| AVS 64th International Symposium & Exhibition | |
| 2D Materials Focus Topic | Thursday Sessions |
| Session 2D+MI-ThM |
| Session: | Novel Quantum Phenomena in 2D Materials |
| Presenter: | Aubrey Hanbicki, Naval Research Laboratory |
| Authors: | A.T. Hanbicki, Naval Research Laboratory M. Currie, Naval Research Laboratory G. Kioseoglou, University of Crete A.L. Friedman, Naval Research Laboratory B.T. Jonker, Naval Research Laboratory |
| Correspondent: | Click to Email |
Monolayer materials such as WS2 or WSe2 are direct gap semiconductors with degenerate, yet inequivalent k-points at K and K’. The valence band maxima for K and K’ have spin states of opposite sense enabling one to selectively populate each valley independently with circularly polarized light. Subsequent valley populations can be determined via the polarization of emitted light. Optical emission is dominated by neutral and charged exciton (trion) features, and changes in emitted polarization provide insight into the fundamental processes of intervalley scattering. We measure the circularly polarized photoluminescence of WSe2 monolayers as a function of excitation energy for both continuous-wave (cw) and pulsed laser excitation sources. Using cw excitation, the temperature dependence of the depolarization of the trion follows the same trend as that of the neutral exciton and involves collisional broadening. However, the initial polarization of the trion is nearly twice the polarization of the neutral exciton at low temperature. When a pulsed laser is used as the excitation source, the initial polarization of the neutral exciton increases and becomes very similar to the trion. We propose either an up-conversion process or screening from the instantaneously large carrier density generated by the pulsed excitation to explain these data. The difference in polarization behavior is linked to the different way energy is deposited in the system during these measurements. We also discuss changes in the photoluminescence induced by pulsed laser excitation.
This work was supported by core programs at NRL and the NRL Nanoscience Institute, and by the Air Force Office of Scientific Research #AOARD 14IOA018-134141.