|M.Sc Student||Preezant Yulia|
|Subject||Excitons and Polaritons in Modulation Doped Quantum Wells|
Embedded in a Microcavity
|Department||Department of Physics||Supervisor||Professor Emeritus Elisha Cohen|
|Full Thesis text|
This dissertation describes a study of the spectroscopic properties of near bandgap elementary excitations in bare, modulation doped quantum wells (MDQW) and similar MDQW’s embedded in a microcavity (MDQW/MC). The GaAs/Al0.1Ga0.9As MDQW’s are 200Å-wide and have a two-dimensional electron gas (2DEG) with densities of ne ~ 1.8x1011 and 0.9x1011 cm-2. The λ-wide GaAs/Al0.1Ga0.9As MC cavity layer is cladded with GaAs/Al0.1Ga0.9As distributed Bragg reflectors (15/25 periods). In the MDQW/MC structures, the MDQW is located in the central antinode of the MC-confined photon electric field.
A comparative study was conducted of the circularly polarized reflection and photoluminescence (PL) spectra, measured at low temperatures (T~2K) and under a magnetic field that was applied perpendicularly to the 2DEG plane (B ≤ 7T). Sharp lines are observed in the MDQW/MC spectra (at B=0). Their energy dependence on the energy of the MC-confined photon (EC) shows Rabi splittings and level anticrossings that are very similar to those observed in undoped QW/MC systems. In contrast, the PL spectrum of the bare MDQW’s consists of a broad band that is characteristic of the 2DEG - free hole recombination, and no sharp lines are observed in the reflection spectrum. Under an applied magnetic field, the sharp line energies of the MDQW/MC structures show a different field dependence from that of the transitions between conduction and valence Landau levels, observed in the bare MDQW’s.
The reflection spectra of both bare MDQW’s and MDQW/MC’s were calculated by the transfer matrix method. For the 2DEG layer, this method used the energy-dependent refractive index function that was calculated from the optical dielectric function. The observed polariton energies of the MDQW/MC structures were analyzed by the coupled oscillators model. A fit the “anticrossing” diagrams that was obtained from the polariton energy dependence on either EC or on B, and it yielded the coupling strengths of the exciton-like transitions to the MC-photons.
The sharp line spectra of the MDQW/MC’s are interpreted as k║ = 0 cavity polaritons that are formed of pair excitations involving electron and hole states with large in-plane wave-vectors (near the 2DEG Fermi edge, namely, k║e = k║h = kF ~ (5-7)x106 cm-1, depending on ne). The coherence between these e-h pair excitations that is required to form polaritons, stems from the strong interaction of each e-h pair excitation with the same MC-confined photon.