Atomic coherence and interference play an important role in the study of the atom-photon interactions. Electromagnetically induced transparency (EIT) is an extensively studied two-photon coherence phenomenon theoretically as well as experimentally. EIT is mainly observed in three-level atomic systems which causes transparency by quenching absorption of the medium. In this paper, based on the lambda type three-level system including energy level |1>, |2> and |3>, a microwave driving field is introduced between the excited-state energy level |3> and another excited-state energy level |4> to form an inverted Y-type four-level system. We theoretically study the two- and three-photon coherence in this system. The results show that the coupling field makes the probing absorption intensity at the resonant frequency have a very narrow line-width depression, i.e., EIT. The microwave field causes a dynamic Stark splitting of the energy level |3> and induces the Aulter-Townes double peaks. Their frequency interval is exactly equal to the Rabi frequency of the microwave field. The presence of all three fields induces wide window of EIT at the line center owing to the enhanced depression results. The transient evolution is also discussed to understand the optical switching process in the system. Our theoretical study will be helpful to get a deeper insight into the three-photon effects in multilevel systems.
Multilayer graphene (MLG) produced by micro-mechanical exfoliation can usually be stacked layer by layer in a Bernal way through van der Waals coupling. During the exfoliation, a partial bilayer graphene (BLG) is folded onto the BLG flake itself to form the exfoliated twisted (2+2)LG. In this paper, we measured Raman spectra of a few pieces of twisted (2+2)LGs with different twisted angles in back-scattering at room temperature with a HR Evolution micro-Raman system. The modes on both sides of G mode were measured to be a signature to distinguish the twisted angle and determine the layer number in twisted (2+2)LGs. The further research was extended to a twisted (3+3)LG and some results obtained in the twisted (2+2)LGs were confirmed. These results provide an applicable approach to probe the interlayer coupling in twisted graphenes and thus benefit the future research studies on their fundamental physics and potential applications.
Tremendous interest has recently focused on the layered TMDs. Layer number is one of the fundamental parameters in TMDs. In this paper, layer-number dependent reflectivity of WS2 and WSe2 flakes on SiO2/Si substrate were measured by a simple and fast reflection spectrum probing technique. Characteristic excitonic peaks, A and B, and some higher energy density of states excitonic peaks were observed and their properties as a function of layer number were studied. Our results are in agreement with the previous reports.