Negatively charged polaritons in a semiconductor microcavity - art. no. 235310

We study by reflection spectroscopy the cavity polaritons in a structure co nsisting of a single GaAs/AlAs quantum well that contains a low density (n( e)) two dimensional electron gas, and is embedded in a h-wide GaAs/Ga1-xAlx As microcavity (MC). For n(e)<5x10(10) cm(-2), negatively charged MC polari tons 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 polari tons have several properties that are distinct from those of neutral polari tons [which are formed from the neutral (e1:hh1)1S (X) and (e1:1h1)1S excit ons] (a) The MC-photon-X(-)coupling strength increases as <root>n(e). 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) Sinc e 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 excito n states are admired in the charged polariton states. The experimental refl ection spectra were analyzed using a model of coupled quantum oscillators r epresenting the excitons and the confined photon mode. From the fitted spec tra it is deduced that the X- coupling strength increases with increasing n (e), and there is an oscillator strength transfer from X to X-. Using the c avity 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 similar to 200 times larger than that of the free electron in a ba re GaAs quantum well. Using the calculated dependence of the effective pola riton 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 times larger than the bare X-.