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.
Figure 1 Schematic illustration of NV center in diamond.
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.
Figure 2 Reconstructed profile of a radially polarized beam.
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.
