Optical Studies of Ultra-thin WSe2

Abstract

Truly two-dimensional systems allow for examination of quasiparticle physics in the presence of strong Coulomb interactions resulting from reduced dielectric screening and under conditions of energy level quantization due to high carrier confinement. Here, we employ optical techniques to study such a system: ultra-thin WSe2 crystals. Due to the overpowering oscillator strength of excitonic transitions, our studies explore the unique behavior of neutral and charged excitons which stem from correlations between spin, crystallographic momentum, and layer occupation. The energy and polarization of the incident and detected optical fields provide insight into these correlations, while application of electric fields allows for tuning of excitonic species as well as spin-pseudospin correlations. Perhaps the most salient result from this work lies in the first demonstration of a superposition state of excitons located in opposite corners of the 1st Brillouin zone. However, other firsts include efficient photon up-conversion in a monolayer semiconductor and spectrally separating intralayer and interlayer excitonic states. Beyond these results, we also find these ultra-thin semiconducting sheets exhibit behaviors such as non-linearity in optically-driven membrane oscillations, electrically tunable excitonic species, and an all-electrical spin Zeeman splitting. Such widespread characteristics and capabilities underscore the importance of continued optoelectronic studies within this class of 2D materials.