טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentRamon Guy
SubjectPolariton-Electron Scattering in Microcavities with
Embedded Quantum Wells
DepartmentDepartment of Physics
Supervisors Professor Ady Mann
Professor Emeritus Elisha Cohen


Abstract

Exciton-electron scattering processes are theoretically studied in semiconductor quantum structures that contain a low density two- dimensional electron gas. Addressing first quantum wells, electron scattering on both neutral and charged excitons (trions) is considered. A microscopic model is presented, taking into account both elastic and dissociating scattering. The model is based on calculating the exciton-electron direct and exchange interaction matrix elements, from which the exciton scattering rates and the resulting linewidths are derived. Electron scattering is found to be much more efficient than both exciton-acoustic phonon and exciton-exciton scattering even for a very dilute electron gas. The dependence of the exciton linewidth on the electron density changes radically upon the transition of the electron gas from a classical to a degenerate distribution. Electron scattering on charged excitons is more effective than on neutral excitons, due to the larger interaction matrix elements and their different dependence on the transferred momentum. Nonetheless, the dependence of the charged excitons linewidths on the electron density is complicated due to the screening effect on the Coulomb interaction. Obtaining excitonic line shapes from

the calculated linewidths, the resulting reflection spectra are compared with experimental reflection measurements taken from a GaAs quantum well.


Next, we consider the effect of electrons on the cavity polariton linewidths in a microcavity with an embedded quantum well containing a variable density electron gas at T=80K. The linewidth dependence on the electron density and on the cavity mode energy is explained by exploiting the calculated asymmetric bare exciton line shape, due to electron - exciton scattering. For a given electron density, the exciton line shape is inserted into the semiclassical linear dispersion theory of the coupled  excitons and cavity mode, yielding the reflection spectra that

are formed of the three polariton branches. The model reproduces the reduced lower polariton linewidth and the complex dependence of the middle and upper polariton linewidths on the cavity mode energy. At the low temperature limit, we study non-linear effects due to electron-polariton scattering in the microcavity system. Calculating the various electron-polariton scattering processes in both inter- and intra-branch channels, the model reproduces the non-linear dependence of the photoluminescence spectrum and integrated intensity on electron density. The enhancement of the integrated intensity, demonstrated for specific cavity mode energies is accounted as well. The model shows that the polariton dynamics is governed by their trionic part, the most effective scattering process being the dissociation of the charged exciton

component in the polariton states.