|Ph.D Student||Ronen Rapaport|
|Subject||Electron-Polariton Interaction in Semiconductor|
|Department||Department of Physics||Supervisors||Professor Emeritus Cohen Elisha|
|Professor Emeritus Ron Arza|
We study by reflection and photoluminescence 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 l-wide GaAs/GaAlAs microcavity (MC). For ne<5´1010cm-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 the neutral polaritons (that are formed of the neutral (e1:hh1)1S exciton (X) and (e1:lh1)1S exciton): (a) The MC photon - X- coupling strength increases as the square root of 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 non-vanishing 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 wavefunctions (that 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 times 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 wavevector and the detuning energy, the maximum distance that the charged polariton can drift under an applied electric field is calculated and found to be at least 10 times larger than that of the bare X-.
We also report on an experimental study of the electron-polariton scattering effect on the cavity polaritons optical linewidth, in the same microcavity structure. A transformation is observed from the strong to weak exciton-photon coupling regime at a very low excitation intensity. Over a surprisingly large microcavity-photon detuning energy range, the linewidth of the lowest polariton branch is much narrower than those of all the higher polaritonic branches. It exhibits a very small increase with increasing electron gas density. At narrow polariton energy ranges, a strong increase in the upper modes linewidth is seen. These results cannot be reproduced by a simple linewidth averaging model. The main question is whether this is a consequence of a distinct electron-polariton scattering process, resulting from the peculiarities of the microcavity polariton dispersion, or is just an energy-filtering effect of the MC on the bare X lineshape. We propose that the latter option is more appropriate. This conclusion is strongly supported by detailed numerical calculations.