Electron-phonon interactions in MoS₂ probed with ultrafast two-dimensional visible/far-infrared spectroscopy

An ultrafast two-dimensional visible/far-IR spectroscopy based on the IR/THz air biased coherent detection method and scanning the excitation frequencies is developed. The method allows the responses in the far-IR region caused by various electronic excitations in molecular or material systems to be observed in real time. Using the technique, the relaxation dynamics of the photo-excited carriers and electron/phonon coupling in bulk MoS₂ are investigated. It is found that the photo-generation of excited carriers occurs within two hundred fs and the relaxation of the carriers is tens of ps. The electron-phonon coupling between the excitations of electrons and the phonon mode E_1u of MoS₂ is also directly observed. The electron excitation shifts the frequency of the phonon mode 9 cm^-1 higher, resulting in an absorption peak at 391 cm^-1 and a bleaching peak at 382 cm^-1. The frequency shift diminishes with the relaxation of the carriers.