Fine structure and lifetime of dark excitons in transition metal dichalcogenide monolayers

By C. Robert, T. Amand, F. Cadiz, D. Lagarde, E. Courtade, M. Manca, Takashi Taniguchi, Kenji Watanabe, Bernhard Urbaszek, Xavier Marie. 2017

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The intricate interplay between optically dark and bright excitons governs the light-matter interaction in transition metal dichalcogenide monolayers. We have performed a detailed investigation of the ``spin-forbidden'' dark excitons in ${\mathrm{WSe}}_{2}$ monolayers by optical spectroscopy in an out-of-plane magnetic field ${\mathrm{B}}_{\mathrm{z}}$. In agreement with the theoretical predictions deduced from group theory analysis, magnetophotoluminescence experiments reveal a zero-field splitting $\ensuremath{\delta}=0.6\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ between two dark exciton states. The low-energy state is strictly dipole forbidden (perfectly dark) at ${\mathrm{B}}_{z}=0$, while the upper state is partially coupled to light with $z$ polarization (``gray'' exciton). The first determination of the dark neutral exciton lifetime ${\ensuremath{\tau}}^{\mathrm{D}}$ in a transition metal dichalcogenide monolayer is obtained by time-resolved photoluminescence. We measure ${\ensuremath{\tau}}^{\mathrm{D}}\ensuremath{\sim}110\ifmmode\pm\else\textpm\fi{}10\phantom{\rule{0.16em}{0ex}}\mathrm{ps}$ for the gray exciton state, i.e., two orders of magnitude longer than the radiative lifetime of the bright neutral exciton at $T=12\phantom{\rule{0.16em}{0ex}}\mathrm{K}$.

Subfields: Electrical and Electronic Engineering, Materials Chemistry

View Paper: https://www.semanticscholar.org/paper/983d4351d9d00408a0469c958fe8434e17591df3