Kavli Affiliate: Farhan Rana
| First 5 Authors: Farhan Rana, Okan Koksal, Minwoo Jung, Gennady Shvets, Christina Manolatou
| Summary:
Optical absorption and emission spectra of doped two-dimensional (2D)
materials exhibit sharp peaks that are often identified with pure excitons and
pure trions (or charged excitons), but both peaks have been recently attributed
to superpositions of 2-body exciton and 4-body trion states and correspond to
the approximate energy eigenstates in doped 2D materials. In this paper, we
present the radiative lifetimes of these exciton-trion superposition energy
eigenstates using a many-body formalism that is appropriate given the many-body
nature of the strongly coupled exciton and trion states in doped 2D materials.
Whereas the exciton component of these superposition eigenstates are optically
coupled to the material ground state, and can emit a photon and decay into the
material ground state provided the momentum of the eigenstate is within the
light cone, the trion component is optically coupled only to the excited states
of the material and can emit a photon even when the momentum of the eigenstate
is outside the light cone. In an electron-doped 2D material, when a 4-body
trion state with momentum outside the light cone recombines radiatively, and a
photon is emitted with a momentum inside the light cone, the excess momentum is
taken by an electron-hole pair left behind in the conduction band. The
radiative lifetimes of the exciton-trion superposition states, with momenta
inside the light cone, are found to be in the few hundred femtoseconds to a few
picoseconds range and are strong functions of the doping density. The radiative
lifetimes of exciton-trion superposition states, with momenta outside the light
cone, are in the few hundred picoseconds to a few nanoseconds range and are
again strongly dependent on the doping density.
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