Ultrafast Anisotropic Exciton Dynamics in Nanopatterned MoS2 Sheets

We study the optical properties of an anisotropic ripple-shaped two-dimensional molybdenum disulfide (MoS2) nanosheet deposited by chemical vapor deposition onto a nanopatterned silica (SiO2) substrate. We unveil a giant anisotropic optical response in the linear and nonlinear regime by a combination of optical extinction measurements, ultrafast broadband transient absorption experiments, and finite element method numerical simulations. In steady state optical measurements, such anisotropy appears as a polarization-dependent extinction in correspondence with the characteristic excitonic peaks (A, B, C, and D) of MoS2. Along with spectral changes, ultrafast measurements strikingly exhibit the onset of an anisotropic relaxation dynamic in the region of the C exciton. Numerical simulations indicate that the observed polarization-dependent optical response is dictated by the nanopattering of MoS2, with peculiar features belonging to the out-of-plane component of the dielectric tensor of MoS2 that is made effective through the rippled configuration. Our findings give a rationale to the anisotropic exciton response and show that morphology manipulation represents a valuable option for the tuning of optical and electronic properties in transition-metal dichalcogenides.