Ultrafast Dynamics of Excitonic Rydberg Series in Single Layer WSe₂

Single layer transition metal dichalcogenides (TMD) such as MoS₂ and WSe₂ have been the subject of intense attention for their optically accessible valley polarization. In these materials, circularly polarized light can be used to break the time-reversal symmetry and lift the degeneracy of the K and K’ valleys. Such optical control is of great importance for possible valleytronic applications. In this work, we will present broadband transient reflection microscopy results obtained from hBN capped single layer WSe₂ flakes. In these high quality samples, it is possible to observe the excitonic Rydberg states in the reflectivity data. We have studied the dynamics of these states after photo-excitation via pump-probe spectroscopy. When the excitation light energy is tuned below the band gap, high quality of these samples enable observation of novel valley selective coherent response in addition to the previously observed optical Stark shift.