M.Sc Thesis

M.Sc StudentGandman Andrey
SubjectFemtosecond Coherent Phase Control of Resonance-Mediated
Three-Photon Absorption
DepartmentDepartment of Chemistry
Supervisor ASSOCIATE PROF. Zohar Amitay
Full Thesis textFull thesis text - English Version


Femtosecond coherent control exploits the broad coherent bandwidth of ultrashort pulses to manipulate photo-induced quantum interferences and state-to-state transition probabilities. This M.Sc. research presents, femtosecond phase control of coupled multi-photon excitation channels, where one channel is coherently incorporated in the other, and the control is achieved using a-priori design of the control field. The study is experimental, however the experiments are accompanied by theoretical-numerical calculations. The scheme under study includes a first channel of non-resonant two-photon absorption, which is coherently incorporated in a second channel of resonance-mediated (2+1) three-photon absorption. The observables under control are the populations excited to the final states of the different channels and their corresponding branching ratio. The overall dynamics and interplay between parallel excitation pathways is extremely rich and coherently combines elements of non-resonant and resonance-mediated multi-photon transitions. The study is conducted in the perturbative regime, where the excitations can be described, respectively, within 2nd and 3rd order time dependent perturbation theory. The physical model system is the sodium (Na) atom. The involved states are the ground state 3s, and the excited states 4s and 7p. In the first part of the work, by proper pulse shaping, the final 7p population is continuously controlled over more than two orders of magnitude, up to 300% of the population excited by a corresponding transform-limited pulse. The control mechanisms are identified as intra-group and inter-group interferences involving two groups of 3s-7p three-photon excitation pathways: Those that are exactly on resonance with the intermediate 4s state and those that are only near resonance with it. The role played by the near-resonant pathways leads to the general feature of resonance-mediated multi-photon transitions of exceeding the performance of a transform-limited pulse already in the perturbative regime. This is contrary to non resonant multi-photon transitions. In the second part of the study high degree of control is demonstrated over the branching ratio between the two excitation channels, i.e., between the populations excited to the 4s and 7p states. Particularly prominent is the methodology we have termed "coherent decoupling", by which, the 7p population is independently controlled over a wide range of values while the 4s population is kept fixed on an arbitrary chosen level. Several cases are implemented experimentally. The results demonstrate the power of femtosecond coherent phase control in manipulating the absolute yields of population transfer and corresponding branching ratio as almost completely independent variables over a very wide control range.