Exploring the light-matter interaction in a new-type hybrid structure: realizing strong plasmon-exciton-cavity coupling at room temperature

Surface plasmons (SPs) are collective electron oscillations of the metallic nanostructure under the excitation of external electromagnetic waves, which have a series of novel optical properties. Surface plasmons can break through the diffraction limit and tightly confine the incident light into sub-wavelength scales, which result in a strongly enhanced local field with ultrasmall mode volume. By utilizing these properties of surface plasmons, light-matter interaction can be significantly enhanced, and the optical absorption and emission of materials can be also promoted.
    In recent years, the interaction between surface plasmons and transition metal disulfides (TMDCs) has generated broad extensive attention and research. According to the strength of interaction, the interaction between surface plasmon and TMDCs can be divided into two categories: weak coupling and strong coupling. When the coherent exchange rate of energy between light and matter is higher than the decay rate, it reaches the strong coupling regime, where a new surface plasmon-exciton coupling hybrid state is formed. This new coupled hybrid state will provide a number of possibilities in fascinating advances such as the low-threshold lasing, Bose-Einstein condensation, chemical reactivity tuning, and optical spin switching, etc. However, so far, the Rabi splitting obtained by the strong coupling between surface plasmon and TMDCs exciton is usually about 100 meV, which is much smaller than that of other materials (such as J-polymer, semiconductor quantum dots, etc.). The small Rabi splitting greatly limits the application of TMDCs in the study of strong coupling. In addition, due to the inherent loss of metallic surface plasmon, the observation of strong coupling is significantly limited.

Schematic of the optical microcavity with an embedded Ag-WS2 heterostructure

    To solve these problems, by fabricating a plasmon-exciton-cavity hybrid structure, Fang's team at Peking University successfully realized strong plasmon-exciton-cavity coupling under ambient conditions, and obtained about 300 meV Rabi splitting. In experiment, the plasmon-exciton-cavity hybrid structure was fabricated by embedding Ag-WS2 heterostructure in an optical microcavity. In the strong coupling regime, three different hybridized modes, as the upper branch, middle branch, and lower branches were generated, and each hybrid branch is an admixture contribution from the plasmon, cavity and exciton. By calculating the linewidths and Hopfield coefficient of hybridized modes using harmonic oscillator model, the criteria of the strong coupling for three oscillators was proposed for the first time. The proposed three-oscillator configuration provides an effective way to develop the strong coupling effect for the practical application of plexciton polaritonic device at room temperature based on low dimensional materials.

Doctor Zheyu Fang is a Professor in School of Physics, Peking University, China. He received his PhD in Physics from Peking University with Prof. Xing Zhu, and worked as Postdoc at Rice University with Prof. Naomi J Halas and Prof. Peter J Nordlander. He has published more than 100 peer reviewed papers with 6000 citations. He joined Peking University in 2012 and was selected as the National Top-notch Young Professionals in 2014. His current research interests are plasmonics, near-field optics, and nanophotonic materials and devices.

Li B W, Zu S, Zhang Z P, Zheng L H, Jiang Q et al. Large Rabi splitting obtained in Ag-WS2 strong-coupling heterostructure with optical microcavity at room temperature. Opto-Electron Adv 2, 190008 (2019).