M.Sc Thesis

M.Sc StudentKhanonkin Igor
SubjectUltrafast Dynamics of Optical Gain Media Based on
INAS/INP Quantum Dots
DepartmentDepartment of Nanoscience and Nanotechnology
Supervisor PROFESSOR EMERITUS Gad Eisenstein
Full Thesis textFull thesis text - English Version


The three-dimensional carrier confinement in semiconductor quantum dots (QDs) provides an optical gain material with vastly improved laser characteristics including low threshold, temperature insensitivity, high speed, low noise and narrow linewidth. It also enables high gain, linear and low noise in semiconductor optical amplifiers (QD SOA).

The superior device characteristics are related to the carrier and gain dynamics, which were investigated by multi-wavelength pump-probe technique, where the response to a pulsed perturbation was measured at any wavelength across the broad inhomogeneously broadened gain spectrum. During the first 2 ps following the pump pulse, we have observed a unique response that takes the form of a plateau in the transmission. It is followed by a recovery to high values due to carrier originating from two photon absorption.

In quantum engineering the QD SOA can act as an effective two-level system, in which basic quantum mechanical phenomena occur. Here we present the first ever observation of Ramsey fringes in an ensemble of QDs and at room temperature. Oscillations in intensity and instantaneous frequency, due to the Ramsey resonance, are accompanied by an unique cumulative effect - an oscillation in the separation between the output pump and probe pulses, which cannot be observed in the more common single quantum dot systems. The oscillation modulation depth decreases with the nominal input pulse delay, which directly maps the process of decoherence and avails the extraction of the coherence time.

To validate numerically the experimental results, a comprehensive finite difference time-domain model was used, that was developed originally to analyze quantum coherent effects induced by ultra-short pulse propagation in QD SOAs and modified to allow calculation of population inversion everywhere in space and at every wavelength. The model was also used to simulate photon echoes for quantum memory applications.