Optical 2D Coherent Spectroscopy of Valley Dynamics in Monolayer Transition Metal Dichalcogenide

Atomically thin layers of transition metal dichalcogenides (TMDs) have unique electronic and optical properties, offering the possibility of novel applications in electronics, optoelectronics and valleytronics. These applications require a fundamental understanding of the valley dynamics involving the carrier radiative and non-radiative recombination, valley polarization, and valley coherence. We will present our study of valley dynamics in monolayer TMD by using optical two-dimensional coherent spectroscopy (2DCS). Compared to conventional one-dimensional spectroscopic techniques, optical 2DCS has many advantages such as separating homogeneous and inhomogeneous linewidths, isolating relaxation pathways, detecting valley coherence and measuring coherence dephasing time. In rephasing 2D spectra, homogeneous and inhomogeneous linewidths are associated with line shape in the cross-diagonal and diagonal directions, respectively. The homogeneous linewidth can be extracted from a cross-diagonal fit to give the coherence dephasing time. The measurement is repeated with various excitation intensities and sample temperatures to extract the intrinsic dephasing time at zero power and temperature. By using various combinations of excitation pulse helicities, our experiment can selectively excite and detect a particular valley population and coherence. This allows to isolate and measure valley exchange and coherence between the two valleys.