Ph.D Thesis

Ph.D StudentKarni Ouri
SubjectNonlinear Dynamical Phenomena and Quantum Coherent
Light-Matter Interactions in Room Temperature
Semiconductor Quantum-Dot Gain Media
DepartmentDepartment of Electrical and Computer Engineering
Supervisor PROFESSOR EMERITUS Gad Eisenstein


The research reported here investigates the ultra-fast dynamics of quantum dot (QD) semiconductor optical amplifiers (SOAs) and lasers operating around 1.55 µm wavelength. Generally, both the gain and refractive index of such SOAs are considerably modified during the propagation of powerful optical signals through them. Nano-structured SOAs, in particular, exhibit large non-linearity originating in the electronic processes occurring in response to the amplified optical signal. This strong non-linearity is useful in all-optical signal processing, which motivates the study of the ultra-fast dynamics of such QD SOAs. The focus on these devices in the telecommunication wavelengths is further emphasized since they were not available up until recently, while on the other hand they have already presented exceptional performance as lasers. It was hence desired to search for the fundamental opto-electronic dynamics in these devices, and to demonstrate extreme non-linear phenomena in them.

This study is mainly experimental, utilizing two systems: A multi-wavelength pump probe system, where a pump pulse triggers non-linear processes in the SOA while another signal probes their transients, and a system where the non-linear phenomena are revealed by characterizing the modified shape of the pump itself after its interaction with the medium. The interpretations made in the later experiments are accompanied by a numerical model developed and upgraded as part of this work.

The thesis first demonstrates the power of the self-pump-probe technique, revealing a unique transient wave-mixing (over sub-ps timescales) between a propagating pulse and the oscillating fields in a laser cavity. Another unique phenomenon revealed by the same system (following its identification in quantum dash SOAs), in cooperation with the numerical model, is the coherent Rabi-oscillations of the electrons in the QDs that coherently interact with an ultra-short pulse. Due to its potential in bringing quantum phenomena to applicative operational conditions, the work continued to evolve mostly around these Rabi-oscillations. The slower electronic processes standing behind the coherent interactions where characterized by a pump-probe investigation over a few ps time. They revealed also that ultra-short pulses experience significant two-photon absorption (TPA) in those SOAs. An ultra-fast investigation was then performed in order to trace the TPA effect in the context of the coherent interactions.

In parallel, the numerical model employed to back the experiments was continually upgraded. First accounting for coherent effects in a homogeneously broadened medium, it was expanded to treat inhomogeneously broadened gain spectra, and was later broadened to include non-resonant effects such as TPA.

The complete comprehensive model supported the subsequent demonstration of coherent control using shaped pulsed excitations. Numerical calculations were first performed, predicting the ability to enhance or diminish the Rabi-oscillations depending on the sign of a frequency chirp imposed on them at the input. The experimental efforts that followed confirmed this prediction.

The prospect of controlling and characterizing quantum coherent phenomena in room-temperature practical devices drives this work further on. It lays the infrastructure for further research in that field, that will bring use to quantum mechanical effects for practical communication and signal processing applications in practical conditions.