Optically Triggered Emergent Mesostructures in Monolayer WS2

The ultrahigh surface area of two-dimensional materials can drive multimodal coupling between optical, electrical, and mechanical properties that leads to emergent dynamical responses not possible in three-dimensional systems. We observed that optical excitation of the WS2 monolayer above the exciton energy creates symmetrically patterned mechanical protrusions which can be controlled by laser intensity and wavelength. This observed photostrictive behavior is attributed to lattice expansion due to the formation of polarons, which are charge carriers dressed by lattice vibrations. Scanning Kelvin probe force microscopy measurements and density functional theory calculations reveal unconventional charge transport properties such as the spatially and optical intensity-dependent conversion in the WS2 monolayer from apparent n- to p-type and the subsequent formation of effective p-n junctions at the boundaries between regions with different defect densities. The strong opto-electrical-mechanical coupling in the WS2 monolayer reveals previously unexplored properties, which can lead to new applications in optically driven ultrathin microactuators.

Controllable Plexcitonic Coupling in a WS2-Ag Nanocavity with Solvents

Strong coupling between emitters and cavities underlies many of the current strategies aiming at generating and controlling quantum states at room temperature. Recent experiments reveal strong coupling between two-dimensional transition metal dichalcogenides (TMDCs) and individual plasmonic structures; however, the coupling strength is quite limited (<200 meV), and the active control of the coupling strength is challenging. Here, we demonstrate the active tuning of plexcitonic coupling in monolayer WS₂ coupled to a plasmonic nanocavity by immersing into a mixed solution of dichloromethane (DCM) and ethanol. By adjusting the mixture ratio, continuous tuning of the Rabi splitting energy ranged from 183 meV (in ethanol) to 273 meV (in DCM) is achieved. The results are mainly attributed to the remarkable increase of the neutral exciton density in monolayer WS₂ as the concentration of DCM is increased. It offers an important stepping stone toward a further study on plexcitonic coupling in layered materials, along with potential applications in quantum information processing and nonlinear optical materials.