Exciton polaritons in transition-metal dichalcogenides and their direct excitation via energy transfer

Excitons, composite electron-hole quasiparticles, are known to play an important role in optoelectronic phenomena in many semiconducting materials. Recent experiments and theory indicate that the band-gap optics of the newly discovered monolayer transition-metal dichalcogenides (TMDs) is dominated by tightly bound valley excitons. The strong interaction of excitons with long-range electromagnetic fields in these two-dimensional systems can significantly affect their intrinsic properties. Here, we develop a semiclassical framework for intrinsic exciton polaritons in monolayer TMDs that treats their dispersion and radiative decay on the same footing and can incorporate effects of the dielectric environment. It is demonstrated how both inter- and intravalley long-range interactions influence the dispersion and decay of the polaritonic eigenstates. We also show that exciton polaritons can be efficiently excited via resonance energy transfer from quantum emitters such as quantum dots, which may be useful for various applications.

Figure 1

Functional behavior of the (a) real and (b) imaginary parts of the intrinsic exciton-polariton self-energy as per Eqs. (17) and (21) for a fixed value of the vacuum light wave number $\kappa$. Red color lines are used for the longitudinal ($L$) and blue for the transverse ($T$) polaritons. Shown with dashed lines are the results for a free-standing monolayer in vacuum, and solid lines are for the monolayer on a glasslike substrate, see text. (c) Schematically, not to scale: Dispersion of the longitudinal and transverse Coulomb excitons as renormalized by the electrostatic Coulomb interaction. At the interface between two media with different refraction indices, Coulomb excitons would be interacting with the transverse photons of the media, whose dispersion is shown by dashed lines.

Figure 2

Acceleration of the decay rate $\mathrm{\Gamma }$ of a randomly oriented electric dipole emitter such as a quantum dot in the vicinity of the monolayer on a glasslike substrate as compared to the spontaneous decay rate ${\mathrm{\Gamma }}_0$ in vacuum. The data computed with Eq. (24) are shown as a function of the emitter frequency $\omega$ and for different distances $h$ to the interface: 5 nm (black lines), 10 nm (red), and 20 nm (blue). Solid lines display results for the nearly dissipationless excitons ($\gamma =0.1$ meV), dashed lines are for the dissipation parameter $\gamma =25$ meV. These illustrative calculations were done with the model parameters $4\pi {\chi }_0=2$ nm, $4\pi {\hslash }^{2}A=0.25{\mathrm{eV}}^2\mathrm{nm}$ in Eq. (18) and $\hslash {\omega }_0=1.9$ eV, $M_0=m_e$ in Eq. (1).