טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentJonathan Shemesh
SubjectPolarization Sensitive Spectroscopy of Charge
Tunable GaAs/InGaAs Quantum Dots
DepartmentDepartment of Physics
Supervisor Full Professor Gershoni David
Full Thesis textFull thesis text - English Version


Abstract

In this work we investigate the photoluminescence emitted from a single self assembled semiconductor quantum dot embedded within a PIN junction. We demonstrate the use of polarization sensitive spectroscopy as an efficient tool for identifying optical transitions between states involving multiple confined charge carriers.

For this goal we built an experimental setup, which allow us to perform high resolution polarization sensitive spectroscopy, with full control over excitation's intensity, wavelength and polarization. We partially control the average charge in the quantum dot, by changing the bias applied on the sample.

We performed full polarization analysis, of each spectral line in the photoluminescence spectrum. Our experimental results are compared with model calculations, based on a full configuration interaction method. This enabled us to identify spectral lines composed of up to six charge carriers. One example is the the doubly negatively charged biexciton (xx-2). Surprisingly,

we discovered that few spectral lines are linearly polarized but not along the conventional crystallographic direction along which the quantum dot is elongated. From the comparison with the model, we found out that this is the case whenever an unpaired P-state charge carrier participates in the initial state of the optical transition. By applying an external electric field we shift the P-levels energy relative to the energy of the exciting light. This way we are able to perform high resolution photoluminescence excitation spectroscopy which is also sensitive to the polarization state of both the exciting and emitted light. This way we succeeded in understanding the fine structure excited P-state of the spectral lines x+1 and xx+1 . We show that the structure of the P-levels is influenced by the spin part of the carriers (as probed by circularly polarized light) and

by the carriers wavefunctions (probed by linearly polarized light). Yet another interesting finding that we report on is the appearance of negative circular polarization memory at quasi resonant excitations of the doubly and triply negatively charged exciton lines (x-2 and x-3, respectively).While, surprisingly, at the same time, the positively charged exciton line

(x+1 ) does not show any appreciable positive memory, as it otherwise shows.