Ph.D Thesis


Ph.D StudentKhanonkin Igor
SubjectCoherent Light - Matter Interaction in Optical Amplifiers
based on InAs/InP Quantum Dots Operating at Room
Temperature
DepartmentDepartment of Electrical and Computer Engineering
Supervisor PROFESSOR EMERITUS Gad Eisenstein
Full Thesis textFull thesis text - English Version


Abstract

Zero dimensional materials, so-called quantum dots (QDs), have attracted major interest in recent decades in both fundamental physics and optoelectronic device applications. Self-assembled QDs are nanometric semiconductor materials that exhibit atomic-like discrete energy levels and delta-function density of states. Moreover, the atom-like character ensures that the QD ensembles behave as a cascaded collection of effective two-level systems, facilitating an optical gain material with vastly improved characteristics compared to higher dimensional materials. Beyond opto-electronic device applications, QDs have become a prominent platform as a solid-state technology for quantum information technologies.

This thesis presents the recent experimental and theoretical demonstrations of quantum optical phenomena, such as, the Ramsey interference, the photon echo and the revival of quantum coherence, induced and controlled in an ensemble of QDs operating at room temperature and at the telecommunication wavelength of 1.55 μm.

Using two time-delayed femto-second resonant pulses, arranged in a pump-probe like configuration, Ramsey interference manifests itself in periodical oscillations of the probe pulse amplitude, it’s phase and the temporal position of the output probe pulse (but with a quarter cycle delay relative to the amplitude variations). This unique phenomenon, which can only be observed in distributed systems, results from the coupling between the real and imaginary parts of the susceptibility where the phase shift is the time-domain manifestation of its complex nature. Following in time the exponential decay of the Ramsey interference fringes (with a time constant of the inhomogeneous transverse relaxation times T2 * ), we have succeeded to observe subsequent periodical reappearance of the QDs quantum coherence. These coherent recurrences result from constructive and destructive interference between the QD set of “modes” that were initially coherently excited. The consecutive revivals decay with the homogeneous transverse relaxation time, T2 > T2 * . A record room temperature T2 of 5 pico-seconds was demonstrated. The long T2 value was confirmed in a photon echo experiment with a three-pulse excitation. Towards practical implementations, we have examined the T2 and T2 * time constants, that define the QDs coherence time, for the two main decoherence mechanisms, carrier - carrier and carrier - phonon scatterings.

An additional quantum coherent interaction related to the so-called tunneling injection (TI) has been studied in QD-based lasers and amplifiers. In the TI scheme, “cold” charge carriers are injected, by means of tunneling through a narrow barrier, directly to the QD ground state, thus avoiding hot carrier injection which improves, in turn, the dynamical properties of QD lasers. Two separate TI processes, resonant and non-resonant, were identified which occur simultaneously at different QDs spectral locations. Various TI designs of QD lasers were proposed that either improve or deteriorate the laser properties, correspondingly reducing or increasing QD coherence time.