All-Optical Probe and Control in Atomically-thin Transition Metal Dichalcogenides

Chaw-Keong Yong^1*

¹Physics, National Taiwan University, Taipei, Taiwan

* Presenter:Chaw-Keong Yong, email:chawkyong@phys.ntu.edu.tw

Atomically thin transition metal dichalcogenides (TMDs) layered materials exhibit fascinating quantum phenomena owing to their unique electronic structure, reduced symmetry, and strong many-body interactions. For example, due to the broken inversion symmetry in monolayer TMDs, a pair of degenerate excitonic states exist at the K and K’ valleys in momentum space, giving rise to a unique valley degree of freedoms in excitonic transitions. The strong light-matter interaction and rich valley physics inherited in TMDs monolayers and heterostructures makes TMDs an attractive platform to explore light-driven quantum phenomena. Yet, the quantum physics in condensed matters depends crucially on the ultrafast dynamics of their atomic constituent. The ability to control the quantum mechanical behaviours of electrons at ultrafast time scales and probe their evolution stroboscopically not only advances the frontiers of physics in mesoscopic condensed matters but also provides crucial aspects to the development of quantum optoelectronic and information technology.

In this talk, I will show you our efforts in probing and controlling the quantum phenomena in atomically thin TMDs materials arise from many-body interactions and Berry curvature, using ultrafast nonlinear optical spectroscopy [1,2]. First, I will show you that the prominent many-body interactions in an atomically thin MoSe2 layer break down the conventional valley selection rules based on the non-interacting exciton picture and provides an additional route to manipulate the valley-excitons in monolayer MoSe2 that leads to rich set of light-driven coherent phenomena [1]. We further show that the Coulomb correlation of quasiparticles in a bilayer system can be systematically controlled by twisting their atomic interface, providing a powerful tuning knob to manipulate the electron dynamics. Secondly, I will show you the Berry curvatures dramatically modify the exciton spectrum in monolayer MoSe2. We implement helicity-dependent intraexcitonic optical Stark spectroscopy to demonstrate that the Berry curvature leads to splitting of the exciton fine-structure with opposite angular momentum in monolayer MoSe2, a quantum phenomenon that has long been predicted by theory but never been observed before [2]. Finally, with extremely high optical field of femtosecond pulses, we demonstrate all-optical manipulation of quantum states of matters that lead to valley-dependent Autler-Townes doublets and exciton transition [2]. Our studies open up new possibilities to explore the intriguing electron dynamics in mesoscopic condensed matters that are related to the many-body physics, valley/spin physics, light-controlled quantum phenomena at ultrafast time scale using femtosecond optical spectroscopy.

1. Yong et. al. Nature Physics, vol. 14, pp. 1092, 2018
2. Yong et. al. Nature Materials, vol. 18, pp. 1065, 2019

Keywords: ultrafast optical manipulation, many-body correlation, valley-pseudospin, quantum optics, two-dimensional materials