M.Sc Thesis | |

M.Sc Student | Gandman Andrey |
---|---|

Subject | Femtosecond Coherent Phase Control of Resonance-Mediated Three-Photon Absorption |

Department | Department of Chemistry |

Supervisor | ASSOCIATE PROF. Zohar Amitay |

Full Thesis text |

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 2^{nd} and 3^{rd} 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.