Controlling Optical Properties of Electron Hole Plasma in Monolayer Transition Metal Dichalcogenides

Recently, monolayer transition metal dichalcogenides (TMDs) have shown the ability to undergo a phase transition between an insulating gas of strongly bound excitons to a conducting electron-hole plasma (EHP). Typically, to reach this phase transition, ultrashort optical pulses are used to create a non-equilibrium high carrier density. However, this EHP state can also be reached via continuous wave (CW) excitation. In most semiconductors, the high carrier density needed for the EHP phase transition cannot be reached using CW light since nonlinear recombination processes limit the equilibrium carrier density. However, the mechanical properties of monolayer TMDs can be used to manipulate the electronic structure to change the carrier dynamics, allowing for high carrier densities to be reached through CW excitation. Specifically, by applying strain, the energy offset between direct K-K and indirect Γ-K transitions can be used to shift the carrier population between valleys. By interchanging different transition metals or chalcogens, we can change the indirect-direct energy offset creating an EHP predominately in the indirect transition or direct transition. This has a profound impact on the optical properties of the EHP, providing an avenue to engineer novel opto-electronic devices.