Dark exciton brightening and its engaged valley dynamics in monolayer WSe2

Abstract

We theoretically investigate the valley Zeeman splitting, dark-exciton brightening, and magnetic-field-controlled dark-exciton engaged valley dynamics in monolayer ${\mathrm{WSe}}_{2}$ subjected to a tilted magnetic field $\mathbf{B}$. In Faraday geometry (out-of-plane ${B}_{\ensuremath{\perp}}$), only bright-exciton emissions emerge, and the valley polarization (VP) as a function of ${B}_{\ensuremath{\perp}}$ for ${\ensuremath{\sigma}}^{+}$ and ${\ensuremath{\sigma}}^{\ensuremath{-}}$ circularly polarized laser excitations features an ``X'' pattern, i.e., the VP is locked to the valley. We also find that the photoluminescence (PL) intensity exhibits a similar behavior to VP. In the Voigt geometry (in-plane ${B}_{\ensuremath{\parallel}}$), however, aside from the bright-exciton emission, the dark exciton becomes brightening. The PL intensity of the dark exciton is enhanced parabolically with increasing ${B}_{\ensuremath{\parallel}}$, independent of the pumping laser helicity. For B along any other direction, as B increases, the dark-exciton emission intensity increases but depends on the pumping laser helicity, accompanied by an decrease of its VP. Furthermore, not only the bright-exciton but also the dark-exciton emission peak splits for laser excitation with different circular polarizations. In addition, the latter demonstrates a much larger peak splitting than the former, which facilitates individual manipulation of each of the two valleys. Our theory paves the way for a study of the properties of bright-dark hybrid states with lifetime orders of magnitude longer than that of the bright states, which is desirable for fields of either spintronics or valleytronics (or their combination).