[ Instrument R & D of Instrumentation Network ] The National Key Laboratory of Mesoscopic Physics, Peking University School of Physics, and the Optoelectronics Frontier Science Center of Academician Gong Qihuang ’s team successfully developed the “femtosecond-nano-ultra-high-space-time resolution” Optical Experiment System ". The experimental system can simultaneously achieve an ultra-high temporal resolution of several femtoseconds and an ultra-high spatial resolution of four nanometers, and it has become a powerful experimental measurement method for meso-tourism and micro-nano photonics research.
Recently, the research team used ultra-high time-space-resolved light emission electron microscopy (PEEM) to reveal for the first time the internal correlation between local surface plasmon near-field enhancement and decoherence time from the near-field microscopic perspective. Related research results are entitled "Correlation" between near-field enhancement and dephasing time in plasmonic dimers "was published on Physical Review Letters (DOI: 10.1103 / PhysRevLett. 124.163901), the authoritative journal of physics, on April 24. For the first time, the research team also fully revealed the ultra-fast electron cooling and relaxation dynamics of single-layer WS2 from the two dimensions of time and energy population evolution. -Resolved Photoemission Electron Microscopy "was published on Nano Letters (Nano Letters, DOI: 10.1021 / acs.nanolett.0c00742), an important journal in the field of nano on April 3
In the surface plasmon photonics experiment, the team took advantage of the high spatial resolution of PEEM to directly observe the near-field distribution of the local surface plasmon mode in the gold nanostructure dimer array system (Figure 1). Excitation light wavelength-dependent light emission intensity measurement and ultrashort pulse-based light emission autocorrelation measurement, respectively, obtained the near-field enhancement and decoherence time of the surface plasmon of the same structure, and found that the correlation between the two depends on gold The nanostructure dimer gap and the polarization direction of the excitation light (Figures 2 and 3) reveal for the first time that this correlation is determined by the near-field and far-field coupling and the localization of the nanostructure. The research results are of great significance for understanding the basic physical problems in surface plasmon photonics and expanding the application research of surface plasmons in high-sensitivity detection and sensing, solar cells and other micro-nano photonic devices.
Figure 1. Schematic diagram of gold nanodisk dimer structure, SEM and PEEM images
Figure 2. Near field characteristics of gold nanorod dimer structure measured by PEEM under longitudinal polarization and ultrafast dynamics of local surface plasmon
Figure 3. Near field characteristics of gold nanodisk dimer structure measured by PEEM under lateral polarization, and ultrafast dynamics of local surface plasmon
In the study of the ultrafast electron cooling and relaxation dynamics of the single-layer WS2, the team found that two time-scale ultrafast dynamic processes exist on the substrate and the suspended single-layer WS2 (Figure 4) With the electron cooling and defect trapping process, two time scale processes can be observed from the attenuation curve, which are about 0.3 ps and 3 ps, respectively. Through energy-resolved PEEM measurements (Figure 5), the team found that the first process corresponds to the cooling of electrons in the conduction band, and the second process reflects the relaxation of electrons at the bottom of the conduction band. By comparing the PEEM measurement of the suspended single-layer WS2 sample (Figure 6), combined with the fluorescence and Raman spectroscopy characterization, it was found that the relaxation process is mainly related to the defect state. This study uses PEEM's resolution capabilities in space, time and energy to reveal the dynamic process of ultra-fast electron cooling and defect trapping of typical single layer WS2 of TMDs. The study also found that the generation of defect states is related to the illumination under vacuum. The generation method of this defect and its significant effect on the dynamic process are notable in general light emission experiments and spectral measurements.
Figure 4. WS2 / hBN / p-Si sample structure and time-resolved PEEM measurement
Figure 5. WS2 / hBN / p-Si sample time and energy-resolved PEEM measurement, the electron energy distribution curve can be fitted by Fermi-Dirac distribution
Figure 6. Time-resolved PEEM measurement of a suspended single-layer WS2 sample
The relevant research work was completed by the team of Peking University, Professor Hiroaki Misawa, Institute of Electronic Science, Hokkaido University, Japan, and Prof. Tan Pingheng, researcher of the Institute of Semiconductors, Chinese Academy of Sciences. Peking University doctoral student Li Yaolong is the first author of two articles. The research work has obtained the key research and development plan of the Ministry of Science and Technology, the National Natural Science Foundation of China, the State Key Laboratory of Artificial Microstructures and Mesoscopic Physics, the Collaborative Innovation Center for Quantum Matter Science, the Extreme Optical Collaborative Innovation Center and the Nanoelectronics Frontier Science Center, and the Ministry of Science and Technology Support from the Provincial and Japan Academic Promotion Association.

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