The generation of electronic coherence of quantum materials is of great significance for multi-body interactions and related regulation. However, this is not easy, and many advanced and sophisticated electrical experimental methods are non-coherent and cannot induce and measure collective excited states. The interaction between coherent light and matter can naturally convey the inherent coherence of the light field to the quantum material and can be used to regulate the coherence of the electron. Whether this coherence transfer can be achieved depends on the form of interaction between light and matter and the electronic structure of matter.
Recently, Ji Jimin, a research associate of the Institute of Physics of the Chinese Academy of Sciences/Beijing National Laboratory for Condensed Matter Physics, and Meng Sheng, a researcher, have cooperated to achieve a 2D layered quantum material MoS2 with bandgap through spatial self-phase modulation. The electronic coherence of the exchange. When no light field is applied, the discrete MoS2 sheets are suspended in the liquid and the electrons are completely independent and have no phase coherence with each other. After irradiation with an ultrafast laser pulse, spatially self-phase-modulated scattering of light and matter (Fig. 1A), the phase of the non-local electron wave function becomes identical to that of the ultrafast laser pulse, and their phases are completely locked. This coherence is a light-induced electronic collective behavior. The author proposes a "wind chime model" to explain this phenomenon of deduction. They further confirmed the existence of this non-local electronic coherence by measuring the time required for spatial self-phase modulated diffraction ring formation (Figures 1B & 1C).
They studied the self-phase modulation based on band gap in different wavelength excitations (Figs. 2 & 3A). It was found that this kind of electronic coherence can be applied to general two-dimensional layered quantum materials with universal characteristics. Their experiments first observed spatial self-phase modulation lower than the band gap, confirming the physical mechanism including two-photon spatial self-phase modulation (Figures 3B & 3C).
They have experimentally demonstrated that this non-local AC electronic coherence can be used to achieve two-color all-optical switches, and has some excellent performance (Figure 4). In particular, the control beam can modulate the phase of the signal beam, change with a small intensity can cause the diffraction ring of the strong signal beam to change in space, and achieve the switching effect of weak light to control strong light. This is the first time that people have realized all-optical switching effects based on spatial self-phase modulation. These work provide the basis for the application of two-dimensional layered quantum materials in photonics.
This work was supported by the Major Basic Research (“973â€) Program of the Ministry of Science and Technology, the National Natural Science Foundation of China, and the Chinese Academy of Sciences’ key cooperation project on foreign cooperation. This work was published in the recent Proc. Natl. Acad. Sci. USA 112, (38) 11800-11805 (2015).
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