China University of Science and Technology has made progress in the study of single photon super-radiation in the molecular chain system

[ Instrument Network Instrument Development ] Recently, Dong Zhenchao and Zhang Yang Research Group, the vice president of the Chinese Academy of Sciences, academician of the Chinese Academy of Sciences, and Professor Hou Jianguo of the University of Science and Technology of China, used the scanning tunneling microscope (STM) to induce single-molecule electroluminescence. For the first time, the technology clearly shows the single-photon super-radiation phenomenon of artificially constructing the molecular chain structure of indigo dye in the plasmonic nano-cavity, and studies the effect of nano-cavity plasmons on the super-radiation behavior of molecular chains. The International Physical Academic Journal "Physical Review Letter" published this result online on June 12.

(a) Schematic diagram of STM measurement of molecular chain luminescence; (b) STM topography of molecular chain system; (c) Real-space electrofluorescence imaging of single-photon super-radiation mode; (d) Photon emission of molecular chains of different lengths Second-order correlation function measurement results.

In molecular systems, multiple illuminants can form delocalized collective states through coherent dipole interactions, which play an important role in intermolecular energy transfer, photosynthesis, and quantum system design. The single photon super-radiation state is one of the particularly interesting delocalized collective states with quantum coherence properties, characterized by the in-phase coupling of all luminescent monomers, and its quantum nature is all possible direct accumulation states (a certain luminescent monomer is excited The quantum superposition of all other illuminants in the ground state. However, since the coherent dipole interaction between molecules is affected by many factors such as the number of molecules, the molecular spacing, and the dipole orientation, it is very difficult to precisely control the molecular system and quantitatively study the related single photon super-radiation state.

China University of Science and Technology's single-molecule science team has long been committed to the development of a combination of STM high-resolution spatial characterization and optical technology with high-sensitivity spectroscopy, especially by cleverly regulating the broadband, localized and singularity of tunnel junction plasmons. The enhanced characteristics greatly enrich the measurement and control methods, expand the measurement limit, and provide a new opportunity for observing and regulating the photoelectric behavior of molecules on a single molecular scale. In this work, they combined STM's single-molecule handling capability, the decoupling of a thin layer of insulating sodium chloride, and the localized enhancement of nanocavity plasmons to construct 2 to 12 indigo dyes. The ordered molecular chain structure of the molecule, and the luminescence properties of these molecular chain systems excited by local tunneling electrons and their evolution with chain length are studied.

Using sub-nano spatially resolved electrofluorescence imaging techniques, they found that the luminescence intensity mode with the highest luminescence intensity and lowest energy corresponds to that all molecular dipoles are coupled together in a collinear in-phase manner, ie, the luminescence mode corresponds to the super-radiation mode. . Further measurement of the statistical properties of the molecular chain photon emission indicates that the luminescence of the molecular chain has a single photon emission characteristic, that is, the entire molecular chain is in a single exciton state and must be treated as a single quantum system. This indicates that when a molecule in the molecular chain system is excited by local electrons, the excitation energy rapidly delocalizes to the entire molecular chain, forming a single exciton super-radiation state, which in turn produces single-photon super-radiation.

In addition, they also found that nanocavity plasmons affect the broadening and intensity of single-photon super-radiation peaks and their evolution along with chain length, but do not affect the intrinsic coherence of super-radiation states established by intermolecular dipole coupling. characteristic. These results not only provide a new understanding of the molecular model of the molecular system and their interaction with local nanocavity plasmons, but also provide a new way to study the interaction between molecules and multi-body interactions.

Luo Yang and Chen Gong are the co-first authors of this article. The research work of this series was supported by the Ministry of Science and Technology, the Fund Committee, the Chinese Academy of Sciences, the Ministry of Education, and Anhui Province.

(Original title: China University of Science and Technology has made progress in the study of single-photon super-radiation in the molecular chain system)