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

如今，获得太赫兹（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、分子振动和磁激发的研究至关重要。长期以来，由于缺乏有效的辐射源和探测器，人们无法探索辐射与物质相互作用较弱的频率范围。通过实现能够与THz辐射相互作用的人造（Meta）材料来解决这一限制。其中，超表面，亚波长平面谐振器阵列，可以控制自然响应之外的电磁波，例如用于超高感测应用的集中电场。太赫兹电子自旋共振（THz ESR）是一种功能强大且用途广泛的技术，用于研究具有未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，由于太赫兹源的功率有限，无法对体积有限的样品进行太赫兹ESR实验，这严重限制了太赫兹ESR的应用。在受限体积中增强磁场的超表面谐振器的实现将极大地解决该灵敏度问题。该项目旨在实现超表面谐振器（MR），以在二维区域中定位THz磁场，从而允许通过THz ESR检测体积受限和薄膜样品。在该项目中，MR将在磁共振实验中设计、制造和利用。作为一个用例，我们将专注于分子自旋量子比特（MSQs）。增强的灵敏度将被用来了解量子比特在沉积在表面后的性质如何变化，这是实现功能量子器件的重要一步。从长远来看，磁性超颖表面将成为覆盖广泛应用的表面敏感磁共振实验的广泛工具。

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.