טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentEilon Poem-Kalogerakis
SubjectRadiative Cascades and Coherent Dynamics in Semiconductor
Quantum Dots
DepartmentDepartment of Physics
Supervisors Professor Emeritus Ehrenfreund Eitan
Full Professor Gershoni David
Full Thesis textFull thesis text - English Version


Abstract

Quantum dots are artificial semiconductor nanostructures composed of nano-meter size regions of semiconductor material of one type embedded in a host material of another type. They localize both types of charge-carriers: electrons and holes, in all three dimensions. This results in a discrete energy level spectrum, similar to spectra of natural atoms. As such, quantum dots are considered as a possible venue for the implementation of quantum bits and quantum logic gates. This venue is particularly compatible with modern opto- and micro-electronics.

One of the advantages of quantum dots is that charge carriers can be generated in them optically. Electrons and holes can also recombine and emit light. The spectrum of the emitted light is composed of discrete lines, and the polarization of the emitted light is intimately related to the spins the recombining charge carriers.

In this thesis I present an experimental and theoretical study of the dynamics of charge carriers confined in single semiconductor quantum dots. To do so I measured time-resolved, polarization sensitive intensity cross-correlation functions between various pairs of spectral lines emitted from an optically excited single quantum dot. From the knowledge of the charge and spin configurations of the initial and final states of each spectral line, I was able to follow the temporal dependence of the system evolution between various many-carriers states.

Comparing the experimental results to classical or quantum rate equation models helped to shed new light on the dynamical processes that the carriers undergo, such as optically induced charging, spin scattering and spin dephasing.

Several new types of "radiative cascades", in which two photons from two different lines are emitted sequentially, were discovered and fully understood.  

We showed that in some cases the intermediate states in these cascades are metastable, "spin-blockaded" from relaxation to the ground state, due to their different spin configurations.

This understanding enabled the measurement of the spin scattering rates of confined holes. It also enabled us to optically access, for the first time, "dark exciton" states, which cannot by themselves absorb or emit light. We showed that the formation of such excitons with a well-defined spin direction is heralded by the emission of a circularly polarized photon from a spin-blockaded metastable two-exciton state. Moreover, this discovery enabled us to follow the temporal evolution of the dark exciton's coherent spin precession. We did that by charge injection and subsequent time dependent detection of a polarized photon.