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The plasmon cavity system formed by the scanning tunneling microscope tip and 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 at the nanometer or even sub-nanometer scale. The plasmon cavity system formed by the scanning tunneling microscope tip and substrate can serve as an advanced platform for studying superradiance phenomena at the ultrafast scale. Methylene blue molecules have a wide range of applications in the field of optics due to their significant light absorption and fluorescence emission characteristics. This article applies macroscopic quantum electrodynamics and open quantum system theory 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 as an 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, one-dimensional chain molecular clusters only have axial symmetry, while two-dimensional planar and cyclic molecular clusters have both axial symmetry and central symmetry. Cyclic molecular clusters also have multiple rotational symmetries, so in the same scanning tunneling microscope cavity, the higher the arrangement symmetry of molecular clusters, the easier it is to generate obvious superradiance pulses. In addition, the scanning tunneling microscope picocavity is more sensitive to changes in external conditions such as excitation wavelength due to its higher spatial resolution and stronger local field enhancement effect. These results indicate that by designing the cavity structure and geometric configuration of molecular clusters reasonably, the occurrence and enhancement of superradiance phenomena can be effectively controlled, and the time scale of superradiance pulses can be extended to the picosecond level, providing new ideas and methods for future practical applications in the fields of optics and nanotechnology.
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Keywords:
- Surface plasmon /
- Scanning tunneling microscope cavity /
- Geometric configuration of molecular clusters /
- Superradiance
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图 1 (a) 针尖尖端半径为42.5 nm, 高度为93.5 nm的银针尖与银衬底形成的STM纳米腔; (b) 针尖尖端半径为1 nm, 高度为112.5 nm的银针尖与银衬底形成的STM皮米腔; 其中腔中的蓝色区域表示不同构形的亚甲基蓝分子团簇
Figure 1. (a) An STM nanocavity formed by a silver needle tip with a radius of 42.5 nm and a height of 93.5 nm with a silver substrate; (b) An STM picocavity formed by a silver needle tip with a radius of 1 nm and a height of 112.5 nm with a silver substrate; The blue areas in the cavity are clusters of methylene blue molecules of different configurations.
图 2 (a) 9个全同亚甲基蓝分子组成的半径为2 nm的环状分子团簇; (b) 连续激光激发下STM纳米腔中的远场辐射谱; (c) 连续激光激发下STM皮米腔中的远场辐射谱
Figure 2. (a) A circular molecular cluster with a radius of 2 nm composed of 9 identical methylene blue molecules; (b) Far-field radiation spectra in STM nanocavity by continuous laser excitation; (c) Far-field radiation spectra in STM picocavity by continuous laser excitation.
图 3 (a) 连续激光激发下Dicke态量子数平均值表示的STM纳米腔中的分子动力学; (b) 不同激发时长时, STM纳米腔中用Dicke态量子数的平均值表示的分子动力学; (c) 不同激发时长时, STM皮米腔中用Dicke态量子数的平均值表示的分子动力学; 其中蓝色虚线表示Dicke态边界
Figure 3. (a) Molecular dynamics in STM nanocavity represented by the mean value of quantum numbers in Dicke states by continuous laser excitation; (b) Molecular dynamics in STM nanocavity expressed by the average of the quantum numbers of the Dicke states at different excitation durations; (c) Molecular dynamics in the STM picocavity expressed by the average of the quantum numbers of the Dicke states at different excitation durations; where the blue dotted line represents the Dicke state boundary.
图 4 (a) 持续时间为28 fs的激光脉冲激励下, STM纳米腔中远场辐射谱; (b) 持续时间为13 fs的激光脉冲激励下, STM皮米腔中远场辐射谱; 插图分别对应STM纳米腔和皮米腔中用集体自旋矢量表示的分子动力学, 其中灰色线表示最外层球面
Figure 4. (a) Far-field radiation spectra in STM nanocavity by laser pulse excitation with a duration of 28 fs; (b) Far-field radiation spectra in STM picocavity by laser pulse excitation with a duration of 13 fs; The insets correspond to molecular dynamics represented by collective spin vectors in STM nanocavity and picocavity, respectively, where the gray line represents the outermost sphere.
图 5 (a) 9个全同亚甲基蓝分子组成的二维平面构形分子团簇; (b) 持续时间为28 fs的激光脉冲激励下, STM纳米腔中远场辐射谱; (c) 持续时间为11 fs的激光脉冲激励下, STM皮米腔中远场辐射谱
Figure 5. (a) A two-dimensional planar configuration molecular cluster composed of 9 identical methylene blue molecules; (b) Far-field radiation spectra in STM nanocavity by laser pulse excitation with a duration of 28 fs; (c) Far-field radiation spectra in STM picocavity by laser pulse excitation with a duration of 11 fs.
图 6 (a) 9个全同亚甲基蓝分子组成的一维链状分子团簇; (b) 持续时间为33 fs的激光脉冲激励下, STM纳米腔中远场辐射谱; (c) 持续时间为9 fs的激光脉冲激励下, STM皮米腔中远场辐射谱
Figure 6. (a) A one-dimensional chain of molecular clusters composed of 9 identical methylene blue molecules; (b) Far-field radiation spectra in STM nanocavity by laser pulse excitation with a duration of 33 fs; (c) Far-field radiation spectra in STM picocavity by laser pulse excitation with a duration of 9 fs.
图 7 波长620 nm到700 nm, 强度为$ I_{\text{ext}} = 10^{3} $ μW/μ$ \text m^{2} $激光激发下 (a) STM纳米腔中环状分子团簇的远场辐射谱; (b) STM皮米腔中环状分子团簇的远场辐射谱
Figure 7. By laser excitation with wavelengths from 620 nm to 700 nm and an intensity of $ I_{\text{ext}} = 10^{3} $ μW/μ$ \text m^{2} $ (a) Far-field radiation spectra of clusters of cyclic molecules in STM nanocavity; (b) Far-field radiation spectra of clusters of cyclic molecules in STM picocavity.
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