Negatively charged polaritons in a semiconductor microcavity

We study by reflection spectroscopy the cavity polaritons in a structure consisting of a single GaAs/AlAs quantum well that contains a low density (ne) two dimensional electron gas, and is embedded in a λ-wide GaAs/Ga1-xAlxAs microcavity (MC). For ne<5x1010 cm-2, negatively charged MC polaritons are photoexcited, as a result of the strong coupling of the MC photon and the negatively charged [(e1:hh1)1S+e] exciton (X-). The charged polaritons have several properties that are distinct from those of neutral polaritons [which are formed from the neutral (e1:hh1)1S (X) and (e1:lh1)1S excitons] (a) The MC-photon - X- coupling strength increases as √ne. This is analogous to the dependence of the confined-photon - atom coupling on the density of free atoms in a metallic cavity. (b) The charged polaritons have a nonvanishing electric charge that is due to the bare X- charge. (c) Since the energy difference between the bare X and X- excitons is smaller than the coupling strength of each one with the MC photon, these two bare exciton states are admixed in the charged polariton states. The experimental reflection spectra were analyzed using a model of coupled quantum oscillators representing the excitons and the confined photon mode. From the fitted spectra it is deduced that the X- coupling strength increases with increasing ne, and there is an oscillator strength transfer from X to X-. Using the cavity polariton wave functions (which are obtained from the model fitting) we calculate the effective charge and mass of all the cavity polaritons as a function of the MC-photon energy. The calculated (e/m)eff ratio reaches a value ∼200 x larger than that of the free electron in a bare GaAs quantum well. Using the calculated dependence of the effective polariton charge and mass on the in-plane wave vector and the detuning energy, the maximum distance that the charged polariton can drift under an applied electric field is calculated. The charged polariton is expected to drift a distance at least 10 x larger than the bare X-