Negatively Charged Quantum Well Polaritons in aGaAs/AlAsMicrocavity: An Analog of Atoms in a Cavity

The negatively charged exciton $\left(X^{-}\right)$ is observed to strongly couple with the microcavity- (MC-)confined photons in a GaAs quantum well containing a two-dimensional electron gas with $0<\mathrm{}{n}_e\le 3\times {10}^{10}{\mathrm{cm}}^{-2\mathrm{}}$. This strong coupling results in a formation of charged polaritons. The coupling strength is found to depend on $\sqrt{n_e}$, in analogy to two-level atoms in a cavity. The analysis of the reflection and photoluminescence spectra shows that $X^{-}$ is strongly admixed with the neutral exciton via their coupling with the MC photons. The linewidth dependence on $n_e$ indicates that electron-polariton scattering is effective.