Detecting the vector of nanoscale light field with atomic defect
Figure 1 Schematic illustration of NV center in diamond.
Figure 2 Reconstructed profile of a radially polarized beam.
The manipulation of light-matter interaction at micro-, nanoscale is an important tool for research in fundamental physics and applied science. With flexible modulation, vectorial optical fields with spatial dependent polarization have drawn wide attention recently. In particular, the tightly focused vectorial optical field are used in areas such as super-resolution microscopy, optical tweezer, nanofabrication and near-field optics. Detecting the light field vector at nanoscale can help to optimize the optical field modulation, and explore the mechanism of light-matter interaction. It needs an optical probe with small size, low perturbation and vector measurement capability.
Recently, atomic-sized single-photon emitters, such as single molecule and single color center, have been used to study light-matter interaction at nanoscale. It provides a potential method to detect light field vector. As one of the most promising candidates, the nitrogen vacancy (NV) center in diamond, which is composed of a nitrogen defect and an adjacent vacancy, shows stable fluorescence emission and long spin coherence time at room temperature. High spatial resolution sensing of electromagnetic field, temperature and stress with NV center has been studied for biology, condensed matter physics and high pressure physics.
Utilizing single NV centers with different symmetry axes as an optical field probe, the research group led by Prof. Fang-Wen Sun at University of Science and Technology of China (USTC) demonstrated the light field vector detection at nanoscale. The vector information of optical field is obtained through measuring the polarization dependent spontaneous emission of NV center. The spatial distribution of vectorial optical field under tight focus is subsequently reconstructed. This work was published in Chinese Optics Letters (Qiyu Wang, Zehao Wang, Xiangdong Chen, Fangwen Sun. Detecting the vector of nanoscale light field with atomic defect[J]. Chinese Optics Letters, 2023, 21(7): 071202) and was selected as the cover of the issue.
The research group chose single crystal diamond as probe, so that the four possible symmetry axes of NV center can be deduced from the shape of diamond plate. The axis of individual single NV center is further determined by measure the electron paramagnetic resonance spectra. In this way, the fluorescence signal from NV centers with different axes can be separated. Since the spontaneous emission of NV center is influenced by the angle between light field vector and the symmetry axis, each NV center will provide part of the information of optical field vector. Combining the information from all four axes, the three-dimensional vector of the optical field is reconstructed with the help of deep learning. In the experiments, the polarization and phase of tightly focused radially polarized light is imaged by scanning the relative position between NV centers and optical field. The results is consistent with theoretical expectation.
Researchers from the group said that, the measurement is highly reliable due to the stable fluorescence emission and well-defined symmetry axis. And the high spatial resolution, is guaranteed by the small size of NV center, though it is limited by the accuracy of scanning setup in this experiment. In the next, the research team will further improve the accuracy by enhancing the fluorescence collection efficiency and introducing NV centers will more axes. The technique can be used to develop solid spin based super-resolution microscopy and quantum sensing.
封面|发光缺陷实现纳米尺度光场矢量测量
封面中为光子准晶光纤（也称为准周期光子晶体光纤），其端面周围的五个插图展示了三种典型的准周期结构及两种代表性的潜在应用，三个白色圆插图（左上、右、左下）分别表示Stampfli型、Penrose型、Sunflower型结构，两个蓝色圆插图（左、右下）分别表示超连续谱、轨道角动量模传输。
Chinese Optics Letters 2023年第7期封面图
图2 利用氮-空位色心对紧聚焦下径向偏振光场的探测与重构
矢量光场
微纳尺度下光与物质相互作用的调控是当前基础物理和应用科学研究的重要手段。由于其灵活的调控能力，具有空间上非均匀偏振分布特性的矢量光场在近年引起广泛关注。尤其是紧聚焦下的矢量光场，被用于光学超分辨成像、光镊、纳米加工、近场光学等领域的研究。对纳米尺度光场矢量的探测将有助于提高光场调控的效率，以及探索光与物质相互作用的机理。实现这一目标需要一种具有小尺寸、低干扰和矢量测量能力等特点的光场探针。
金刚石氮-空位色心
近年来，原子尺寸的单光子源，如单分子、单个发光缺陷，被用于研究纳米尺度下的光与物质相互作用，为探测纳米光场矢量提供了一种潜在方法。金刚石中氮-空位色心是一种最具代表性的发光缺陷。它是由一个氮原子杂质和相邻的空位组成，具有稳定的自发辐射荧光和室温下长自旋相干时间等优点。基于氮-空位色心的高分辨率电磁场、温度和应力传感在生命科学、凝聚态物理和高压物理等研究中得到广泛应用。
纳米尺度光场矢量的测量
中国科学技术大学孙方稳教授团队利用金刚石中不同轴向的单个氮-空位色心作为光场传感器，通过对偏振依赖的自发辐射的测量，得到光场的三维矢量信息，实现紧聚焦下矢量光场空间分布特征的成像与重构。相关工作发表在Chinese Optics Letters第21卷第7期（Qiyu Wang, Zehao Wang, Xiangdong Chen, Fangwen Sun. Detecting the vector of nanoscale light field with atomic defect[J]. Chinese Optics Letters, 2023, 21(7): 071202）被遴选为该期封面。
封面中未知的矢量光场从上方泵浦单晶金刚石中的氮-空位色心，不同取向的氮-空位色心自发辐射携带了光场的偏振、强度等信息，对多个氮-空位色心荧光信号的分析重构出光场矢量的空间分布特性。
研究团队选择单晶金刚石作为光场探针，其中的氮-空位色心有4种可能的对称轴取向。进一步测量电子顺磁共振谱，可以确定每一个氮-空位色心的对称轴取向，从而分离出不同取向的氮-空位色心在同一光场泵浦下的自发辐射信号。由于氮-空位色心的自发辐射强度受到泵浦光场矢量与对称轴之间夹角的影响，对每一种对称轴取向的氮-空位色心的测量便可得到光场矢量的部分信息。综合4种对称轴取向的氮-空位色心信号，并借助深度学习可重构出光场的三维矢量。实验中通过振镜扫描光场与氮-空位色心之间的相对位置，得到径向偏振光在紧聚焦下的偏振与相位分布，与理论预期相符。
展望
该文通信作者陈向东副研究员表示：“得益于单个氮-空位色心极稳定的荧光辐射和确定的轴向，矢量光场的测量具有高可靠性。而氮-空位色心的小尺寸使测量结果能够达到很高空间分辨率，该工作中测量结果的分辨率主要受限于扫描装置的精度。在下一阶段的工作中，研究团队将通过提高荧光采集效率、引入更多轴向的色心等方法进一步提高测量精度，并将该技术用于发展基于固态自旋体系的光学超分辨成像和量子传感。”