上（Lei Kang, Huaguang Bao, Douglas H. Werner. Interference-enhanced optical magnetism in surface high-index resonators: a pathway toward high-performance ultracompact linear and nonlinear meta-optics[J]. Photonics Research, 2019, 7(11): 1296-1305）。
研究结果表明，用金替代玻璃作为基底材料可使无定形硅亚波长谐振结构中的近红外波段磁响应强度提升约一个量级。进一步的概念验证实例研究显示，增强的光频磁性不仅可用以实现无定形硅超构表面的低光抽运非线性调制，还可用于设计高效率定向表面等离子体波耦合器。
Werner 教授认为，该工作有效地利用了干涉机制增强米氏磁谐振，是电介质超构表面研究向着实现超紧凑、高性能的线性及非线性超构器件迈出的重要一步。
未来工作将致力于实现具有干涉增强光频磁性且可工作于透射模式的电介质超构器件。为此，可考虑利用反射及透射频带均可设计和优化的布拉格反射镜替代金属基底。