Ultra-strong interaction of metamaterial plasmons with photons in a terahertz photonic crystal cavity

腔（量子）电动力学研究物质与光子在共振腔中的强相互作用。长期以来，人们一直在深入研究物理学中的基本量子特性，如薛定谔的猫态和光子纠缠，并探索量子信息处理的方法。最初，强耦合仅在自然原子中进行研究，而近年来，它已在各种材料系统中通过实验实现，包括半导体量子威尔斯和量子点中的带间和子带间跃迁，磁性材料中的自旋共振，以及玻色子激发，如二维电子气中的回旋跃迁和聚合物中的分子振动跃迁。最近，我们已经证明了强大的光-物质耦合的超颖表面，一类人工材料，太赫兹光子在一维光子晶体腔。我们采用了等离子体金属结构，这些结构在文献中已被广泛研究，尤其是因为它们具有实现化学和生物传感器、光学滤波器和调制器的潜力。由于它们巨大的偶极矩，超颖表面的晶胞与腔中的光子有效地相互作用，形成等离子体-光子极化激元，其上、下极化激元分支具有巨大的拉比分裂。尽管这是一个经典的电动力学系统，但事实证明，腔量子电动力学中使用的术语是有用的和适用的。一个是致力于更好地理解和增强这种形式的光-物质相互作用。我们想回答关于巴比特互补超材料表面与光子的意外强耦合的开放性问题，然后研究“暗原子”与腔光子通过超材料中的单元间相互作用的耦合。为了实现更强的耦合--达到超强耦合状态--我们然后重点开发太赫兹法布里-珀罗腔，这些腔的基模体积比我们迄今为止研究的光子晶体腔更小。随着模式体积的减少，我们的目标是证明比迄今为止获得的极化激元分支更强的拉比分裂。 在第二条工作线中，我们开发了主动切换交互强度的能力。文献已经示出了通过外部控制参数来修改超颖表面的光学性质的方法。这在腔中的强相互作用下应该工作得更好。我们将专注于从开口环谐振器构建的超颖表面，并采用概念来改变其属性的偏置电压施加到单位细胞，或通过吸收的激光辐射撞击他们。通过这项研究，我们打算为将来在应用程序中使用的交换平台做准备。

Cavity (quantum) electrodynamics investigates the strong interaction of matter with photons in a resonant cavity. It has long been studied intensively to investigate fundamental quantum properties in physics such as Schrödinger's cat states and entanglement of photons, and to explore approaches for quantum information processing. Initially, strong coupling was only investigated with natural atoms, while in recent years, it has been realized experimentally in a variety of material systems, employing interband and inter-subband transitions in semiconductor quantum wells and quantum dots, spin resonances in magnetic materials, and bosonic excitations such as cyclotron transitions in 2D electron gases and molecular vibrational transitions in polymers. We have recently demonstrated strong light-matter coupling of metasurfaces, a class of artificial materials, with terahertz photons in a one-dimensional photonic crystal cavity. We have employed plasmonic metallic structures which have been studied extensively in the literature, not least because of their potential for the realization of chemical and biological sensors, optical filters and modulators. With their huge dipole moments, the unit cells of the metasurfaces interact efficiently with the photons in the cavity, forming plasmon-photon polaritons with an enormous Rabi splitting of the upper and lower polariton branches. In spite of this being a classical electrodynamic system, it has turned out that the terminology employed in cavity quantum electrodynamics is useful and applicable.Based on our findings, we intend to pursue two lines of research. One is devoted to a better understanding and an enhancement of this form of light-matter interaction. We want to answer open questions regarding the unexpectedly strong coupling of Babinet-complementary metasurfaces with the photons, and then study the coupling of ‘dark atoms’ with the cavity photons via inter-unit-cell interaction in the metamaterial. In order to achieve even stronger coupling – reaching high into the ultrastrong-coupling regime – we then focus on the development of terahertz Fabry-Perot cavities which exhibit a smaller fundamental-mode volume than the photonic crystal cavities studied by us so far. With this reduction of the mode volume, we aim to demonstrate an even stronger Rabi splitting of the polariton branches than obtained until now. In the second line of work, we develop capabilities to actively switch the interaction strength. The literature has shown ways to modify the optical properties of metasurfaces by external control parameters. This should work even better with the strong interaction in a cavity. We will focus on metasurfaces built from split-ring-resonators, and adopt concepts to change their properties by a bias voltage applied to the unit cells, or by the absorption of laser radiation impinging on them. With this research, we intend to prepare switching platforms for future use in applications.