Coherent Control of Metastable States - A View from Behind the Computer Screen

Creating, understanding, and controlling metastable states of quantum matter is highly interesting due to the prospects of enabling ultrafast and energy efficient devices with novel functionality. Recent estimates indicates that non-thermal pathways to metastable phases may require several orders of magnitude less energy than a thermally driven process. In addition, hidden states of matter may be accessed if a system out of equilibrium follow trajectories to a state inaccessible, or nonexistent, under normal equilibrium conditions. In this talk, I provide a theoretical framework for describing photo-induced transitions to metastable states covering a range of photon energies from THz to UV. Further, I exemplify this theory by showing how similar transitions may be invoked in the transition-metal dichalcogenide WTe2 by different mechanisms, contrasting a THz driven transition to an optically driven transition. In the Weyl semi-metal WTe2, the intriguing electronic topological properties can be traced to the break of inversion symmetry resulting from the ground-state stacking sequence of the WTe2 layers. Previously it has been shown that THz light may be used to manipulate the interlayer stacking order, leading to the formation of the metastable centrosymmetric 1T* phase. Strategies for perturbing the stacking order by optical means instead is of high interest since it holds the potential for integration in ultrafast switches in future device designs. In a collaboration between theory and ultrafast electron diffraction experiments, we provide a mechanistic insight into switching from Td to 1T* phase of WTe2 using a 515 nm optical pump.