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UBC Theses and Dissertations

Quantum coherent control of laser-kicked molecular rotors Bitter, Martin


The objective of this dissertation is the experimental study and control of laser-kicked molecular rotors. Nonresonant rotational Raman excitation of linear molecules by periodic sequences of ultra-short laser pulses allows for the realization of a paradigm system - the periodically kicked rotor. This apparently simple physical system has drawn much interest within the last decades, especially due to its role in the field of quantum chaos. This thesis presents an experimental apparatus capable of producing long sequences of high-energy femtosecond pulses. Rotation of diatomic molecules, the most basic version of quantum rotors, is investigated under multi-pulse excitation. In the case of periodic kicking, the wave function of the quantum rotor dynamically localizes in the angular momentum space, similarly to Anderson localization of the electronic wave function in disordered solids. We present the first direct observation of dynamical localization in a system of true rotors. The suppressed growth of rotational energy is demonstrated, as well as the noise-induced recovery of diffusion, indicative of classical dynamics. We examine other distinct features of the quantum kicked rotor and report on quantum resonances, the phenomena of rotational Bloch oscillations and Rabi oscillations. In addition, multi-pulse excitation is investigated in the context of creating broad rotational wave packets. Another goal of the reported study is the coherent control of quantum chaos. We demonstrate that the relative phases in a superposition of rotational states can be used to control the process of dynamical localization. We specify the sensitivity to external parameters and illustrate the loss of control in the classical limit of laser-molecule interaction. Our work advances the general understanding of the dynamics of laser kicked molecules and complements previous studies of the quantum kicked rotor in a system of cold atoms. The results encourage further studies, e.g. of quantum phenomena which are unique to true rotors. The possibility of control in classically chaotic systems has far reaching implications for the ultimate prospect of using coherence to control chemical reactions.

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