Ultrafast optical absorption and coherent phonon generation in monolayer and bilayer transition metal dichalcogenides

When ultrashort laser pulses are applied to the solid-state material within a time scale less than the period of a particular phonon mode, the energy and momentum of the absorbed light are used by photo-excited carriers to interact collectively with the crystal lattices and generate lattice oscillations coherently, known as the coherent phonons. The Fourier transform of the absorption modulation due to the coherent phonon oscillations gives the coherent phonon intensity. In this work, we theoretically investigate the shape of coherent phonon intensity for transition metal dichalcogenides as a function of the polarization angle of the laser pulses. In particular, we focus on the E2g and A1g phonons in monolayer and bilayer molybdenum disulfides. Within the simplified theory of displacive excitation of coherent phonons, we find that the laser pulses uniquely select the electronic states in monolayer and bilayer molybdenum disulfides, resulting in a rich feature of the polar plots of the energy-dependent coherent phonon intensity of these materials.