Metasurface-Enhanced Terahertz Magnetic Resonance to Address Molecular Spin Qubits on Surfaces

如今，访问太赫兹（THz）频率的技术进步对于超高速通信、安全扫描仪、基于指纹的材料识别、分子振动和磁激励的研究至关重要。长期以来，缺乏有效的源和探测器阻碍了探索辐射物质相互作用弱的频率范围。通过实现能够与太赫兹辐射相互作用的人工（元）材料，实现了这一限制的解决方案。其中，亚波长平面谐振器阵列的超表面可以控制超出自然响应的电磁波，例如用于超高传感应用的集中电场。太赫兹辐射可用于激发磁跃迁：太赫兹电子自旋共振（THz ESR）是一种强大而通用的技术，用于研究具有不成对电子自旋的材料，与较低的标准频率（9-35 GHz）相比，它允许更高的光谱分辨率和更广泛的材料和现象。然而，由于太赫兹源功率有限，无法在体积有限的样品上进行太赫兹ESR实验，严重限制了太赫兹ESR的应用。超表面谐振器在有限体积内增强磁场的实现将极大地解决这一灵敏度问题。该项目旨在实现超表面谐振器（MRs）在二维区域内定位太赫兹磁场，从而通过太赫兹ESR检测体积有限的薄膜样品。在该项目中，MRs将被设计、制造并用于磁共振实验。作为一个用例，我们将关注分子自旋量子位（msq）。增强的灵敏度将用于了解量子比特在表面沉积后的特性变化，这是实现功能量子器件的重要一步。从长远来看，磁性超表面将成为一种广泛应用于表面敏感磁共振实验的工具。

Technological advances to access Terahertz (THz) frequencies are nowadays of crucial importance for ultra-fast communication, security scanners, identification of materials based on their fingerprint, investigation of molecular vibrations and magnetic excitations. For long time, the lack of efficient sources and detectors prevented to explore this frequency range where radiation-matter interaction is weak. A solution to this limitation was achieved by the realization of artificial (meta)materials able to interact with THz radiation. Among them, metasurfaces, arrays of subwavelength planar resonators, can control electromagnetic waves beyond natural responses, for example concentrating electric fields for ultra-high sensing applications. THz radiation can be used to excite magnetic transitions: THz electron spin resonance (THz ESR) is a powerful and versatile technique for investigating materials with unpaired electron spins that, compared to lower standard frequencies (9-35 GHz), allows a higher spectral resolution and the access to a wider range of materials and phenomena. However, the limited power of THz sources precludes THz ESR experiments on volume-limited samples, which seriously limits its application. The realization of metasurface resonators enhancing magnetic fields in a confined volume would greatly solve this sensitivity issue. The project aims to realize metasurface resonators (MRs) to localize THz magnetic fields in a two-dimensional area, thus allowing detection of volume-limited and thin film samples by THz ESR. Within the project, MRs will be designed, fabricated and exploited in magnetic resonance experiments. As a use case, we will focus on molecular spin qubits (MSQs). The enhanced sensitivity will be exploited to understand how the qubits properties change after deposition on surface, which is an essential step towards the realization of functional quantum devices. In the long term, magnetic metasurfaces will become a widespread tool for surface-sensitive magnetic resonance experiments covering a broad range of applications.