Ph.D Student | Ronen Rapaport |
---|---|

Subject | Electron-Polariton Interaction in Semiconductor Microcavities |

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
(n_{e}) two dimensional electron gas, and is embedded in a l-wide *GaAs/GaAlAs* microcavity (MC).
For n_{e}<5´10^{10}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
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 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 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 n_{e},
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.