The plasmon cavity system composed of a scanning tunneling microscope tip and a substrate has attracted much attention due to its ability to break through the diffraction limit, enhance the electromagnetic field by hundreds of times, and localize it on a nanometer or even sub-nanometer scale. This plasmon cavity system can serve as an advanced platform for studying superradiance phenomena on an ultrafast scale. Methylene blue molecules have many applications in the field of optics due to their significant light absorption and fluorescence emission characteristics. In this work, macroscopic quantum electrodynamics and open quantum system theory are used to explore the radiation dynamics of methylene blue molecular clusters with three different configurations: cyclic, two-dimensional planar, and one-dimensional chain, in specific scanning tunneling microscope nanocavity and picocavity. Taking the cyclic molecular clusters for example, the radiation effects of different external field excitations on the molecular clusters in the cavity are studied. The research results indicate that for the same molecular cluster configuration, the scanning tunneling microscope picocavity has a more significant superradiance intensity, while the scanning tunneling microscope nanocavity has a longer duration of superradiance. From the perspective of symmetry, the one-dimensional chain molecular clusters only have axial symmetry, while the two-dimensional planar and cyclic molecular clusters have both axial symmetry and central symmetry. The cyclic molecular clusters also have multiple rotational symmetries. Therefore, within the same scanning tunneling microscope cavity, the higher the symmetry of the arrangement of molecular clusters, the easier it is to generate significant superradiance pulses. In addition, because of its higher spatial resolution and stronger local field enhancement effect, the picocavity of scanning tunneling microscope is more sensitive to changes of external conditions such as excitation wavelength. These results indicate that the occurrence and enhancement of superradiance can be effectively controlled by reasonably designing the cavity structure and geometric configuration of molecular clusters, and the time scale of superradiance pulses can be extended to the picosecond order, which provides new ideas and methods for practical applications in the fields of optics and nanotechnology in the future.
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