טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentPreezant Yulia
SubjectThe Interaction of Fermi-Edge Polaritons and Exciton-
Polaritons in a Modulation Doped
Microcavity
DepartmentDepartment of Physics
Supervisor Professor Emeritus Elisha Cohen
Full Thesis textFull thesis text - English Version


Abstract

A detailed spectroscopic study is presented of two types of polaritons in a variety of GaAs/Al0.1Ga0.9As quantum well - microcavity (QW/MC) structures: a. Excitonic polaritons, b. Fermi-edge polaritons. These are elementary excitations formed of interband electron-hole pair (e-h) transitions that resonantly interact with the MC-confined photon. In type a, the e-h pair is bound into excitons, and in case b, the pair is unbound due to the presence of a two dimensional electron gas (2DEG). This research focused on the interaction between excitonic and Fermi-edge polaritons that were formed in a specifically designed QW/MDQW/MC structure consisting of an undoped, 18nm wide GaAs/Al0.1Ga0.9As QW and two 20nm wide GaAs/Al0.1Ga0.9As modulation doped quantum wells (MDQW) embedded symmetrically in the same 2l-wide MC. The electron density in the MDQW’s was  n 2DEG=1.1x 1011cm-2. The undoped QW was separated from the MDQW’s by 103nm barriers, sufficiently wide to suppress conventional tunneling of electrons from the MDQW’s into the empty QW .

Photoluminescence and reflection spectra were measured at T=2K, of the QW/MDQW/MC and of various QW/MC and MDQW/MC structure, that served as reference esystems. The spectra were taken over a wide range of the MC-confined photon energy, thus varying the interaction strength between the electronic excitations of the QW and MDQW's, and the confined photon.  The elementary excitations of the QW/MDQW/MC structure were observed to be mixed polaritons of the two types, whose properties (energy, intensity and linewidth) are strongly dependent on the MC-confined photon energy. The extracted polariton energies were analyzed using the coupled oscillators model. This analysis indicates that electrons accumulate in the undoped QW, and their density varies with the MC-confined photon energy, increasing from 2x108 cm-2 when it is far from the resonance region to 4x1010cm-2 near resonance . This most interesting finding is interpreted as resulting of electron transfer from the MDQW’s into the undoped QW, that is enhanced by the resonant interaction between excitonic polaritons and Fermi-edge polaritons.

Confocal spectroscopy studies were conducted on the QW/MDQW/MC and on a bare QW/MDQW structures, by measuring the photoluminescence and reflection spectra, scanning the detection over areas of 80x 80 mm with a spatial resolution of about 2x 2 mm. It was found that in these structures, the exciton spatial distribution is very uniform, and the spatial distribution of the electron density in the MDQW's showed a 4% inhomogeneity, in the form of islands of the size 4-10mm.