Ph.D Thesis

Ph.D StudentCapua Amir
SubjectDynamical Processes in Nanometric Semiconductor Lasers and
Optical Amplifiers
DepartmentDepartment of Electrical and Computer Engineering
Supervisor PROFESSOR EMERITUS Gad Eisenstein


In this work we investigate the dynamic behavior of  InAs/InP nanostructured quantum-dot and quantum-dash (wire-like) gain media. The fabrication of these nano-species is based on self-assembly methods which result in size variation and hence in inhomogeneously broadened gain spectra which exhibit extremely wide gain bandwidth and very complex cross saturation dynamics.

The thesis starts by studying the energy dependent response of the gain media following a perturbation by a short 150 fs pulse. For that purpose, a unique ultrafast multi-wavelength pump-probe setup was constructed having a temporal resolution of 150 fs, determined by the width of the perturbing pulse. In addition to the quantification of the energy dependent dynamical response in the nanostructured gain medium, a unique instantaneous gain mechanism was discovered which is induced by nonlinear two-photon excitation. The investigation reveals also that the two photon absorption coefficient in these nano-materials is larger by almost two orders of magnitude compared to common semiconductor gain media.  The large nonlinear coefficient allows another unique phenomenon: the induction of sufficient gain to initiate laser oscillations. Next, the research explored the light-matter interaction on shorter timescales namely, during the pulse itself. This study is carried out by measuring the phase and amplitude of the electro-magnetic field of the 150 fs pulse after propagation. The data was obtained from an extremely sensitive cross frequency-resolved optical gating (X-FROG) system which we have constructed. Intricate details of the carrier and gain dynamics are extracted and in addition, with this system, a novel cascaded four-wave-mixing interaction between the short pulse and an oscillating quantum-dot laser field was discovered. The experimental work is accompanied by a detailed theoretical finite difference time domain (FDTD) model which describes the co-evolution of the electron wave function in the active material together with the electromagnetic field according to the Maxwell and Schrödinger equations. The model accounts simultaneously for a vast number of effects such as gain saturation, carrier diffusion, cavity quantum electrodynamics, wave mixing, self-phase modulation, spatial hole burning and more. The combination of the powerful FDTD model and the X-FROG apparatus brought this research to its highlight: the direct observation of the co-evolution of the electronic wavefunction in the semiconductor together with the electro-magnetic field with a nearly single femto-second resolution. These are revealed in the form of Rabi oscillations and self-induced transparency, all occurring at room temperature.