The strong coupling between light and matter is of great scientific significance as a basic quantum optical phenomenon. When the coupling intensity between the quantum emitter and the optical cavity exceeds the average loss of the two, strong coupling occurs, forming a polarized excitator state of some optical part of the substance, which has important application value in the fields of Bose-Einstein condensation, polarized excitator laser, and quantum information. The mode volume of the dielectric cavity is constrained by the diffraction limit, which limits the coupling intensity it can achieve. Surface plasmons can limit the light field to the nanoscale space, to achieve light field manipulation that breaks through the diffraction limit, so the plasmon nanostructure becomes a nano-optical cavity with an ultra-small mode volume. The reduction of the pattern volume makes the coupling of the plasmon nanostructure and the quantum luminous body stronger, and strong coupling can occur at room temperature, and even strong coupling at the single exciton level can be achieved.
Figure 1. (a) Schematic diagram and light microscope image of a silver nanowire-monolayer WSe coupling system. (b) Transmission and fluorescence spectra of monolayer WSe on a glass substrate. (c) Scattering spectra of silver nanowires on the glass substrate (I, III) and on the monolayer WSe (II, IV).
At present, the experimental study of the surface plasmon and exciton strong coupling system mainly uses scattering, reflection and transmission spectra, and strong coupling will lead to the splitting of these spectra, reflecting the formation of each polarized excitator state. However, in photogenesis spectroscopy (fluorescence spectroscopy), the spread of the spectrum or the polarized excitator state of low energy is mainly observed. The origin of the fluorescence line features of these strongly coupled systems remains unclear. The fluorescence process is more complex than optical processes such as scattering. In a coupled system, partial fluorescence is emitted through scattering of surface plasmons. In addition, the detected fluorescence signal usually also contains the fluorescence of excitons that are not coupled to the surface plasmon. Due to the complexity of the fluorescence emission process and signal composition, it is more difficult to extract strongly coupled information from fluorescence spectra.
Figure 2. Fluorescence spectra (a, d), normalized fluorescence spectra (b, e) and corresponding scattering spectra (c, f) of the silver nanowire-monolayer WSe coupling system.
The Research Group of Researcher Wei Hong of the Institute of Physics of the Chinese Academy of Sciences/Beijing National Research Center for Condensed Matter Physics cooperated with the research group of Professor Xu Hongxing of Wuhan University to conduct research on the above problems. The research team designed a coupling system between silver nanowires and monolayer WSe, because silver nanowires support the transmission of surface plasmons, making it possible to distinguish fluorescence signals from different sources, providing an ideal system for analyzing the basic physical process of surface plasmon coupling and exciton coupling and the spectral characteristics of strong coupling.
Rabbi cleavage caused by the coupling of one or two surface plasmon patterns and excitons in the scattering spectrum of the coupling system (Figure 1), the size of the rabbi cleavage exceeds the average loss of the surface plasmon and excitons, indicating the occurrence of strong coupling. The fluorescence emitted through the surface plasmon was extracted by the transport of the surface plasmon on the surface of the nanowire, and the results showed that the fluorescence spectrum and the scattering spectrum had the same linear characteristics, and the fluorescence spectrum also showed two polarized excitator states produced by the strong coupling of the surface plasmon-exciton (Figure 2). The fluorescence spectrum of the coupled system was calculated by using the coupled oscillator model and combining the fluorescence spectrum of the monolayer WSe, and the results of the computational data analysis were consistent with the experimental results (Figure 3). This study reveals the relationship between the fluorescence emission process and the surface plasmon scattering process in the surface plasmon-exciton strong coupling system, clarifies the reasons for the formation of spectral characteristics of fluorescence, and provides new methods and new ideas for in-depth understanding of the rich spectral phenomena of surface plasmon-exciton coupling system.
Figure 3.(a) The relationship between the energy of the experimental fluorescence spectrum (solid point) and the calculated fluorescence spectrum (hollow point) fitted peak as a result of the energy of the low-energy plasmon pattern. (b) The relationship between the intensity ratio of the low-energy polarized exciton peak and the exciton peak in the experimental fluorescence spectrum.
The research was published in Physiological Review Letters 128, 167402 (2022) under the title "Unified Scattering and Photoluminescence Spectra for Strong Plasmon-Exciton Coupling". The first author is Doctoral Student Niu Yijie. The research was funded by the National Natural Science Foundation of China and the Chinese Academy of Sciences.
Article link: https://link.aps.org/doi/10.1103/PhysRevLett.128.167402
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